THE PROBLEMS
T he pro blem s which follow have been d ivid ed into grou ps. Each problem in a gr ou p
is related to th e other problem s in the same gro up, as indi cated by the group title. Howeve r, the solut io ns o f the problem s within a particular grou p will not necessar ily be sim ilar ;
ill gene ral, the solu t ion of each pro bl em in a group ill ustr ates a d ifferen t aspect of the top ic
specified in the grou p t itle. T herefore, the stude n t who wishes to obtain a thoro ugh b'Tasp
of ele mentary chemistry sho uld end eavor to become fami liar , by discussion wit h his classmates, wit h the solu tions to their p ro bl ems. I n this way his knowledge of each to pic will be
broade ned an d h is ability to u nd erstand mater ial discussed in the classroom increa sed .
Group A. Physical Changes
l. Pu t seve ral small pieces of dry Ice into a rubber balloon. Tie the neck tigh tly
wit h a small piece o f stri ng. W ait.
' Vhy d id the balloon b u rst?
2. W rap a sma ll piece of filter paper aro un d the bul b of a the rmome ter. Soak the
filter paper wit h acetone ; then bl ow gently on the filter paper .
\ \'hy d id th e thermometer indi cate a drop in tem pera ture?
3. H ea t 2 g- o f w pper sulfa te pc nt ah ydratc in a crucible, turn ing it oyer occa sionall y
wit h a st irr ing rod , unti l it be com es light bl ui sh-gray in color. Pu t approximately half of
the prod uct in a clean , dry test tube and sto p per it wel l. Put t he re mainde r on a watch glass.
Examine both samples 24 ho u rs, or more, later.
Wh y di d one sample become bl ue an d the o ther reta in its ligh t bluis h-gray color?
4. Set fire to a small p iece of paper in an evaporating d ish. \ Vith a ham me r and small
na il, pu nc ture the cap of an linch ill cd bottl e of car bona ted soft d r in k. I nver t the bottl e
ove r the fire an d extingu ish the flames.
Wh y was the lire extingu ished ?
5. Pu t approxi mately I g of calc ium chloride on a watc h glass. Let it stand u nd isturbed for a few days.
\\' hy d id a l iq u id collect on the wat ch glass?
6. \ \'eigh out 25 g of calcium sulfate hcm ih vdratc and mi x wit h enough water to
form a th ick paste . Form the paste into a ro ugh conical shape an d wa it .
'\' hy d id the pasty m ixt ure harden ?
7. Prepare about 100 ml, each, of I-M soluuons of C 1:.! H:l:.! O l t , K C I, an d Zn (N <h k
U..ing· a mixt ure of icc, rock salt , and wat er, determ ine the freezin g po int (usc only a few
milliliters) of eac h solu tion .
" ' h y arc the freezing po int s of these solu tion s not 00 C?
8. Stretch a r u bber band as ta ut as possibl e without ca using it to break an d , by to uching it to you r li ps, note its approxi mate tempera ture, Release the tension an d again sim ilarly
note th e approx imate tem peratu re of the flaccid r u b ber.
' Vh y d id the tem perature change?
37
9. Fi ll a te st tu be almos t to the brim with water. Cut ou t a r ectan gu lar p iece of paper
a bou t i by in H U ll " Fold it in to a wi ng ed V, as ill ustra ted in Figure 7. P u t it into the test
lu be so t ha t th e lower end o f th e V is u nder the su r face o f the water. P ut one cr ystal of
pota..sium pcr llla nga nate on th e pa per, a.. illust rat ed . \\'ait.
\\' h y did the red colored liq u id fl ow in a r ibbon -like manner to the bo tt om of th e
tc vt tu be ?
Fi r m ly affi x a sm all test t ube insi de a larger te st tu be. Fi t a cor k in the mouth of
the larger lest tube. previously bori ng a proper-sized hol e- to fit th e sm all te st tube- in the
cor k. I nsert the sm all test tube in the ho le and make su re that the sma ll test tube is approximatel y centered imide the larger test t ube. Now prepa re a mixture of ice, r ock salt , am i
wate r in an to\OO-ml beaker. T h e temperat ure o f rhi.. mixt ure shou ld not be lower th an
_ j O C. P ut 5 111 1 of a 0 .1 % K ~ I n O .. solution in the sm all test tube and p lace the test tu be
assem bly in the mixture. A ft er a film of solid approx imat ely I mm thick ha s formed o n th e
wa lls o f the sma ll te st t u be. remove the rest tube asscm b ly from the mixture and pOllr out
th e unfrozen K ~ J nO .. solu t ion . Carefully rinse the solid rema in in g in the small te st tube
wit h a few mi lli lite rs of icc-cold wa ter and d iscard the washings. Let the remaining ..olid
melt and mmpare the co lo r o f th is liqu id wit h that o f the K~l n O .. solut ion previou cly
poured Iroru the sma ll test tu be .
t vh y were the colors of t he two liq u ids di fferent ?
10 .
Group B. Ions in Solution
l. I n a tc..t t ube ad d 5 m l of 0 .5 -,\( C H :\CO O :'\a solu t ion to 5 ml of 0 .5 ·M Ag:'\O:\
solu tion. Stir the mi x ture and note the co lor o f the preci p itate. Add 5 m l o f 0 .5 -AI N a l
so lu tion: s topper the test tube and sh ake it.
\\' hy d id the preci pi tat e ch ange color?
2. Sm ura te 10 m l o f wate r with h yd rogen ch lor ide . Sat u ra te 10 m l of d r y tol uen e
with hydroge n chl or id e. P u t a sm all p iece of mag nes iu m r ib bon in each se lu tio n .
\ \' h y W;\S there a d ifference in th e behavior o f the ma gnesiu m?
3. P re pa re . by m ix in g proper qu anti ties o f Ag-N O ;l an d C H :\CO ON a solution s,
a pprox im atel y 0.3 m illimole of silver ace tate. \ Va..h th is prec ip itate with a sm all q u ant ity of
wa te r and tra nsf er it to a test tube con tain ing to m l of water. \V ar m the te st tube and
it s co nt en ts for 15 m inu tes, using a wa ter bath hel d at 40° C ± 5° C , stirring inter m itteml y. Cool the co n te nts to 0 ° C. Then filter. D ivide the filt rate int o two a p proxi m at el y
eq u al port io ns. To o ne port ion ad d 0 .5 m l of 3-M C lb COO i'\ a solu t io n: to the o ther add
n..: . lil t of 3-M C l h C O O H solution. \\Tait.
' Vhy d id a p reci p itate for m in one case an d not in the o ther?
4. Pre pare a I-M ..olut ion o f lead (II) n itrate and a I -M solu tion of coppe r . (II)
..u lfare . P ut 200 111 1 of one solution in a 250-1111 beaker and 200 m l of the ot her solu t ion in
a secon d 250-m l bea ker. Ca refu lly clean and weigh two sn-ips (ca. 1 by 5 cm) of lead and
t wo str ip s o f co p pe r. \\~ i t h the lead str ips in the P b (:'\ 0:1)2 solution an d the copper st ri p s
in the Cu SO .. solu t ion , con nect the t wo so-constr uc ted {ell s in ser ies wi th each o ther. us ing
len gths o f wi re and alligat or cl ips, as illustr at ed in Fig- ure 8. Con nect the rema in ing copper
electrod e to the po sitive po le of a d irect current ..ou rc c and the remaining lead electrode to
38
th e negati ve pole. After several mi n u tes, stop the flow o f curren t and de te rm ine the loss in
weight o f o ne of the lead and one o f the <:op pcr el ectrodes.
\ Vhy di d the co pper and lead ele ctrodes not lose the same amou nt of 'weight?
5. M ix 5 ml of O.l -Al Agr'\' O:l so lu tion wit h 5 ml of O. I·M H e l solu tion . Add 3-.Af
N H 40 H solu tion, d rop by drop , with stirr ing , u n til the prec ip itate just d issolves. T hen
ad d , stirring, 10 m l of O. l -At H el solu tion . Add 5 ml of 0.15-M Na ZS20 3 solut ion. St ir.
\Vhy d id th e precipi tat e d issolve, reappear, an d then d issolve agai n?
Fig ure 8
Fig ure 7
6. Prepare 100 ml o f a sat urated solu t ion of Ca (O H )2 an d d ivide it into th ree
ap pro ximatel y equal port ions . T o oue portion add 10 ml of O.1 -Al FeCt , solu tion ; to
anot her add 10 ml of O.I -M Cu (N O :!) z solu tion ; to the ot her ad d 10 m l of O. I-M Pb(NO a)z
solut ion. Gen tl y bo il eac h mi xtu re.
\ Vhy d id a precipitate form and red issolve in some cases and not in other s?
7. Xlix 5 ml of O. l -.M (C H:!CO O) zPb solut io n with 5 ml of O. I ·M Na 2S0 4 solu t io n.
T hen ad d 5 m l of 0.2-M K t Cr0 4 solut ion. Sti r.
\ Vh y d id the prec ipita te chan ge color?
8. Add I-AI H e l solu t ion, dro p b y d rop , with stirr ing, to 10 ml of O. l -M Pb ( N O:~ ) ~
solu t io n u n ti l the p recipita t ion o f lead ch loride is complete. Filter the mi x ture . Add a few
d ro ps of 6-M sulfuric aci d to the fi ltr ate.
W h y di d a preci pitat e ap pear when sulfu ric acid was ad ded to the filt rate?
9. Usin g a variable voltage powcr sou rce , electrol yze a I-N Na C I solu t ion with a lype
46 or 4i (6- to 8-volt ) m ini at ure lamp bu lb in series with the elect ro lytic cel l. Adjust t he
voltage so tha t th e bul b glows wit h me d iu m in tensi ty. Rep lace the salt solu t ion with a
I-.N C H ::CO O H solu t ion . Do not alte r th e voltage adjus tmen t.
\Vh y d id th e b ulb glow less briglul y dur ing the electrolysis of the acetic acid ?
10. I n the hood, pass hydrogen sulfide in to a n.5-At solut ion of N iS0 4 . Similarly trea t
a
n.s-'\[ solu tion o f CUS0 4.
\ Vh y d id a p recipitat e form in o ne case an d not in the o ther?
39
Group C. Properties of Gases
1. Place an u n infla ted ba lloon ins ide a 500 -ml Erlen me yer flask. I nflate the balloo n
with enough a ir so th at it occupies a volume o f a p pr ox im ately 250 m1. Fill th e re mai n ing
spa ce in the flask wi th ox ygen an d sto p per the flask with a one-ho le ru b ber sto p per. Th e
hole in th e rubber sto p per sh ou ld pr eviousl y han bee n fill ed ,»ith a sh on length of glass
tube wi th a rubber pol iceman over the outer , open , end. I n the hood fill another 500 -ml
Erl en meye r flask wi th n itrogen monox ide and sto p per it sim ilarl y. Rem ove the two policem en fro m the en ds of the gla ss tu be s and quic kly re p lace th em wi th a single sha n length
of r u b ber tube, th us joining the flasks togethe r. ' Vail. (T he ap pea ra nce of the phenomenon
to be ob served m ay be h astened by warming the wall s of th e flask conta ining the oxygen
with the pa lm s of yo ur hand s.)
' Vhy d id the ball oon expa nd?
Cau tion: Do no t pre par e oxygen b y h eating a ch lorate or perch lorate. This m ethod
is d an ge rous. I nstead , prepare this gas b y gent ly heat ing a fr eshly pre pared 3.5% solut ion
o f sod iu m perborate almost to the boi ling po in t.
Fig ure 9
2. Cons truct an ope n-ended m anomete r from a length of glass tub ing an d pa rt ly
fill the manom eter with par affin oi l. Pu t 3 IIlI of concen trated am m on ium hydrox ide solu t ion in a filt ering flask. Connect on e arm of the m anometer to the side an n of th is filt ering
lIask wi th rubber tubin g". T horo ughl y m o isten a p iece o f filter paper wi th wa ter an d lay
it over the m ou th of the filt er ing flask. Con nect the side arm of a second filt er ing flask to
the o ther end of the m anometer wit h r u bbe r tub ing. Invert the secon d flask over the first,
as ill us trated in Figure 9 , be ing sun' th at a good "ca l is for me d between the two filteri ng
40
flasks by the filter pa per se par at ing them . N o air sh ou ld leak in or ou t o f t hi s jo in t ; t he
filt er paper sho u ld act as a gasket.
\ Vh y d id a pressu r e d ifference de velo p be tween the two flasks?
3. Fill a co n ical filter ing flask a bout three-fourths fu ll wit h water. Form a nozzle
011 o ne en d of an l x-cm lengt h of glass tube. Fi t the tube w ith t wo one- hole stoppers so
t hat the sto ppe r nearest t he nozz le fits an Erl enmeyer flask an d th e ot he r sto p per fits the
f ilter ing flask. Fill an Er lenm eyer flask w ith am monia an d insert the n ozzle end of the
glass t ube into th is flask , secu r ing th e stop per firm ly. N ow fit the assem bly of flask, tubes,
and slo p pe rs into the filt ering flask as illustrat ed in Figure 10. If no thi n g ha p pe n s, p ut a
small p iece of ice o n tOP of th e inverted Erlen m eyer flask.
W h y d id the wat er flow into the u pper flask?
Figur e 10
Figur e 11
1. Fill a test tu be with h yd rogen chloride . Close th e m out h o f the test tu be wit h a
slOp per and p lace the d osed mout h o f the test tub e unde r wa ter. Rem ove the sto ppe r.
Wh y did the rest tu be fill with wate r?
5. P ut 10 1111 of wa te r in a test tube. Add o ne drop of phenolphth alein solu t ion .
Pu t )0 m l o f i .5-M :'\ H 4 0 H solu tion in a second test tube. Con ne ct th e tu be s as illu st rat::d
111 Figure I I. \\7a it.
W h y d id the wate r turn pin k?
6 . Ligh t a cand le . lin en a sma ll beaker a nd lower it over the ca nd le flam e, ho ld in g
it in this posi tion fo r a few secon ds.
" 'h y was the flame ext ingu ished ?
4 1
7. Cut two small p ieces of filter paper, about 2 cm sq u ar e, from a filter pa per circle.
Moisten one of these pieces with conce n trated h yd rochloric acid and d ro p it in to a 25 0-ml
Erlenmeyer flask , mak in g sure that it ad her es to th e bo ttom. Lay a p iece of pa per , a bo ut
5 em sq ua r e, over the mouth of the llask. Simi larl y moisten the seco n d p iece o f filter paper
with concentra ted am m on iu m hydr ox ide solu t ion and affix it to the bottom of a secon d
Er len meyer llask. Co ver the m o u th of t his flask wit h a piece of p ap er a bout 5 cm sq u are.
Now wait ten m inutes; do not di stur b the flasks. T hen , keeping the cover over the mout h ,
in vert the flask conta ining the filter paper moistened wi th h ydroch lo ric acid over th e o ther
flask. Xlake sure that the m o uths of t he two Basks are j ux ta posed , in line with each oth er ,
as acc ur ately as possible. Th en pu ll ou t the two covers wh ich sep ar ate the flasks. N ote
th at "s moke" first form s in the upper flask. Now, using two fres h Erlenmeyer flasks and
two more p ieces of filter paper, repeat the exper im en t, exce pt that this ti me the flask
conta ining t he filter pa per wet with ammon ium h yd rox ide solu tion goes on to p. X otc t hat
the "sm oke" first forms in t he lower flask.
' Vhy di d the "s m oke" form lirst in the llask contai n in g the filter pa per m oi stened wit h
concen trated h ydrochloric acid, without respect to its relat ive posit ion?
8. F it a 250· m1 Erlenmeyer flask wit h a on e-hol e sto ppe r in which an inverted U sha ped glass mbc h as been fixed. Arrange th is setu p so tha t the ou ter en d of the U tube d ip"
u nder th e su r face o f several mi lliliters of wa ter in a beaker. R em ove th e sto pper from the
flask an d p ut 8 g of sod iu m h yd roxide , 8 g of am mon ium ch loride, an d 15 ml o f wat er in
the flask. Re p lace th e stop per, ma k in g sure tha t th e ou ter end of the glass tube is below
the surface of the wat er in th e beaker. ' Va il.
' Vh y did water pass from the beaker int o the flask?
9. Open a bottle of concen trated h ydrochlor ic acid . ' Vail.
\V h y d id fum es issue fro m t he open mouth o f the bott le?
10. Fi ll a buret to the brim wit h water and, with yo ur th u mb over th e op cn end , invert
the bur et, placing the th u mb-ca p ped end u nder th e su rfa ce of the water in a panl y filled
large beaker. Remove your thumb. O pen the sto pcock and admit a bou t 10 ml of air in to
the buret. Attach a fu n ne l, by means o f rub ber tu b ing, to the b uret spou t. P ut 15 m l of
d ict hy l eth er in the fu n ne l. Carefu ll y ad m it approximatel y 5 m l of ether in to the b uret.
'Va il ten min utes. Note the volu m e of gas now present . Admit 2 m ore ml of e th er . Note
the volu me o f gas. Admi t 2 more ml of ethe r. Note the volu m e of gas. Admi t 2 more ni l
of ether. N ote the vo lu m e of gas present in the buret.
\ Vh y, after the add ition of the first 2-m l port ion of ether , d id th e gas vo lu me rem ain
approxim at ely co nstan t wh en more ether was added?
Group D. The El e me nts of Group I a nd The ir Com pounds
.
1. T o 0.5 g of co pper tu rn ings or copper wire , ad d 5 m l of b rom ine water. I n the
hood , h eat the mi xture gen tly unti l all the bromine present is driven from the liqu id . Then
add 3 ml of 0.5-M K"Fe (C N )1l solu ti o n.
' Vh y d id a precipi tat e form ?
2. Saturate 150 m l of concentrated ammoruu m hydroxi de solution with sodi u m
ch lor ide. Drop sm all p ieces of dry ice into the m ix tu re.
42
\ Vh y d id a precip itate form?
3. In a su itab le ap paratus, di st ill half o f a 20-ml sample of 2-N (N H 4) 250 4 solution.
co llecting the con de nsate.
\V hy was the conde nsate ba sic?
4. T o 10 m l of a.I -N AgN 0 3 solution add one drop o f co nce n trated n itric acid solu t ion . Then add sufficient !\' aCI solu tio n. of any su ita b le co ncentrat ion , to just prec ipi tate
all the silver as silver ch loride. Do not add an excess o f p reci pi ta t ing reagen t. Ad d 7-N
ammon iu m h yd ro xid e solut ion , d ro pwis e, with stirring, u nti l the preci p itate is just red issolved . Do not add an ex cess of ammonium h ydro xide. T h en ad d a few d ro ps of 2-N
7.n50 4 solut ion.
\ Vh y was the sil ver ch loride re-pr ecip ita tcd ?
5. T o 5 ml of O. I-M Cu5 0 4 solu rion. add 0.5 ml of 15·M N H 40 H so lu t ion. Stir well
and no te the co lor. Th en ad d 15 m l of a.2-M KC ;'\" solut ion, m ixing well.
\ Vh y d id th e co lor disappear?
6. Put 10 ml of I-AI CuC l 2 solu tion in a test tu be . Pu t 10 ml of I-M Cu50 4 so lu t ion
another test tube. P u t a 2-cm length o f recently cleaned, b ri gh t copper wire in each
sol ut ion .
\ \' hy d id on ly on e of the wire s beco me co ated with a wh ite deposit?
111
7. Pu t severa l d ro ps of S-N 7\ H 40 H solutio n in to 10 ml of I -N C US04 solutio n.
\ Vhy di d the col or tu rn da r k bl ue?
8. P ut 15 ml of clean mercu r y in a sma ll beaker. Po u r 50 ml of half-sat urated 1\ aC 1
solut io n on top o f the mercury. Using a graphite anod e an d the mercu r y as the cathode .
electro lyze the salt solu tion for 30 m in u tes at a poten t ial o f approxima tely to volts. Turn
off th e curre nt.
Tra nsfer the mercury to a beak er con tain ing 50 ml of O.l-N l\'aO H solution. I nsert a
p iece of graph ite into the al kaline solu tion, lower ing it un t il it to uches the me rcu ry .
\ Vh y 'was a gas evolved fro m the liq uids in the second beaker?
Cau tion: Do n ot be careless with mcrcu ry. Ask your ins tructor for directions con cor ning the di sposal of the mercury when you arc finished wit h this p roblem . If mercury
is sp illed on th e la boratory bench or floor , clean it up thorough ly and ask your instructor
10 inspec t the area. Mercury evaporates and its vapors are ve ry po isonous.
9. P re pare approximat el y 5 X 10- :1 mo le o f silver ch loride and pl ace the material in
a ll eva po rat ing d ish . T he silver ch lor ide shou ld be mo ist but washed free of re sidua l im p uri t ies. Put a \"Cry sm all p iece of mossy zinc in the center of the moist silver chlor ide. P ut one
drop of concen tra ted su lfu r ic acid o n lOp of thc zinc. Cover the evaporating dish with an
opaq ue obj ect, to excl ude l igh t, and exa mine fro m time to time .
\ \'h y did a reactio n occur?
10. D issolve 5 g of sod iu m ch lo r ide in 50 IIl I of water and ad d fou r d ro ps of phenol ph thale in solution and 10 ml of 0.5-"\'[ KI solu tio n. Sti r. Fill a U tube almost full with thi s
liqu id . Pu t a pl atin u m el ectro d e in each ar m and el ect ro lyze, usin g about 10 to 15 volts, DC.
\ Vh y di d different colo rs appcar in the arms o f the U tube?
43
Grou p E. The Elements of Group II and Their Compounds
l. Put I ml of I-M
N a2 S 0~
sol uti on in each of two cen tr ifug e tu bes. To one tube
add , approximately 0.5 ml at a tim e, gently boili ng after each addi tion, 8 ml of D.3 -At BaCh
solu tion. To the other t u be add 8 ml of D.3-At CaC1 2 solution. St ir each m ixtu re wit h
separate stirri ng rods. Centrifuge both m ixtu res and decan t the d ear liq ui ds in to separa te
test tubes. Mi x 2 ml of D.3-At CaCh solut ion with 2 ml o f 0.3-At BaC t , solut ion. Pour
2 m l of th is m ixture into o ne decantarc and the remaind er into th e other d ecan tate .
' Vhy '...' as a precipitate formed in one case and no t in the other?
2. Crumple a strip of ma gnesium ri b bon 10 em lo ng into a cr ude ball an d place it
in a cr uci ble. Cover the cr ucible and heat it, gently at first, the n mo re stro ng ly. R emo ve
the cruc ible cover fro m t ime to time, to ad m it air. \ Vhen all react ion has ceased let the
cruc ible an d conte n ts coo L Be su re tha t the cruc ible an d its co n te nts are coo L Add a few
dro ps of water to the powd er in the cruc ible and note the odor.
' Vhy is an odo r prod uced?
3. Pu t 10 ml o f 3-M Zneb solu tion in each of two test tubes. Ad d a few drops o f
3·M I':aO H solut ion to on e test tube ; a precipitate will form. Ad d a few drops of 3-M I':a O H
solut ion to the other test tube, and with stirring, co ntin ue to add more N aO H solution
u ntil the p recipitate j ust d issolves. Now ad d 3-M H e l sol ut ion, drop by d ro p, with stirri ng ,
to both tes t tubes.
W h.y d id the prec ipitate in the fir st test tu be d issolve ami that in the secon d reap pear?
4. T o 10 ml of I-At Hg (1'\ 0:1)2 solution add, drop b y drop, with st irring, several
mi llilite rs of I-AI KI solut ion.
' Vhy d id the p recipitate re disso lve?
5. Put 10 ml of I-At n itr ic acid in ea ch of two test tubes. Put a small q uan tity of
reagen t grade zinc in one of these test tu bes and a sma ll q uan tity of reagent grade magnes ium
in the other. (Use that weight of each metal which will react wit h approximately 8 m l of
I·AI nitri c acid. ) \\' hen the metals han compl etel y reac ted, add 5 ml o f 6-AI N aO H solut ion to eac h test tu be and sto pper each tes t tu be wit h a one-hole sto ppe r. Pu t a strip o f
mo isten ed red litmus pa per, folded length wise, in eac h sto pper ho le. Do not dog the hole.
Pu t both test tu bes in a boiling water bath for a few seconds.
'V hy d id one l itm us paper becom e blue mo re rapid ly than the other?
6. By passing, in the hood , hydrogen sulfide into 10 ml of O.D5-.M ZnCh solut ion ,
precipi tate all possible zinc out , as ZnS. Filter, and test the filtrate for com plet e p recip itation by ad ding mo re h ydroge n sulfide . If prec ipitation was com plete add 3 1111 of 5-M
C H :!C O O0:a sol u tion to the fi lt rate.
W h y did a prec ipitate for m?
7. Di vid e approxi ma tely I ~ of magn esium carbonate into two approx imately eq ua l
portions. Place these po rt ions III two cr ucible s. Heat the conten ts o f one cr ucible to re el
heat for fiftee n min utes. Let it coo l. Now add I ml of 3-N hydroch loric acid La each
crucible.
W h y d id the two portions react d ifferent ly?
44
8. Fi ll a test tu be half full of d ist illed water. Fill ano ther ha lf fu ll with a sat urated
solut ion of Ca (O H)z. Pass C O 2 into th is solut io n unti l the pre ci pita te, which forms at
first, redissolves. Add I m l of a 2% soap (repeat: soap. not detergen t) solution to each test
tube. Shake both vigorously.
\ Vh y di d a foa my frot h fonn in one test tube and not in the o ther?
9. H eat 3 g- of ZnC0 3 in a test t ube fitted with a del ivery tu be , the end of which is
im me rsed in a satu rated solut ion of Ca (D B ) 2.
' Vhy was a precipitat e formed ?
10. Put 5 IIlI of brom ine water in a test tu be . Ad d 1 g of gran ular zinc.
\ Vhy d id the color cha ng c?
Group F. O xidctlon-Reducticn Reactions
I. Pu t 10 m l of O.5-M AgNO a solution m a test tu be . Measu re the tem perat ure of
th is solut ion. Ad d I g of finel y d ivided ZI nc.
\ Vh y d id the tem pera ture rise?
2. O bt a in a length of 6- or , -mm d iameter Pyrex or Kimax tu bing, abo ut 30 cm long.
Close one end by constric tio n in a gas flame. Let the tu be cool. Pu t I g of d iarsenic trioxide
in the tube , pack ing- it down in the closed end. Put I g of powdered charcoa l o n top of th e
white powder, packing it down. I ncline the tube , but keep th e o pe n en d higher than th e
closed end. Very gently heat the t ube ncar the top of the charcoal. If the charcoa l moves
abo ut as heat is applie d, reduce the heat and, while heating more gently, tap the t ube to
help the charcoa l ret urn to its or igi na l posi tion. G rad uall y increase the hea t on the charcoal. Xow, keepin g the cha rcoal ho t, apply heat to the po rt ion of the tu be containi ng t he
d iarsenic tr iox id e.
' Vh y was a silvery de posit formed at the upper end of the t u be?
3. Put approx imately 150 rnl of I-AI Na l\'O :1 solu tion in a 250-m l beaker. Partiall y
imm erse str ips (ca. I by 5 cm) of zinc and of coppe r in this solut io n. Make sure that th e
two pieces of metal do 110t touch each o ther. Co nnect the zinc strip to the positive pole of
a source of d irect current and the coppcr str ip to the ne gat ive pole. \ Vhile gently st irring
the solut io n, elect rolyze for a few min ut es at 5 to 10 volts. N ote the preci pi tat e whi ch form s.
Xow reverse the po lar ity on the metal str ips an d cont in ue the electrolysis for a few mor e
min utes, stirr ing occasiona lly. Note the appeara ncc of the material in the bea ker.
Wh y were the resu lts di fferen t when the po larity was reversed?
-I. Put ap pro xima tely 100 ml of I-.M N H t C I solut ion in a 150-m l beaker. Firmly
fix strips (ca. I by 5 em) of copper , lead , and zinc so that, not touching each other, tite y
arc partially immersed in the i\ HtCI solut ion. ~I ca s u re the potent ial d ifference between
the lead and zinc str ips an d between th e lead and copper strips.
\\'h y was the po lari ty of the lead strip different-
5. I mmerse a small piece of magnesium r ibbon
,rhy di d the r ib bon change color?
45
III
a 0.5-M CuSOt solution.
6. Usc the hood for this proble m. D issolve a small drop of mercury in a few milliliters of concentra ted nitric acid. Then di lute the solut ion wit h an approximately equal
volume of water. Clean a sma ll piece of co pper. a p iece abou t the size o f a penny. until it
sh ines. Dro p it into the so lu t ion . \ Va it 15 seconds and then flush th e liq u id Out o f the
container by fill ing it to overflowing with water.
" 'h y d id t he p iece of copper become silver-colored?
7. D issol ve 0 .5 g of iod ine in 10101 o f ethy l alcohol. P repare 50 101 o f a l -M :'\a 2 5 2 0 ::
so lu tio n. .\f ix the two solu t io ns.
\\'h)' did the co lor d isap pear?
8. T ri turat e I g of potassium iodide wit h 0.5 g of po tat o starch an d enou gh wat er
to mak e a th in paste. D ip a str ip of fi lter pa per (ca. I by 8 em ) in to the pa ste an d withdra w
il carefu lly so th at some of the paste d in gs to the filter paper. Lay th e treated filter pap er
o n a p iece o f alu m in u m foi l abou t 10 on sq u are . \ Vith a wire. att ach th e foil to the nega t ive te r m in al o f a low-voltage direct-cu r re nt source . Attach a seco nd wire to th e posit ive
ter m in al an d lightly touch the othe r end o f th is wire to the upper face of th e filter paper
stri p.
\\'h y d id the wire ma rk th e filter paper str ip?
9. P repare 50 1111 of th e following four solutions. A . B. C. and D (all arc 0.05-.\'):
Iron (I I I) ch loride , A .. u-ipotassium hexacyano fcrrate, B ; ammonium disulfate hcxaaquoFer-rate (II) , C ; and potassium th iocyanate. D. M ix A and B in a beaker. 'f ix C and D in
anoth er bea ke r. Xl ake a salt bridge. fi lling it with O.I -N XaC I solution. and p lu g the end ,
loosel v with small t ufts o f cotton.
P ut 10 m l of the A ·B m ixture in a test tube an d 10 101 of the C-D mixtu re in an other
test tube. Adj ust the test tubes so that the level o f liq ui d in eac h is the same d istance from
the labora tor y bench top. T hen inser t the legs of the salt bridge in to the two test tu bes,
ma king sure th at the ends arc u n der the su rfa ce o f the li qu id in eac h tes t tu be. P ut a
p lat in um wire electro de in each test tu be. Co n nect the plat in um wire in th e A -B test t u be
to the n egative ter min al o f a sou rce o f d irect cu rrent and the p lati n u m wire in the C·})
test tube to the posit ive te rm ina l. Elect rol yte for several m in u tes at a pot en t ial o f 3 to [)
volts.
\ \' h y d id the conten ts of th e two test rubes t urn d ifferent co lors?
10. I n the hood, put 2 g of iod ine in a test tube. Add 10 ml of water. t van n the
mixture slightly. Pass hyd rogen su lfide into the warm liquid .
W h y wa ... a precipitate formed?
Group G .
The Eleme nts of G roup 111 a nd Their Com pounds
l. P u t I g of alumin um , tu r n ings or granu lar, into 10 ml of 2-M KO H solut ion .
H eat the mixtu re gent ly if necessary.
\ Vhy was a gas evolved ?
2. Di ssolve 10 g o f bora x in 50 m l o f water. gen tl y warming if n ecessar y. D ivide the
46
solut ion in to two ap proxim ate ly eq ual po rtions. ' 1'0 on e portion add 10 m l of 3-M hydrochloric acid. Let both po rtions stan d u nd isturbed .
\ Vh y d id a precipitate form in bo th cases?
3. Pu t 5 ml o f 3-Al AtCl, solu t ion in each of two test tu bes. To one test tu be add a
few d ro ps of 3-M Na O H solu tion. T hen add more N aO H solu t ion, with stirring, unti l th e
p reci pitat e d issolves. Treat the ot her test tube sim ilarl y. but usc 3-M N H ..O H solu t ion to
ob tain p recipitation an d then add more N l-i.. O H solut ion.
\ Vh y d id the precipitate d issolve in one case an d no t in the ot her?
4. Cle an a sma ll p iece (ca. I by 10 em) o f alum inu m foil with benze ne to remove
an y grea se that may be on t he su r face. T hen ligh tly burn ish o ne side of the fo il with a
sma ll piece of steel ' . . 001. Dip one end of the foil in to a few m illi li ters of O.25-M H gC l :!
solution and remove fro m the solu tion . hold ing the foil in the air for a few mi nutes.
\Vhy was a d eposit formed 0 11 the foil?
5. T arni sh a small piece of silver about the size of a d ime with h yd ro gen sul fide.
Dissolve 3 g o f d isodi u m car bo na te in 150 ml of wat er and p u t a 5-cm sq uare piece o r
alu m in u m foil in thi s solu tion. Pu t the tarnished piece of silver in the so lu tion, ma king
su re that it to uches the alu m in um foiL
\Vhy did the tarnish d isap pea r?
6. Put 30 mg of magnesiu m in a small ba lloo n and carefu lly fix the mouth of th e
balloon ove r the mou th of a small test t u be filled to the brim wit h 3-M h yd rochloric ac id.
I nsofar as possibl e press the re sidual air ou t of the balloon. Be su re, up to this po in t, to
keep the ac id in the test t u be an d the magnesium in the balloon. Now carefully in vert the
test tu be and, after all actio n has ceased, me asure the vol ume of the d isten ded ball oon.
R epeat . using 60 mg of alu mi n um and 9-1\f NaO H so lut io n.
\V hy, when al um inum was used , was the volu me of the di stended balloon not approximat ely twice th at of the ba lloon used with the mag nesiu m?
7. Prepare IO ml o f an 80 % satu rated solution of borax . Dilu te I m l o f thi s solu tion
wi th 10 IIlI o f water. st ir, th en add 1 ml o f 0.5-N AgN O a solu tion. T o the remain ing 9 ml
o f the bo rax sol u rion , ad d I lU I of O.S -N AgI\' O:1 so lut ion and st ir.
\ Vhy were the H \ 'O prec ipitat es not the same?
8. P u t 50 ml of wate r an d 4 g of tr ih ydrogcn bora te in each of two beakers. T o one
beaker add 2 g of po tassiu m hydroxide. T o the o ther add 2 g of sod iu m h yd roxid e. Hea t
both gently for a few mi n u tes u n til the cont ents are d issolved. (Ta ke care th at the heating is not prolo nged sufficien tl y to cause marked loss of water by evaporation. ) T hen cool
the co n ten ts of bo th beakers by placing the beakers in an ice-water bath , st irring
occasionally.
.
\ Vh y d id a p recipitate form in one beaker an d not in th e other?
9. Xl ix 50 ml of water with 50 m l of 95 % ethanol. P ut 3 g of borax in the m ixture
an d heal it to ap pro ximatel y 50 ° C, ho ld ing it at this temperatu re and st irr ing to ob tai n a
satu rated solut ion o f borax . \ Vhcn no more borax will d issol ve, le t the solid settle an d
d ecant most o f the warm solut ion in to two sma ll Erlenmey er flasks, or test tubes, divid ing
47
it approximate ly eq ually in the se two containe rs. I m m ediatel y ad d I g of bor ic acid to
one o f the port.ion s of d eca nta tc. Stopp er both containers an d shake them u nt il the b or ic
acid in th e on e has d issol ved . T hen cool bot h so lut ions to 0 0 C.
\ Vhy di d a pr ecipi tate n ot for m in both solu tions?
10. .\ fix I g o f po wde re d Pyrex or Kimax borosilicate gla ss (soft glass will n ot do)
with I I{ of sod iu m car bonate. P ut the mix tu re in an igni tio n tu be fitt ed wit h a one-hole
stopper in whi ch a ga s deli very tube has be en inserted . H eat the mixture to a du ll red and
pass th e gas wh ich is evo lved into a satu r ate d solut ion o f Ba(O H )2.
Wh y was a precip itate formed?
Group H. Hyd rolysis and Equlibrium
1. To 5 till o f a I-M .\f gSO -t solut ion, add 2 ml o f I -Af N H -tOH solu tion. To 5 m l
o f a I-M .\Ig-SO-t solu t ion ad d 5 ml of a l -.M I"\ H -lCI solu t ion ; then add 2 ml o f I -M
;\ H-tO H soh uion .
' Vhy did a precipi tate form in one case an d n ot in the ot h er?
2. P u t 15 1111 of I-Af C H :1COON H -t solu tion in a sma ll be ak er. Add two d ro ps 01
m ethy l ora ng e indicator. Stir. P u t 15 ml of water in another sm all be aker an d add two
d rops o f m eth yl oran ge in di cat or. St ir. Deter m ine the vo lu m e o f 1-M h yd rochlor ic aci d
req u ired , for each of the two liq u id s, to cau se the m eth yl oran ge to chan ge color. Put 15 ml
of 1-1\1 C H 3COO N H -t solut ion in an ot her sm all beake r an d IS m l o f wa ter in a four th sm all
beaker. T o each , ad d t wo d ro ps o f ph enolph thalei n indica tor an d st ir. D eter min e t he
volum e of I·AI I"\a O H solution re qu ir ed , for each of the two l iquids, to cause the phe no lp h thale in to becom e pink .
' Vhy d id the am mo n iu m acetat e solut ion requ ir e mo r e aci d an d more ba se than was
re qu ired by the water in ord er to effect a ch ange in t he colors of the indica to rs?
3. Add 0.25 m l o f glacia l acet ic aci d to 200 m l o f wat er. Add 0.25 1111 of gla cia l ace tic
acid to a solutio n o f 0.5 g of sod iu m aceta te in 200 ml o f water. To bot h acidi fied solu tions ad d t wo d ro ps o f thy mo l blue in di cat or. Ad d just enough. drop by drop , wi th st irring,
o f O. l -M I-h SO -t solut ion to th e acet ic acid solu t ion to cause the in di cato r to ch ange to a
p in k colo r. Ad d an ex act ly equ al amo un t o f O.I -i\f H 2SO j solut ion to the acet ic acid sodium ace tate solut io n .
\Vh y d id the color rema in th e sam e in the acetic aci d-sod ium acetate solu t io n?
4. To 100 m l of recent ly boiled . cool d istilled water add o ne drop of 3-M 1\'aO Il
solut ion and two drops o f phe no lp htha le in indica tor. Blo w your breath through this solu tion for seve ra l m in u tes.
\V h y d id the color d isappe ar?
5 . To 8 m l o f O. l -M CaC f , solution add 2 m l o f O.S-AI (CO O H ) 2. St ir; cen tr ifu ge.
( Be sure to ba la nce the centri fuge wi th a co u nt er po ise b efore turn ing on the motor. Ask
you r inst r uctor for assistance if in doub t.) De cant an d S,l\'C the dcc antatc. P ut 10 m l o f
fl.l-Al Ca C h solution in a test tube. Add O.S m l of 1-M N H ~O H solution to t he liq ui d
ltl the test tu be a nd to the de cantatc.
' Vhy d id a precip itate for m o nly in the dcc an ta rc?
.8
6. Add a few drops of ph en olph thalein indicato r to 25 ml of J-N N a2S0 3 solut ion .
Xotc the co lor. Fr om a buret ad d I -N H 2S0 4 solu tion d ropwise, with st irring. u n til the
ind ica tor co lo r just d isappears. To be su re en ough acid has been ad ded , add t hree morc
d ro ps o f l -.N H 2S0 4 solu tion. St ir. Note th e co lor. Now add a few drops of brom cresol
green in di cator to the m ix ture. St ir . Note the co lor.
\ Vh y did th e color of the brom cresol green indicator ind ica te that th e solut .iou was
basic, whereas the loss of the ph en olpht halein p in k co lor ind icated th at the solu t ion was
acid ic, even before t he th ree ext ra dro ps of ac id were ad ded ?
7. Deter mi n e th e a ppro ximate p H o f half-saturated solutions o f CaCh o
K:! CO :l. C H 3 CO O :'\'a, and LiC !.
\ Vh r did not all of th c solutions have a p H of ap prox ima tel y 7?
C u S O~ .
8, ~I i x I ml of 3-M C H 3C O O H solut io n with 10 ml o f O.05-At BaC h solution. ~I i x
I ml o f 3-.\1 H C I solut io n with another 10-ml port io n of O.05 -,M BaC h solut ion . T o each
mixt ure ad. I I ml of 0.5-.\1 K:! Cr 20 . solut ion and st ir.
\ Vh r d id a precip itate form in one case and not in t he other?
9. P u t a sma ll p iece of am mon ium car bonate o n a watch glass. Add one dro p o f
water to th is solid an d note the odor.
Wh y di d the WC l salt smell like ammonium h ydro x ide?
10, I n a su itab le ap paratus, d isti ll 10 nil of I-N FeCh solu tio n. collect ing th e co nd ensat c.
\ Vhy was the co nde nsa te acid ic?
Group I. The Ele me nts of G roup IV a nd The ir Compou nds
I. P repare an el ect rol yti c cell, uving a 250-m l beak er , two str ips of lead (ca. I b y 5
em). 100 m l o f a 4 % sol u tion of K CI( h , an d 100 ml o f a 1% solu t ion of X a:! CO;l. Using
the lead stri ps as electrod es, electrolyze at abo u t 8 volts, DC .
W hy was a precip itat e formed ?
2. P ut I ml of O,25-.l\.[ (C I-b CO O):!l' b solution in eac h of two ce n trifuge tu bes, K
an d N . T o tu be K add 4 ml o f 0.15-" 1 K :! CrO ~ solu tion. T o tube N add 4 ml of O. 15·'\[
X a ~CO :1 -olur ion. St ir each m ixture with separat e stirring rods. Ce ntrifuge. Ad d a d rop
o f O.15-M K ~ C ro.f solu t ion to the liquid in K and a drop of 0.15-..:\1 X a:!C( h solu tion to
the liqu id in X . If p recip itation is fo u nd to be comple te, decant the liq ui d fro m K in to a
thi rd ccnu-i fugc tu be and d eca nt th e liq u id from N int o the same cent rifuge tube. Stir.
Cc nt rifuge t h is m ixtu re. (Be sure to bala nce th e cen trifuge wit h a co u nterpo ise befo re
tur ning O il th e mo tor. Ask you r instr u ctor for assistance if ill do ub t.}
W h y di d a p reci p itate for m in t he thi rd cen tri fuge tube?
3. P rep are a few grams o f lead tart rate by a suitab le proced ure. H eat 0.5 g of th is
salt in a lest t u be, gen tly at fi rst , u n ti l no fu mes are evolved an d t he su bstance becomes
black , SIO\d y pour out the con te n ts of th e test tube ill such a way tha t th e bl ack powder
falls th ro ugh the air.
W h y \ "3 \ ligh t emi tted ?
49
4. M ix I g of lead monox id e w it h 0.5 g of an hy d ro us sodium carbonate and enough
water (a ile or two d r ops wi ll su ffice) to form a thi ck pa ste. Pu t t he pa st y m ixt ure 011 a
char coal block and h eat th e mixture in th e red ucing flam e of a blowp ipe.
\ Vhy wa s the lead monoxid e re duced ?
5 . Xl ix I ml of I-M lI e l solu tion wit h 5 m l of O. l -Af KzCr zO, solu tion. T o th is
mix ture add 10 ml of 0.5-M SnC h solut ion. St ir.
'Vhy di d the liq ui d ch ange color?
6. Add a few dro ps of ph enolph thalein indicator to 100 m l of water. Sti r, the n add
0.5 g o f po ta ssium cyan ide . Sti r.
wh y did the solut ion become pin k-colored ?
7. Add, in the h ood , 15 m l o f 6-Af H I\'0 3 solu t io n to I g of t in. W ait u ntil th e
reaction is comple te .
\V hy was a n in soluble r esid ue for med ?
8 . T o 5 m l of D.5-AI 5nC I :! solut ion ad d 2 ml of O.5-M Na Z C0 3 solut ion . Stir. To
anoth er 5 m l p or ti on of D.5-At SnCh solut ion add 2 ml of 0 .5-M Na HCO s solut io n.
W hy di d th e r esu lt s d iffer?
9. P u t a few drops of universal indica tor in 10 m l of I-AI Na H CO a solut ion . Stir.
Sim ilarly trea t 10 m l of I -M NatC O ;l solu ti on . Gen tl y boil both solu t io ns for a few m inutes
an d the n let th em coo l.
Wh y di d the color of on ly one solu tio n change?
10 . Pass car bon diox id e into 10 nil of n. l -AI (C H .,CO O)t P b solu tion for seve ral
minutes. Simi larl y tr eat 10 m l of O. l -At P b (I\'0 3)z sol u tion.
Why did a precip itate form more readi ly in o ne case th an in th e other?
Group J . The El ements of Gr oup V and Their Co mpo unds
1. P lace 0.5 g of an t imon y in a d ean mo r tar an d gr in d it to a ver y fine po wd er. I n
th e h ood , spr in kle a li ttle of this po wdered ant imony int o an Erl en meyer flask filled wi th
chlo r in e.
" Thy wa s ligh t emitted ?
2. Fill a 25 0-llll Erl enm eyer flask wit h am moiu a an d close the m ou th of the flask
with a s(O ppe r. Sel ect two approximatel y eq ua l sized p ieces o f ice (ca. 1 g each) an d p u t
one p iece in the flask con ta in ing the am mon ia , b riefly re m oving th e stop per in orde r to
do so . P ut the oth er piece of ice in a sim ilar sto p pered flask fill ed with air.
' Vh y d id on e p iece of icc melt m ore r a p id ly tha n the other?
3. Di ssolve 3 g of arse n ic (I II) oxide in 5 m l of conce n trated hyd roch lor ic ac id aIHI
d ilute to 100 rul wi th water. P re pa r e a saturated solution o f t in (II) chlor ide in concentr ated h ydrochl oric aci d . To 5 ml of con centrated h yd roch lor ic acid ad d 0 .2 ml o f the
arsenic (I II) ox id e solut io n and 0 .5 m l of th e tin (II) chl oride solut ion . m ixi ng wel l. \ Vair.
\ Vhy d id a precip itat e for m?
4. In the hood, drop a few sm all pieces of calcium phosp hide in to a 250 -ml beaker
50
ha lf-filled wit h water. Co ver the mouth of the beaker with a watch glass.
'Vhy were smoke rings for me d ?
Caution: Aft er re moving calciu m pho sp hide from its con tainer, make sure that the
con tainer is tj ~h t l y re scaled to preven t ad m ission of moist ure . If this is not do ne, a very
po isonous gas will be produced wh en moi st air en ters the con taine r. Ask you r instructor
to check the container to make su re it is properl y rescaled .
5. Usin g a dry test tu be, d issolve I g o f bism uth trichlorid e m 10 m l of 3·N H CI
solut ion. Add 1 ml o f water. Xotc tha t the precipitate which forms d isap pear s when the
m ixture is st irred. Ad d anot her m illi liter of water, then st ir. Contin ue to add wat er in
l -ml incremen ts, with st irring.
Why d id the pre cipitate fina lly fail to d isap pear?
6. P ut 1 g o f sod iu m iod ide in a d ry test tu be. Add I g o f bismuth (II I) chloride.
Then ad d, drop by drop, 1 1111 o f water.
\ Vhy d id the con te nts of the test tube become dar k-colored?
7. Pu t 5 ml of I-M Na:\O~ solu t ion and 2 m l of carbon tet rachlor ide in a test tu be.
Sim ilarly, in to a seco nd test tube pu t 5 ml of l -M i'\ aNO ~ solu t io n and 2 1111 of carbon
tetrach lori de. Add o ne drop o f 3-M l-h S0 4 solutio n to 1 1111 of l -.l\f KI solut ion and pour
this mixture in to the first test tube. Sha ke it vigorously. Ad d one d ro p of 3-.M Na O H
solutio n to 1 011 o f l -.M KI solu tion and pou r thi s mixture into the second test tu be. Shake
it vigoro usly.
\ Vh y were the resul ts diffe ren t?
8. I n a d ry test tube d issolve 2 g" o f ant imo ny (III ) chloride in 10 ml o f 3·M B Cl
sol ut io n. Add 1 ml o f water. Sti r. Add another m illiliter o f water. Stir . Cont in ue in this
fash ion.
\\'h y di d a precipi ta te for m an d th en d isappear?
9. Pu t 10 m l o f water in each o f two test t u bes an d dissolve 3 g of sod iu m ruu-nc
III the water in each test tu be. Cool the con tents of o ne test tube to 00 C and allow the
o th er to remai n at room tem perature. T hen, in th e ho od , add 5 1111 of glacial acetic acid
to the cold solutio n. Com pare t he color of this mixtu re with the color of the untreated
sodi u m n itrite solu tio n.
\\'h y were the colors of th e two liq u id s diff eren t?
10. Pu t 1 m l of water ill a d r y 2S0-1ll1 Erlen meyer fl ask and , in the hood , d isplace
the air ill the flask with n itrogen di oxid e. Add 1 ml o f mercury to the flask, stopper it .
and shake it vigoro usly.
\Vh y d id the b rown color d isapp ear?
Group K. Ox ygen a nd Sulfur, of Group VI, a nd Their Compounds
I. Mix 5 1111 of 3-M ( N H ~) ~ S solut io n with 5 ml of 0.25· M Pb (1'\0;1)2 solu t ion.
Fil ter the mixt ure and d iscard th e filtrate. Pou r 10 to 15 ml of a 5 % H ~ ( h solu tion into
the filter paper cone conta ining the precipitate, and as this liq u id passes th rough th e filter
51
paper. ad d an ad di tio nal In to 15 ml portion of f-L O :! solution . Repeat severa l times un t il
a ch an ge in th e precipitat e ca n be not ed .
\\' hy di d the pr ecip itate chan ge in appeara nce?
2. P ut j m l of I-.H :'\ a:!SO :,: solution in each of two tc..t tu bes fitt ed with on e-hole
swppers wit h L'sha pc d gl ass ru bes in the ..topper holes. T o one re..t tu be add 10 ml o f water
and :\ ml of I-M lI e l solut ion. Replace t he sto p pe r and heat gent ly. in the h ood . bubbling
the gas wh ich is formed t hrough a sat ura ted so lu tio n of Ca (O Hk Ad d 10 m l of 10%
H :! O :! solu tion a nd 3 m l of 1-,\1 H e l solu t ion to the ot her test tu be am i t reat it similarly.
' Vh )' was a precip itate for med in on ly one case?
3. I n the hoo d. add 3 ml of conce nt rated
I lea r gen tly u n t il a reacti on beg ins.
' Vh y was hyd rogen not evo lved ?
H :! SO ~
to I g of co pper t urn ing.. or w ire .
4. Add I 1Il1 o f 3·M H :!S0 4 solut io n to 5 m l o f O.00 4-M K:\ l n0 4 solut ion. Stir. T o
the m ixture ad d 10 ml of 3% H 2 0 2 solu t io n. St ir. T o a second 5 nil po rtion o f 0.0 0·1-,\/
K:\ In O " solu tion add I lU I of 3-M :\aO H solu tio n . ..tir . and ad d 10 1111 of 3% H 1 0 1 solut io n.
St ir aga in.
\\' hy d id t he results d iffer?
5. Sat urate 200 111 1 of water with sulfu r di ox ide . Do th is in the h ood . \\'rap the end
of a p iece of copper wire tight ly aroun d the remer of a str ip of l ine. Let the ot her end of
the wire free , th us forming a T -..haped object . Place th is cop per-zinc co u p le in to the 200
1111 of liq u id . " 'a it.
W hy wa... a preci pita te formed ?
6. Add 5 IIlI of co nce n trated n itric acid to 0.5 g of su lfu r . H eat gent ly, in the h ood.
for several m in ut es. rep lacing acid lost b y eva poration , as ncces...ary . Decan t mo st of the
liq u id from the solid. collec ting the liq u id in a test tube . T r y to m inimize , in sofar as possib le , an y tr ansfer o f u n reactcd su lfu r. Add 2 m l of O.2-M BaCb solu t io n to the d ecantate .
W h y was a precipitate formed ?
7. I II the hood . add a small p iece o f iron (II ) sulfide to 10 ml o f 6·1\' H CI sol utio n.
Ii o id a sma ll p iece of silver. abou t th e sill' o f a d ime , in the fum es.
' Vh y d id th e silver lose its polished . me tall ic sheen?
8. I n the hood . pa.... su lfur d ioxide in to a n. I-M K;\[n 0 4 solu t ion.
' Vhy d id the colo r ch ange?
9. In the hood . pa..s dih ydrogen su lfide in to a O. I·M K2 Cr 2 0 , solut ion.
" ' hy did the co lo r chan ge?
10. I n the hood, pa..s ...ulfur di ox ide into to ml of a 0.05 -M K 2 Cr 2 0 , solution co ntaining a few drops of co ncent ra ted su lfu r ic acid.
\\' h y did the co lor cha nge ?
Group L. Orga nic Chemistry
l. Ad d , with ..urri ng. 50 m l o f for malin (an aq uco us solu t io n ca . 40% for ma ldeh yde ,
60% water) to 50 ml of a saturate d so lu tion of an ilinc h vdrochclor id c. Slo wly po ur th is
52
mixtu re in to a be ak er conta ining 100 m l of co ncent rat ed ammonium hydrox id e solu t ion .
w hy d id a solid form ?
2. Add I drop of me thyl orange in di cator , with st irring, to 5 ml of O.OO I-Al He l
sol ut ion. T h e mixture shou ld be red : if it is no t, prepare th e d ilu te aci d rno rc car ef u lly
an d begin again . Ad d 10 m l of 1% egg al bumin sus pe n sion to the acid ic l iq ui d . Srir. Note
tha t th e color o f the methyl oran gc no w indicates th at thc solut ion is Icss acid ic. (T o co n firm th is, yo u m ay add two or thr ee drops of an y 3-Al acid , no ti ng the cha nge of color as
th e acid is addcd. ) Now add I drop of phenolp htha le in in di cato r , with st irring, to 5 m l of
O.OOI -JH Na Of-I solu t ion . Note the pink color. Ad d , 'with stirring , 10 m l of 1% egg alb umin
suspension to the b asic liquid. Note the color change .
W h y d id the egg alb um in react wit h the acid an d with the base?
3. Di ssolve I g of 1\'aOH in 25 ml of water. H eat to boil in g and add 1.5 g of
powdered su lf u r. Con tinue to boil t he mixture gently , replacing water lo st by evap orat ion,
un t il all , or almost all. o f the su lf ur h as d issolved . Let the mixture cool to 700 C and add,
q u ickly, 5 m l of 1,2-dich loroe thane . Sti r vigorou sly and rapid ly.
' Vh y d id a preci pi tate for m ?
4. Di ssolve 2 g of pot assiu m iod id e and I g of iod ine in 10 ml of water and add 0.1 ml
of 3-A1 :-':aO H solut ion. Add 0.2 ml of ace to ne. Sti r. H eat for 2 minutes at 60 0 C. Then
ad d 3-Al Na O H solu t io n, dropwise. \\-i111 st irri ng , until th e iodine color just d isappears.
' Vh y was a precipit at e formed ?
5. Ad d 1.5 m l of toluene to 3 ml of a 2% K~ l n O ~ solu tion. Sim ila rl y ad d 0.5 m l of
benzene to 3 r nl o f a 2% K:\ f nO -l solu t ion. Gently boil both m ix tures for five minutes.
'Vh y did the two m ixtu res behave d iffer en tl y when ge ntly boiled ?
6. P u t 2 g of sodium acetate in a test tu be . Add I m l of co ncentrated su lf uric acid
and warm ge n tly. H o ld a moistened p iece of b lu e li tmus paper n ear th e mouth of the test
tube.
'\'hy di d th e litm u s pape r tu rn red ?
7. P u t 4 1111 of vinegar ill a test tube. Not e th e odor. Add 1 m l of eth yl alco hol and
n ote the od or. Add seven drop s of conccmra rcd su lf uric acid . st ir well , an d war m gent ly.
Xo te the odor.
' Vh y di d the odor change af te r the mixt ure was wa r m ed ?
8. Carefully ad d th re e dro ps o f ace tyl chlori de to I m l of water. T o ano ther I m l of
water , carefu lly add th r ee d rops of benzoyl ch lor ide .
\\' h y did these two su bst ances react d iffere n tly?
Not e: Xlake dou bly sure that t he con tainers are completely res ealed after u sc. T he
two organ ic ch lorides use d in th is problem are d ifferent from m ost of the reagents you have
handled pre vio usly. T r eat them with re spect.
9. Ligh t a ca nd le; let it burn for at least one m inute. Ligh t a m at ch or flint ligh ter ;
blowout the can dl e an d immediatel y hold the flame of th e b urn in g match , or ligh ter,
a p prox ima tely I em a bove th e top t ip o f the smok in g wi ck o f the candle , in the sm oke o f
the wick .
' Vhy di d the cand le r el igh t?
53
10. Set up a bu nsen bur ner , closin g the air ports at the b ase. T urn on the gas and
light the burner.
' Vhy was the flame lu m ino us?
Group M. O rgan ic Chemistry (continued)
1. D issolve 0.4 g of s- n ap h thol in 2 ml of methanol. Slo wly add 1 m l o f concent rated
su lf u r ic aci d . H eat gen tly to boil in g. Let the m ix tu re cool.
' Vhy did a p rec ip it ate for m?
2. P re pare 20 m l of a 20 % solutio n o f su crose. D ivide the solu tion in to tw o a pproximatel y eq ual port ions. Add a few drops of o ne port ion to a test t u be con ta ining 1) m l of
Fe hli ng's solu tio n. H eat th is mix ture ge ntl y. To th e oth er portion add ten dr ops of co ncentra ted h ydrochlori c acid and hea t ge ntly to bo il ing. Bo il very m oderatel y for three
minu tes. Then ne ut r alize the acid ified solution wit h solid sodiu m carbona te. Add a few
d ro ps o f thi s ne u tralized solut ion to a fr esh 5-m l po rtion of Fehling's solution . H eat th is
mix ture gcn tly.
\ Vh y d id a pre ci p ita te form in one case an d not in the ot he r?
3. Add bromin e wate r, drop by drop, wi th sh ak ing, to a few m ill ilite rs of a 2%
phenol solut ion.
' Vhy d id th e brom ine water lose its color?
4. I n a test tube, mix I g o f salicylic acid , 1 ml of m eth yl alcohol, and 0 .5 m l of
conce ntra te d su lf u r ic acid . N ote the od or o f the m ixtur e. H eat it gcn tl y an d no te the
odor again .
\ Vlr y d id the odor ch ange?
5. In a test tube, slowly, wi th st irr ing, add 5 m l of concentr ate d sulf uric aci d to 15
m l o f wat er. \ Vh en th e solu t io n has cooled , add 0.2 g of potassium iodate to it an d stir u n t il
di ssolved . T hen ad d 2 m l of »-am yl alcohol. Stir. St oppe r th e test tu be loosel y an d heat
it to a pproxi m ately 100 ° C by means of a boilin g wat er bath . H old the test tube an d conten ts at th is temper a tur e for several minutes.
Why d id th e m ix ture becom e colored?
6. Add 2 m l of concen tr at ed su lf uric acid to 0.5 g of 2 ,4-di l1 itr op hc n yl h ydrazin c.
\Vith st irri ng. slowly ad d 4 m l o f water; con tin ue to stir u n til solut ion is com p let e. T h en
add 12 1111 o f eth anol (95%) . Stir agai n . Xow add , wit h stirring, I m l of ace to ne. Let the
m ix ture stan d u ndisturbed.
\ Vh)' d id a precip itate for m ?
7. I n a d r y test tu be, add 5 drops of acetyl chlor ide to 0.5 ml o f d r y an ili ne . AHd 5
ml of wa te r an d stir.
'Vh y was a p reci pi tate for m ed?
N ote: Bc su re th at the ace tyl chlor ide con ta iner I S r esealed a fter usc . T r eat acety l
chlo ri d e with respect wh ile you ar e u sin g it.
8. wash
~5
ml of carbon tetra chlorid e. Place
54
It 111
a sepa rato ry funnel wit h 10 m l
of G-Al N' aO H solu t io n. Sha ke th e fu n nel and conten ts vigorou ..ly for one m in ute. \ Vithdraw th e ca r bo n tetrach lo r ide, rin se the fu nnel, r e place the car bo n tet rachl or ide in the
fu n nel a nd wa ..h it simi larly, twice m ore, with 10-101 por t io ns o f wa te r. P ut a pprox im atel y
half o f the washe d car bo n te t rach loride ill o ne test tube an d the remain der in another te st
tu be . T o the first test tu be ad d 5 ml of I-M I-I I'Wh solution and I ml of O. I·M AgI\ O ;\
solution . Shake this mixtur e and ..c t it aside. In a third test tube put a pprox imately 0.5 g
of soda-li me a nd heat it to redness. Th en add, in the hood , a few d ro ps of carbon tetrach loride fmm thc second tes t tu be. I n the hoo d , reheat the soda-lime to red ness and add,
..lowl y at lir..r. the remai nder o f the ca rbon tetrach loride from the seco nd test tube. Now add
:') IIlI o f 1-.\1 11 :'\ 0 3 solu t ion and I IIlI of O. I-.\[ Ag:'\0 3 solution. Shake th is m ixture.
\\' hy did a precipitate form in one case and not in the ot he r?
9. Dissolve 0. 1 g o f azoxybenzene in 2 IIlI of benzene. Add 0. 1 g of anh yd r ous alum inum chl oride to thi.. mixture .
" 'h y did a colored product form ?
Sote: Th e alu m in um ch loride bott le shou ld be we ll -sealed after u se. Ask your
in structor to check yo ur resealing tech n ique.
10. In a larg-e te s t tube place 2.5 ml o f form ali n, 2 g of phenol, and 5.5 m l of glacial
aceric ac id. Stir gen tly. and slow ly add 2 ml of concentrated h ydroch lo ric acid while stirring .
W ait.
W hy did a solid form ?
Group N. The Halogens, of Gr oup VII, an d The ir Compounds
I.
Pu t I g of sodiu m su lfite in a 250-ml Er lenm eyer flask. Slowly add b ro m in e water
to the Ila..k u mi l this liq u id is no lo nger dccol o ri zcd as it is ad de d an d a fain t b rown color
pcr..ists in the liq u id in the flask. In the hood , ge nt ly boil this liq u id u ntil all the excess
bro min e is dri ven off. T hen , aft er the solu tio n ha s cool ed , slow ly add , wit h stirring, 20 m l
01' con rc rura tcd sulfu r ic acid .
\ \' h y di d th e solutio n become col ored again ?
2. no t h is in the hoo d : Dissol ve on e small d rop of bromi ne in 10 m l of carbon tetrachloride. (Cau l ion: If not Hu sh ed olI inu u ccl inwly, with in secon ds, bromi n e will damage th e
skill, prod uc ing a n ann oying and slow-hea ling wo und.) Pass ace tyle ne into the solut ion
for scvcral m inutes. (Caution: Acetylen e for ms exp losive mix tures with air. A ll, r epeat :
. 1/. /.. flame .. in t he laboratory m ust he exti ngui shed before ge ne rating acetylen e and for at
lea ..t te n min utes afte r you h ave fin i..hed. C heck wit h yo ur in stru ctor befo re ge nera t ing
acct ylcuc. )
W h y d id the solut ion become colorless?
3. Ad tl I g of iodi ne to 10 m l of wa ter in a test t u be . Sha ke the m ix tu re vigo rously
unti l yo u arc ..nrc that no more iodine will d issolve, tha t a satu r ated solu t io n has been prepared . "\<111 2 g of pota....ium iod ide to th is mixture and sha ke it vigorously again.
\ \' hy did the remainder of the iodine go into sol ution?
-t. Pu t I g of potassium b romide
111
a test tube. Add Qu e drop of water to the
55
po tassium bromi de. Add th ree or four drops of concen trated sulfu ri c acid to the mo ist salt.
Blow across the mou th of the test t u be.
\ \' hy were white fum es formed?
5. Dissolve, by ge n tle heating, 0.5 g of potassium chlorate
solut io n. Slow ly add th is hot liq uid to 5 ml of I-AI KI solu ti on.
\ Vh y d id the color chan ge?
10
10 m l of 3-M
H 2 S0 ~
6. I n the hood , pre pare a solut ion of 1':aOCI b y passing chlor ine into a I-M solution
o f NaO H solution, coo led to oe C:, until no ma rc will react. Next, mix 3 ml of O. I·M K I
solut ion and 3 ml o f O.l -.M KBr solu tion in a test tu be. Add 5 ml of car bon tet rachloride
to the test tu be co nt a in ing th is m ix ture. Then add 0.2 ml of the NaOCI sol u tion to the
same test tu be and shake it for o ne or two seconds. Le t it stand u nd isturbed and no te the
color o f the lower layer. Now add 3 ml of the l\' aOCI sol ut ion and shake the m ixt ure u n til
the lower layer , on sta nding, is colorless. Lastl y, ad d 1 1111 o f 3-M H :! SO ~ solution to th e
mi x ture an d shake it again. Let it sta nd u nd istu rb ed. Xote the color o f the lower layer.
\ \'h y d id the carbo n tcu-achlor idc layer beco me colored, lose its color , and, finall y,
beco me colored agai n?
7. T o 10 m l of I-M KBr solu tion, add 1 1111 of Ii-M Ib rO~ . To 10 ml of I -AI KI
solutio n, add 1 1111 of Ii-M I b P O ~ . Ad d 5 ml o f I-M Fe (1"\ 0 ;: ):\ so lu tion to each mi xture
an d st ir.
\ Vhy was only one halide ion oxidi zed?
8 . Put 10 ml o f l·M KHr solu tion in a test tu be. Ad d 5 m l of car bon rcrrach lori de.
Sha ke the mi xtu re vigorously and set it aside. I n to a secon d tes t tube , in the hood , p u t two
sma ll crystal s o f po tassiu m chlorate ; aci d .5 m l o f concentrated hydroc hlori c acid. After a
gree n-colored gas fills thi s test t ube, ad d 10 IIlI o f water ami sha ke the co n ten ts for a ile
mi n u te. Pou r th is liq u id in to th e first test t u be, shake it vigorously, then le t it stand
u n cl ist u rbcd.
\ Vhy d id the car bon tet rach loride beco me colored?
9. Dip a piece of fi lter pa pcr . ca . I by 3 e-m , ill war m tu rpent ine . (Cau tion: do not
heat the turpc n t .i uc over an opcn flam e. Usc an el ect ric ho t pla te or, at least . a water bath. )
R emove the fil ter papcr fro m the turpenti ne an d iuun cdi atclv drop it in to a bou le of
chlorine, p re pared in the hood . \ Va il.
W h y d id a bla ck deposit for m?
10. Fill , in the hood , a gas-collecting bo ttl e wit h chlori ne and cover the mou th of
th e bo u le with a glass plate. Fill an ot her boule with hyd ro gen iodi de and cover the mo uth
with a glass plate. (Caut ion: If you decide to prcpare h ydro gen iod ide by hyd rol yais o f
phosp horous iodi de , chec k wit h yo u r inst ructor before carrying ou t thi s prepara t ion. ] \V ith
the covers in place. inver t the ch lorine-con taini ng bo ule over the bou le of hyd rogen iodi de.
Make su re tha t the mou ths of the bott les arc juxta posed . Remove the tWO cover pla tes and
allow t he t wo bo ul e mout hs to to uch.
\ \' h y d id a reac tion occu r?
56
Group O. Rates of Reaction, a Stud y of Kinetics
l. Prepare 500 uil of a 7-N
H ~ S () ~
solu tion. T o this solution add enough po tassiu m
pcrm angan ate to ma ke it 0.0 3-N with respect to potassium permangana tc . (Not e clearly:
These instruct io ns d o not d irect yo u to make 500 m l of the acid solut ion an d 500 m l o f the
potassium pcrmangana te solut ion an d the n to m ix the two, o b taini ng 1000 m l of solu t ion .)
Sti r the m ixt ure well to in sure that all the po tassium permanga na te is d issolved and that
the co mposition o f the solution is un iform th roughou t.
Pu t one Scl fi nish ing nai l in each o f f ive test tu bes and p ut all the test tubes in a la rge
beaker abo ut half-filled with water. H eat this assembly u nt il the \.. . ater is at a tempera ture
of 700 C . H old it at this tem pera tu re as closely as possible. Add 10 ml o f th e acidi fied
potassium permanganate solu tion to one test t ub e. N ote the t ime req u ired for the solution
to beco me colorless.
Lower the tem perat ure to 60 C C and ho ld it at this temperat u re. Add 10 ml of the
acidi fied po tassiu m pcrmanga natc solut io n to on e other test t ub e. Note th e ti me requ ired
for the solut ion to become colorless, R epea t at 500, 40 °, an d 30 ° C.
' Vh)' d id the rate o f d ecol ori zat ion d epend u pon the tem peratu re?
2. Except as directed , thi s ex per imen t should be carr ied ou t in su bd ued light. D issolve 3 g o f u -i potassium hcxacyan ofc rratc (II I) and 2.S g o f the b rown or green variety of
the "co mpou nd" called ferric ammoni um citrate in 20 ml o f water. Add 1 ml of 28 %
C ll:;COO H solution an d mix wel l. Sat ura te a piece o f filter paper wit h th is solu tio n an d
le t it dry in com plete d ark ness. W hen the paper is dry, p ut a key, pape r clip, or ot her
o paq uc objec t on it and expose the paper to a stro ng l igh t, with the opaque o bj ect between
the paper and the ligh t. Co n ti nu e the exposure for several m in u tes, u ntil the pap er beco mes
dark green in color. R emove the opaq ue ob jects an d rinse the paper in tap water.
' Vh y di d some po rtions o f the fi lter paper become wh ile ?
3. Pu t a single large piece of cop per sul fate pentahydrate wh ich weighs about 2 g
in a test t ube . Pu t a n eq ua l weight o f finel y powdered copper sulfate pcntah ydrate in
a nother rest tu be. Pu r 10 ml of wate r ill each test t u be, sto pper, an d sha ke bo th test tubes
wit h equal "igor for several m inutes. examinin g them from time to time.
' Vhy d id the powd ered solut e d issolve more rap id ly?
-I. Clea n an d d ry five sma ll bea kers and pm a dean, dry st irring rod in each beaker.
Put 25 m l o f water, measu red with a p ipet, not a grad uate, in each beaker. Using a b u ret,
put 5 1111 o f 3-.N H CI solut ion in one beaker, 6 ml in th e seco nd bea ker, 7 ml in the th ird,
and so on . M ix th e co ntents o f eac h beaker thoroughly. O bt ain five pieces of magnesium
rib bo n, each ap pro x imately I CIll long, all of exactly the same length . Pu t a piece of t he
rib bon in each beak er and, sti rring occasion ally, note th e t ime req uired, for each piece,
for co mple te re action.
\Vhy d id th e rib bon react more rap id ly in the mo re co ncentrated acid so lut ions ?
5. " Dissolve" 2 g of potato starc h in 250 ml o f bo iling water : then add 7.5 g of
Na :!S:! O:i an d 2S0 m l of water. Sur u nt il d issolved. T his is solut ion P. T o ma ke solut ion
D , d issolve 1.5 g of HgCb in 500 ml o f wate r. Solution (2 is 7.5 g of KIO .1 d issolved in 500
ml of water. Pu t an y de sired volume of soluti on P in an Erlen meyer flask and add an eq ual
57
volume of solut io n D swirling to insu re complete m lxtng. T hen add, with sw irl ing, an
equal volum e of solution Q and stop swir ling as soo n as all o f th is volu me of solut io n Q
is added.
W hy di d the co lor ch ange a few seco nds after the potassiu m iod ate solu tion was add ed?
6. Set u p your Bu nsen burner. Pl ace a wire gauze over the mouth of the b u rn er.
A djust th e air ports an d gas flow so that when the bu rner is lit a me d iu m-sized lu m inous
flame will be ob tai ne d . L igh t the gas above t he wire gauze. Slow ly li ft the gauze.
\V hy d id the flame r ise as the ga mc was lifted ?
7. Prep ar e 500 ml o f a 3.5 -M H 2 S0 ~ solution . T o this solu t io n add en ough potasSlu m per manganate to make it 0.006-1\-1 wi th respect to po tassium permanga nate. (N ote
clearly: These inst ructions do nor d irect you to p repare two solu tions each 50 0 ml ill
volu me and th en to m ix the two. If yo u do th is, your solution wi ll be half as concentra ted
as is dcs ircd. ) St ir th e mi xt ure we ll to ins ure that d issolutio n is co mplete an d that th e
com pos it ion of the solu tion is u ni form throughout. P ut 10 ml of th is solu tion in each of
five test tu bes. T o the first tes t tu be add one Sd fi n ish ing n ai l, to the secon d two fin ishi ng
na ils, to the thi rd three, an d so on. \Vait.
W h y di d the solut ion in the te st tu be with five nails be co me decolo rized morc rap id ly
th an th c on e with fou r na ils, and it in turn more rap id ly tha n the one wit h th ree nai ls,
and so on ? (N ote : If the resu lts di d not turn out as in dicated by the q u estion , yo ur solution
was not u ni form in com position or the na ils were greasy or rusty. St ir the re main ing 450 ml
o f solut ion ve ry thoroughly, clea n the n ails, and beg in aga in .)
8. P ierce a pa per (book ty pe ) match with a straight pi n j ust below the head, an d
u sin g the p in for su p po rt, h ang th e match , wit h the h ead upright, in the cen te r o f the
mouth of an u nligh ted Bu nsen b urner. O pen the air por ts at the base o f th e b u rner abo u t
ha lfwa y. T urn on th e gas su p p ly abo ut h alf way. Using- another match or Hin t ligh ter ,
light the burner.
\Vh y d id the match in the ce nter of the b ur n er rema in u nlit ?
9. P u t 10 ml of O.l -N H gCL sol ution in a test tube. Pu t I ml of O. l-N SnCL solut ion
anoth er test tu be. R ap id ly pour the tin (II) ch loride solution into th e mercur y (II)
ch loride solu t ion and im med iatel y sha ke the m ix ture .
P ut I ml of O. l -N H g·C L solu tion in a test tu be . Pu t 10 m l of O. l -N SnC!:! solution ill
an other test tube. Slo wly , d rop by drop, wit h st irring, po ur the mercu ry (II ) ch lor ide
solution in to the li n (II) chloride solut io n.
Why di d the re su lts d iffer?
111
10. Add 2 ml of u ni versal ind icato r an d 5 ml of satu rated Ba (O H)2 solu tion to 200 1111
of wa te r in an Erlen meyer flask. St ir. Ad d 10 ml of eth yl aceta te, sto ppcr, mi x we ll, and let
stand for several h ours, observin g from ti me to time .
\\' hy did the color of the m ixture slow ly ch ange?
58
Group P. A Few Transit ion Elements, Cr to Ni, and Their Compounds
I. P re pare a few mi lli liters o f iron (II) su lfate solut ion . I-M , frec o f an y Fe ' "" ions .
T o 10 m l o f th is solut ion in a te st tube , ad d one d rop of I ·M potassiu m thiocyanate solut ion . Make a fine noz zle on the end o f a length of glass tu b in g . \V ith th is nozzle , bubble
ai r th rough the iron sulfate-potassium th iocyanate solu t io n for sever al minutes.
" ' hy d id the liq u id become re d ?
2. Select a test tube. For m a nozzle on the end of a shor t length of glass tubin g.
I nsert th is tub ing into the ho le of a one-hole sto p pe r of the proper size to fit the test tube
and allix a shor t leng th of ru bber tubing to the glass tube. Pu t a screw damp (a p inc h
d amp wi ll not d o) on the r ub ber tu bing and close the tu be with the damp . Put a sma ll
wad of Xo . 00 steel woo l at the bottom o f th e test tu be and ad d 0.25 m l o f water to the
stee l wool. Stopper the test tube as illustrated in Figure 12. Make sur e that no air can
Figur e 12
get into or ou t of the test tu be, scaling all joints w ith melted wax if n ecessar y. " ·a it o ne
week or longer. T hen fill a beaker with wat er. invert the assembl y so that the loo se e nd of
t he rubber tube is un der the sur face of the water in the beaker. Loo sen the scr ew damp
and . if necessa ry. rna-sage the rubber tubing to open it aga in , keeping the lom e end und er
water.
W hy d id water sq ui rt into the tu be?
3. P u t 5ml o f 1).25 ·M
~l n SO t
so luti o n ill a large IC:-t tube . Ad d o ne m l of 3-M XaO H
59
-ol n ti on . sri r, and note (h e color of the pren pn ale. Sto pper the test tube an d shake it
, ' i ~om u sl y for severa l m inu tes.
" 'h y d id the col or o f the p reci p itat e change?
4. Sclert fo ur lest tu bes and dcsigumc them G, II , E. an d M . T o C and }{ add 10 ml ,
each . of a 1·.\1 FeCh solut ion . T o E anti ,\I add 10 m l. each , of a I-M (:\' H 4)2Fe (50 4)2
cohnion . Ad d Ollt' drop of a 0. 1-.\1 K :~ Fe ( C~ ) , ; solu t ion to G and E. Stir. Add one drop
of a 0 .1-.\1 K 4Fe(C :-': )" solu ti o n to II and M . Stir.
W h y did a pr ecipitat e not form in e\"Cry test t ube?
5. P ut IO-ml po r ti ons of 0.25 -.\1 :'\iC12 solut io n in cadi o f two test tubes. T o one add
ml o f 3·.\ 1 :'\ aO H colur ion. T o th e oth er add 1 ml o f 3-M =" 11 40 1-1 solut io n .
\ " h y did a precipi tate not form in bot h cases>
6. Ad d I m l o f 3-X H e l solut io n to 3 m l of 2-M K2Cr 20 i solut io n in a test tu be. T o
th i.. mixtu re ad d I m l of ethanol . \\'an n ge m ly.
" ' hy was an u nusual odor produced?
7, By hea t ing gcm ly in a test tu be , d issolve 2 g of cobalt (ll) ch lor ide he xahyd r ate
an d 2 1-:" o f ammo n iu m ch lori de in ~ m l o f wat er . Po ur the liq u id in th e tes t tube into a
25 0·m l Er lenmeyer flask. (At thi s time it may be noted th at no t all o f the solutes d issolved
in th e ·1 111 1 of wa ter. If o nly a n r y cr ual l q unnr itv rema ins u nd issol ved, allow it to r em ain ,
if powihl e. ill the test lube a nd procccd .} Add !) m l o f ronce utra tcd am m on ium h ydrox id e
to the l iq u id in t he Ha vk. Sto ppe r the Ilask an ti shake it vigo ro usly for a few mi nutes.
\" h)' di d a precip ita te form ?
8 . P ut 10 m l o f O.OI-M K .\ lnO ~ sol u tion in each of four te st lubes. T o the first , add
10 111 1 0 1" fi -M ~ a () 11 sol ution: to t he second . add O . ~ 5 1II1 o f 6·M :\' aO Il solu t ion : to the
th ird , add 1 III I of 9 -,\1 I b S O ~ -olu tion . St ir each o f t he se three m ixtures wit h se para te
st irr ing rods. To all fo u r liq u id .. add a few cr ystals o f so lid sod iu m su lfite.
\\'h y d id th e resul ts d iffer ?
9. Di v.o lvc (J. rJ g of an u nonium di ch ro ma te and 0.:>g- o f potassium h yd rox id e in 5 ml
of wa ter and heat /!;l·lll ly. C a utiouvl v note th e odor. Nex t , gcu tly heat 0.5 g o f a nnuon iuu i
d ichrouuu c in a tes t t ube . Caurio uslv not e the odor.
\\'h y was th e od or di ffer e nt in the t wo ca,e, ?
Ill . .\ dd 5 IlII 0 1 G-X II CI -ol u uou 10 I g of potassium chrom at e.
\ r h y d id the color cha nge?
Group Q. The Chemistry of Surfoces, Colloids, ond Diffusion
It is particularlv ne ('(.'s,ary tha t the app;n ;lt ll" beak ers . te st tubes. st ir r in g rod ...
\\ ';l ~ h yo u r a p para llh thorough ly
with soa p . dc tcrgcru-. sco uri ng powder. elbow grea se. an d pcrvistcm e . R im e all clean ed
a ppa ra tlh free o f r esidual deterge n t . L·tL , with co p io us quam itie s of tal' water. R inse the
I;I P water away wit h d isti lled wa ter. Be cure th at such apparatus a, beakers, te ..1 tu bes .
an d fla..ks arc cleaned ins ide and o u t . w hen fi ni-hcd . eac h pi ece of a pparatus sh ou ld a lmost
SOft' :
e tc. . n -ed ill t he prob lem.. in ihi -, group be , ery dea n .
60
" glow" with cleanl ine ss. Ce rtain ly, a properly clean ed p iece of glassware wi ll have a
di fferent a p pcaran ce from the oth er, casu all y clean ed , glasswar e in you r possessio n .
l. I n a 250-m l Erl enmeye r flask d issolve 25 g of da rk brown su gar (impure suc rose,
C 12 H22(11 ) ill 100 ml of wa ter. Add 5 g o f finely pow de re d activate d charcoal to the solu ti on ; stop per the flask a nd sh ake it vigoro usly for severa l m in utes. T h en filter the mi xt u re.
W hy was the filtr at e co lorless?
2. T o 20 0 m l of water in a 250 -m l bea ker add 10 nil of a 1% gel atine solut io n, I m l of
0.5-M K 2Cr2() , solut ion , and I m l of 3-M A g ~() ;l solu tio n. Stir. To a seco nd 25 0-m l
beake r add 200 ml o f wa ter, 1 m l o f 0.5-M K 2Cr 20 , solutio n, an d 1 m l of 3-AJ Ag:L\ 0 3 solutio n. St ir. D igest bot h solut ion.. for 10 to 15 m inutes an d lilt er each sepa r ately.
W b y was the d igest ion in cfl cct .i vc in on e case an d n ot in the oth er?
3. Xl ix well 40 1111 o f a 400 Be. sod iu m silic ate so lut ion wit h 120 m l of water. (T he
weigh t r at io of th e solu te in terms of N a- O to Si0 2 sh ou ld be a bo ut 1 1O 3.25.) D rop o ne
cr ysta l of each of the fo llowi ng in to the dil u ted sodi u m silicate solut ion: ni ckel (II ) n it r at e ;
cobalt (II) n itra te ; ch ro m iu m (II I) ch lor ide ; copper (II ) su lfate; m anganese (II) ch lori de ;
ammo ni u m su lfate ; potassium d ichroma te.
\\'h y d id growt hs form s 011 some of the cr ystals and n ot o n others?
an d
l' ut
test
an d
4. By b u bbling h yd rogen su lfide throu gh a saturat ed solut ion of d iarscn ic tr iox id e
filt er ing the yellow liq u id wh ich re su lt s, prepar e ap prox im ately 150 m l of an A S2S :l sol.
10 m l of th is sol in each o f two test tu be s. Ad d 3 m l of I-AI N a N 0 3 solu t io n 1O the first
t u be a nd stir thorough ly. Add 3 m l of I -M Al (N 0 3 ) 3 solut ion 1O the seco n d tes t tub e
stir th oro ugh ly. Let these two test tu bes stand u nd istu rbed for several m inu tes.
\ Vh y d id the sols in the two test tu bes precip itate at d iffer ent r ates?
5. P ut I g o f Dowe x 50 cation exch an ge resin (hyd rogen for m ) in to a beak er. Add
30 m l of 4 -M N aC I solut ion an d t wo d rops of m eth yl orange ind icator. Stir ge n tl y for five
mi n u tes.
W h y d id the liq u id change color?
No te : Do n ot d iscar d the resi n . It is ex pe nsive and can be regene rat ed for furt her usc.
Ask yo ur instructo r for d irect ions concern ing the d isposal of the r esin .
6. Add 9 g of iro n (Il l) ch lo r id e h exa hydra te to 10 m l of water and h eat gen tl y unt il
the solid has "dissolved ." Th e result ing liq u id will proba bl y be cloud y. Pour I 1111 of thi s
liq u id slowly in to 50 1111 o f bo ili ng water. Rem ove th is m ix tu re fro m the heat so urce u sed to
m a inta in bo iling a nd let it cool.
\\'h y d id the prec ipitate th at formed not sett le o ut?
Xo t e: 1£ the procedure is carried o ut properl y, accor d in g to th ese d ire cti ons , the
preci pitate that for ms will not be readi ly o bserva bl e.
7. P re pare 15 m l of a 2% d extro se so lu tion . Add m l o f Feh li ng's solution an d st ir.
Let the m ixtur e stan d and observe th e colors o f the prec ip it ate th at for ms.
\ \' hy did the col or of the precip itate cha nge ?
8. D issolve 0.06 g of K 2Cr20 , and 0 .25 g of CuS0 4-5 H zO in 150 m l o f wate r. Ad d
conce n trated ammoni u m hy d ro x ide solu tion , drop b y d ro p, with st irrin g, u n t il the prcc ip i61
rate whi ch first form s just redissolves. Pass 50 ml of this green solut ion slowly th roug h a
cat io n exchange column filled with a slurry of 8 g of Dowex 50 catio n excha nge resin
(h yd rogen form ).
W h y did the color change?
N ote: Do no t d iscard the re sin. It is expensive and can be regenerated for fu rther use.
Ask your instructor for di re ct ions concern ing the disposa l of th e resin.
9. I n a test tube, mix 10 ml of benzene and 10 ml of water by agitation. Let t he
m ixt ure stand undisturbed. I n a second test tube, mi x, by agi tat ion, 10 ml of a 3% solut ion
of oleic acid in benzene with 10 ml of a 0.4 % solut ion of :\aO H in water, Let this mixt ure
stand un di sturbed unt il two layers are visible.
\Vh y d id the second mi x ture not separate into two layers of dear liquids?
10, I n 80 ml of hot water d issolve 2 g of aga r agar and ad d, with st irring, 0.05 g of
tet rapotassium hexacya no ferrate (II), one drop of 3·M :\aO H solution, and six d ro ps of
phe no lp ht halein ind icator. Fill a large test tu be app ro xi mately three-fourt hs full with t hi s
m ix ture an d let it set to a gel. Then fill the remaini ng space in the test t ube wit h a 0.5-..:\1
FeCb solution . Let it stand und isturbed for severa l hours.
\\rhy was th e gel multicolored?
.2