Experiment 1 : Identification of Compound
Objective :
1. To identify a compound on the basis of its chemical properties.
2. To determine identity of given unknown compounds based on their chemical properties
Introduction:
Experimental work involves using the scientific method. Making observations is the key step
in this method and observations can be both qualitative and quantitative. Quantitative
observations are measurements while qualitative observations involve chemical test to observe
chemical changes. All data from experiments will help to identify the identities of unknown
solutions. There are several signs that chemical reactions has occurred. These signs are :
(a)
(b)
(c)
(d)
(e)
Colour changes.
Formation of precipitate.
Evolution of gas forming bubble.
Disappearance of precipitate
Creation of heat or cold.
From this experiment, we know that accurate observation, properly recording them and
deductive logic to draw conclusions are very crucial to identify unknown compound without
difficulty.
Chemical and apparatus :
0.2 M NaCl, 0.2 M Na2CO3, 0.2 M MgSO4, 0.2 M NH4Cl, 0.2 M AgNO3, 0.2 M NaOH, 0.2
M HCl, Solution 1, Solution 2, Solution 3, Solution 4, test tubes, dropper
Method :
A. Observations of Chemical Properties of Known Compounds
Observations with silver nitrate test reagent.
1. Four small, clean test tubes are labeled.
2. 5-10 drops of each of the five ‘known’ solutions are placed into the labeled test tubes.
3. A dropper pipette is used to deliver several drops pf silver nitrate test reagent to each
labeled test tubes.
4. Observations are recorded in Table 1 .
5. If after adding several drops of silver nitrate test reagent, a chemical change is observed,
and then 5-10 more drops is added to see additional changes.
6. The test solutions are saved as reference for Part B.
7. Chemical equations for reactions that show a chemical change are written.
Observations with sodium hydroxide test reagent.
1. Four small test tubes are cleaned and labeled.
2. 5-10 drops of each of the five ‘known’ solutions are placed into the labeled test tubes.
3. Several drops of sodium hydroxide test reagent are added. Observations are recorded
in Table 1.
4. The test solutions are saved as reference for Part B.
5. Chemical equations for reactions that show a chemical change are written.
Observations with hydrochloric acid test reagent.
1. Four small test tubes are cleaned and labeled.
2. 5-10 drops of each of the five ‘known’ solutions are placed into this third set of labeled
test tubes.
3. Several drops of hydrochloric test reagent are added to the solutions.
4. Observations are recorded in Table 1.
5. The test solutions are saved as reference for Part B.
6. Chemical equations for solutions showing a chemical change are written.
B. Chemical Properties of Unknown Compounds
1. Four ‘unknown’ solutions labeled 1 to 4 are given.
2. Using the given reagents, experiments are performed to determine identity of your
unknowns.
3. Observations are recorded in Table 2.
4. Each of the four ‘unknown’ solutions are identified.
Data :
Table 1 : Observations for known solutions.
AgNO3
NaOH
HCl
NaCl
White
precipitate is
formed.
Na2CO3
Yellowish
precipitate is
formed.
MgSO4
NH4Cl
White
Clear solution is
precipitate is
formed.
formed.
Pungent smell
Clear solution is Clear solution is Cloudy solution
and clear
formed.
formed.
is formed.
solution are
formed.
Clear solution is
Bubble is
Clear solution is Clear solution is
formed.
formed.
formed.
formed.
Table 2 : Observations for Unknown Solutions.
AgNO3
NaOH
HCl
Solution 1
Solution 2
Solution 3
Solution 4
White
Yellowish
White
Clear solution is
precipitate is
precipitate is
precipitate is
formed.
formed.
formed.
formed.
Clear solution is Cloudy solution Clear solution is Clear solution is
formed.
is formed.
formed.
formed.
Clear solution is Clear solution is
Bubble is
Clear solution is
formed.
formed.
formed.
formed.
Identification of unknown Solutions
Solution 1 is NaCl.
Solution 2 is MgSO4.
Solution 3 is Na2C03.
Solution 4 is NH4Cl.
Questions :
1. The following experimental observations were collected :
+
Na
Ag+
Ba2+
Mg2+
Cu2+
NH3 (aq)
No change
No change
No change
White precipitate
Blue precipitate /
deep blue solution
with excess
HCl (aq)
No change
White precipitate
No change
No change
No change
H2SO4 (aq)
No change
No change
White precipitate
No change
No change
(a) Identify a reagent that distinguishes the chemical properties of Mg2+ and Na+.
Explain.
The reagent involved is NH3. This is because when Mg2+ react with NH3, it produces
white precipitate while when Na+ react with NH3, it shows no change.
(b) Identify a reagent that distinguishes the chemical properties of Mg2+ and Ag+.
Explain.
The reagent involved is HCl. This is because when Mg2+ react with HCl, it shows
no change while when Ag+ react with HCl, it produces white precipitate.
(c) Identify a reagent that distinguishes the chemical properties of Cu2+ and Ba2+.
Explain.
The reagent involved is H2S04. This is because when Cu2+ react with H2SO4, it
shows no change while when Ba2+ react with H2SO4, it produces white precipitate.
(d) Identify a reagent that distinguishes the chemical properties of Ba2+ and Cu2+.
Explain.
The reagent involved is NH3. This is because when Ba2+ react with NH3, it shows
no change while when Cu2+ react with NH3, it produced blue precipitate.
(e) Identify a reagent that distinguishes the chemical properties of HCl and H2SO4.
Explain.
The reagent involved is Ag+. This is because when HCl react with Ag+, it produced
white precipitate while when Ag+ react with H2SO4, it shows no change.
2. Three colourless solutions are placed in test tubes with no labels. Lying beside the test
tubes are three labels : 0.10 M K2CO3, 0.10 M H2S04 and 0.10 M NaOH. You are to
place the labels on the test tubes using only the three solutions. Here are your tests :
Test 1 :
Test 2 :
A few drops of the solution from test tube #1 added to a similar volume of
the solution in test tube #2 produces no visible reactions but the solution
becomes warm.
A few drops of the solution from test tube #1 added to a similar volume of
the solution in test tube #3 produces carbon dioxide gas.
On the basis of your observations above, how would you label the three test tubes.
In Test 1, when acid react with base, it produces salt and water. Clear solution is formed
and becomes warm. In Test 2, when acid react with carbonate, it produces carbon
dioxide gas, water and salt.
Test 1 : H2SO4(aq) + 2NaOH(aq) → Na2SO4 (aq) + 2H2O (l)
Test 2 : H2SO4 (aq) + K2CO3 (aq) → K2SO4 (s) +H2O(l) + CO2(g)
Thus, test tube #1 is H2SO4, test tube #2 is NaOH and test tube #3 is K2CO3.
Discussions :
From Method A,
Observations with AgNO3 :
NaCl (aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl (s).
Expectations : white precipitate formed
NaCl will react with AgNO3 as a double replacement reaction. The cations Na+ and
Ag+ will swap places. White precipitate is formed because AgCl is insoluble in water.
Na2CO3 (aq) + 2AgNO3 (aq) → Ag2CO3 (s) + 2NaNO3 (aq)
Expectations : Yellowish precipitate formed
Na2CO3 will react with AgNO3 as a double replacement reaction. The cations. Na+ and
Mg2+ will swap places.Yellowish precipitate is formed because Ag2CO3 has very low
solubility.
MgSO4 (aq) + 2AgNO3 (aq) → Mg(NO3)2 (aq) + Ag2SO4 (aq)
Expectations : no precipitate formed but reaction occur
MgSO4 will react with AgNO3. However, there was no precipitate formed because
Ag2SO4 is soluble in water.
NH4Cl (aq) + AgNO3 (aq) → NH4NO3 (aq) + AgCl (s)
Expectations : white precipitate formed
When aqueous solutions of the two are mixed a double replacement reaction takes
place. White precipitate is formed because AgCl is insoluble in water.
Observations with NaOH:
NaCl (aq) + NaOH (aq) → no reaction
Expectations : clear solution formed
The cations in both compounds are the same (Na+). Thus, no chemical reaction is going
to occur.
Na2CO3 (aq) + NaOH (aq) → no reaction
Expectations : clear solution formed
The cations in both compounds are the same (Na+). Thus, no chemical reaction is going
to occur.
MgSO4 (aq) + 2NaOH (aq) → Na2SO4 + Mg(OH)2
Expectations : cloudy solution formed
NaOH will react with MgSO4 as a double replacement reaction. The cations. Na+ and
Mg2+ will swap places.
NH4Cl (aq) + NaOH (aq) → NaCl(s) + H2O(l) + NH3(g)
Expectations : pungent smell produced
Ammonia gas and water is produced because NH4Cl is ammonium salt react with strong
alkali which is NaOH.
Observations with HCl :
NaCl (aq) + HCl (aq) → no reaction
Expectations : clear solution formed
HCl is a strong acid and NaCl is neutral salt thus they dissociates completely in water.
No precipitate are formed.
Na2CO3 (aq) + 2HCl (aq) → 2NaCl(aq) + H2O(l) +CO2(g)
Expectations : bubble produced
Both HCl and Na2CO3 will dissociate completely in solution. The H+ will bind with
oxygen of the carbonate produce water, carbon dioxide and dissolved NaCl.
MgSO4 (aq) + 2HCl (aq) → MgCl2(aq) + H2SO4(aq)
Expectations : clear solution formed and reaction occur
HCl will react with MgSO4 as a double replacement reaction. The cations H+ and Mg2+
will swap places. MgSO4 and HCl dissociates completely in water.
NH4Cl (aq) + HCl (aq) → no reaction
Expectations : clear solution formed
NH4Cl and HCl are both acids. Double replacement cannot occur.
From Method B,
To know identity of Unknown Compounds, the result of Method A and Method B are
compared. Solution 1 is NaCl. This is because result of Solution 1 are same as NaCl. When
Solution 1 react with AgNO3, it will produce white precipitate (AgCl). AgCl is insoluble in
water. Solution 2 is MgSO4. This is because result of Solution 2 are same as MgSO4. When
Solution 2 react with NaOH, it will produce cloudy solution. Double replacement reaction
occur. Solution 3 is Na2CO3. This is because result of Solution 3 are same as Na2CO3. When
Solution 3 react with AgNO3, it will produce yellowish precipitate. Ag2CO3 is insoluble in
water. Moreover, when Solution 3 react with HCl, it will produce bubble formed. Carbon
dioxide is produced. Solution 4 is NH4Cl. This is because result of Solution 4 are same as
NH4Cl. When Solution 4 react with AgNO3, white precipitate is formed. AgCl is insoluble in
water. Moreover, when Solution 4 react with NaOH, it will produce pungent smell. Ammonia
gas is produced. Qualitative observations is very important to differentiate each compounds.
However, there were any weakness in the experimental design that affected result. The
weakness is not exactly sure of the exact yellowish and white colour that we are looking for.
The other weakness is not exactly sure of the exact smell that easily disappeared.
In that case, weakness in the experimental design must be overcome. The yellowish
solution and white solution need to put beside each other to differentiate the colour. The
experiment must be done in the close room to avoid the smell easily disappeared.
Conclusions
The purpose of this experiment was to identify a compound on the basis of its chemical
properties. Each compounds have different chemical properties when react with acid, base and
salt. Some of them are soluble in water, undergoes double replacement reaction, producing
carbon dioxide and ammonia gas. In this case, solubility and double replacement reaction can
be observe when NaCl and NH4Cl react with AgNO3, white precipitate are produced., when
Na2CO3 react with AgNO3, yellow precipitate is formed and when MgSO4 react with NaOH ,
cloudy solution is formed. The release of carbon dioxide can be observe when Na2CO3 react
with HCl. The release ammonia gas can be observe when NH4Cl react with NaOH.
The next purpose of this experiment was to determine identity of unknown compounds
base on their chemical properties. First NaCl, what is believed to be the unknown Solution 1
were reacted with AgNO3, which produced a white precipitate because it was a double
displacement that produced AgCl which is insoluble in water. Second, MgSO4, what is
believed to be the unknown Solution 2 were reacted with NaOH, which produced a cloudy
solution because it was a double displacement that produced Na2SO4 which is insoluble in
water. Third Na2CO3, what is believed to be the unknown Solution 3 were reacted with AgNO3,
which produced a yellowish precipitate because it was a double displacement that produced
Ag2CO3 which is insoluble in water. Solution 3 too when react with HCl will produced bubble
because of the presence of CO2. Fourth NH4Cl, what is believed to be the unknown Solution 4
were reacted with AgNO3, which produced a white precipitate because it was a double
displacement that produced AgCl which is insoluble in water. Solution 4 too when react with
NaOH will produced pungent smell because of the presence of NH3 gas.
References
Hill, J. C., Brown, T. L., & LeMay, H. E. (2015). Chemistry: the central science, 13th Edition.
Boston: Pearson.
Essays, UK. (November 2018). Lab Experiment to Identify Unknown Compound. Retrieved
from https://www.ukessays.com/essays/chemistry/unknown-ionic-compound.php
Wired
Chemist.
(n.d.).
Retrieved
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from