CHEMISTRY LAB: DENSITY OF SOLUTIONS
(DENSOLN.DOC)
1. Introduction: The more solid material that is dissolved in a solution, the more dense
is the solution. If given the opportunity, the more dense of two
solutions naturally settles to the bottom. (After a long period of time,
natural random motion of the particles mixes the two solutions until no
difference can be detected.) These facts can be used to detect which of
several solutions is the most dense, and least dense, and the correct
order of all the solutions of intermediate density.
2. Safety: Do not drink or taste any material while in lab. These chemicals are generally
considered safe, but you never know who has used what before you. If a spill
stains your clothing, rinse it out immediately with tapwater; any residue of
food coloring dye should come out in the wash at home. Wear your lab
apron to protect your clothing just in case. Goggles are not necessary in this
lab, but the saltwater would sting if it got in your eyes: rinse it out
immediately with tapwater. Do not return any chemical solutions to any
stock bottles; this will avoid contamination for others. You may dispose of
any leftover solutions down the drain. Rinse the glassware with tapwater.
Rinse your hands before you leave the lab.
3. Materials and Supplies Needed:
a. From the prep area:
Samples of four salt solutions, color-coded with food coloring
One clear glass tubing section about 15 to 20 cm long, fire-polished,
or a translucent drinking straw
b. From the lab station supply cupboard:
Test tube rack with test tubes
4. Procedure:
a. Getting Ready. Use four test tubes to obtain samples of the four colored salt
solutions from the preparation area. A test tube approximately half full will be plenty of
solution. Place some tap water in a fifth test tube to use to rinse the glass tube or straw
between uses.
b. Learning the Technique. Samples of the solutions can be collected in the
glass tube as follows. Place your index finger over the upper end of the open glass tube
or straw. Grasp the tube between your thumb and the other three fingers. Now you can
hold the tube and open or close the top all with one hand. If you insert the tube into a
solution to a depth of a centimeter or so and then open it by lifting your index finger, the
solution will rush in. If you now seal it with your fingertip and lift, you can remove a
small sample of that solution from that test tube. You can now add a second solution: if
you lower it about two centimeters into the second solution and release your finger, the
second solution will rush in to the tube which already contains the first solution. If the
second solution is more dense than the first, two distinct stable colored layers will be
seen. (It may help if you hold a piece of white paper behind the tube.) But if the first
solution was more dense than the second, it will try to change places with the second
(lower) solution, and mixing will occur. You will then not see layers.
c. Conducting Tests of Density. Repeated testing of pairs of solutions should
enable you to work out which solution is most dense, next most dense, second lightest,
and least dense.
d. Final Check. As a final test of your mastery of logic and technique, fill one
tube with all four solutions so that the layers can clearly be seen, and call me over quickly
to see it. But you will already know if you are right because if you have any in the wrong
order, they will mix and you won’t see four layers very long!
5. Data: Record your data on your data sheet.
6. Clean-up:
Liquid: Dispose of all liquid down the drain.
Test-tubes: Rinse test tubes with water, using test-tube brush as needed.
Return all other materials to the central area or as your teacher directs.
Remember to wash you hands when you are finished.
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Density Rainbow Lab- Data Sheet
Results of Tests of Two Solutions at a Time:
Trial:
Top:
Bottom:
Stable/Mixed:
Conclusion:
1
2
3
4
5
6
7
8
9
10
11
12
Conclusion:
Based on an analysis of the results above, and as confirmed by the stacking of all
four solutions, the order of the solutions from top to bottom is:
Color:
Position:
Relative Density:
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
*** Certification: I saw the tube with the four solutions in it, all in the correct order.
Teacher’s Signature _________________
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LAB SET-UP- Density Rainbow
Translucent plastic straws actually work better than 6 mm glass tubing, and are nonbreakable. In two-liter bottles, make up four color-coded saltwater solutions. Use food
coloring. Use kosher salt, or chemically pure sodium chloride, to prevent the cloudiness
caused by the anti-caking agent in regular table salt. Since this is not a quantitative lab,
measurements can be approximate. For two liters of each solution, use the indicated
number of scoops of salt. (The "scoop" is a 50-ml beaker.)
Color of solution:
blue
red
yellow
green
Scoops of NaCl per 2000 ml:
none
one
two
three
LABELS:
Density of
Solutions:
Blue
Density of
Solutions:
Red
Density of
Solutions:
Yellow
Density of
Solutions:
Green
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