Name
Lab Day
Solutions
Introduction:
A solution is a homogeneous mixture of two or more substances.
Solutions may be gaseous, liquid, or solid and the components may be in
any physical state. For example, sugar is a solid which mixes with water
to form a liquid solution; and air is an example of a gaseous solution
primarily composed of oxygen and nitrogen.
This experiment focuses on liquid solutions formed by dissolving
solids or liquids in water. When two liquids mix to form a solution, the
liquids are said to be miscible and if they do not mix then the liquids are
immiscible. A solid or liquid that is dispersed in a larger amount of liquid
in forming a solution is called the solute and the liquid serving to
dissolve the solute is called the solvent.
Liquids that are miscible can be combined in all proportions; but
there is a limit to the solubility of a substance in water (or other liquids).
A solution which contains the maximum amount of solute capable of
dissolving in a given amount of solvent is said to be saturated. Solutions
in which further solute may be dissolved are said to be unsaturated.
Usually increasing the solvent’s temperature, increases the amount of
solute that may be dissolved. By cooling a saturated solution, a mixture
is formed that is still a solution, but it is unstable. There is too much
solute dissolved in a given amount of solvent. This is called a
supersaturated solution, and it is unstable. The dissolved solute in a
supersaturated solution may crystallize (i.e., form the solid compound)
until the remaining dissolved solute forms a saturated solution.
There is an old adage which is handy in trying to decide which
substances are soluble in a liquid: “Like dissolves like.” This means that
if the solute is polar, then it is likely to dissolve in a polar liquid (like
water). Whereas, a non-polar material is unlikely to dissolve a polar
liquid. For example, the non-polar compound, chloroform, does not mix
with the polar compound, water, but is thoroughly miscible with the nonpolar liquid, benzene.
The rate at which one substance dissolves into another material
depends on: (1) the particle size; (2) agitation or stirring of the mixture;
(3) the temperature of the solution; and (4) the concentration of solute
already present in the solution. The term solubility relates to the amount
of solute that can dissolve in a given amount of solvent. Solubility is
often qualitatively expressed by terms as: (1) very soluble; (2) soluble; (3)
slightly soluble; and (4) insoluble. Quantitative solubility may be
expressed in a variety of terms which expresses the concentration in
some numerical form. For example, a 10% sucrose solution means that
in 100 grams of solution, there is 10 grams of sucrose and 90 grams of
water.
2
Experimental:
Determining the concentration of a saturated K2Cr2O7 solution.
Clean, dry, and weigh an evaporating dish. Record the weight to the
nearest 0.01 grams on your report sheet. Fill a 250 mL beaker about ¾
full with water and place the beaker on a hot plate. Place about 6 to 10
mL of saturated K2Cr2O7 solution into the evaporating dish and weigh
the dish containing the solution to the nearest 0.01 grams. Place the
evaporating dish on the partially-filled 250 mL beaker and boil the water
until the material in the evaporating dish looks dry (about 30 minutes).
Remove the evaporating dish from the beaker and place it directly on the
hot plate. Heat the evaporating dish on “high” for 1 to 2 minutes. Remove
and allow the dish to cool. Weigh the dish containing the solid K2Cr2O7 to
the nearest 0.01 grams. Reheat and reweigh until a constant weight is
obtained. Using the recorded weights, calculate the percentage of
K2Cr2O7 dissolved in the saturated solution.
Miscibility of Liquids. Using three dry test tubes, prepare the
following mixtures: (1) 1 mL water and 1 mL isopropyl alcohol; (2) 1 mL
isopropyl alcohol and 1 mL kerosene; and (3) 1 mL of water and 1 mL of
kerosene. Stopper and shake the test tubes and allow them to set at least
5 minutes. Report your observations on the report sheet.
Relative Solubility of a Solute in Two Immiscible Solvents. Add 5 mL
of water to a test tube. Pour in 2 mL cyclohexane. Notice the separate
layers. Stopper and shake the tube, then allow the liquids to separate.
Add a couple of crystals (just the tip of your lab scoop) of solid iodine to
the mixture of cyclohexane and water. Stopper and shake well. Notice the
color of the cyclohexane layer.
Rate of Dissolving Versus Particle Size. Place 0.5 grams of fine
crystalline NaCl in one test tube and 0.5 grams of coarse crystalline NaCl
into another test tube. Add 20 mL of water to each tube, stopper, and
shake noting the time required to dissolve each amount of NaCl.
Solubility and Temperature. Label one test tube “NaCl” and another
“NH4Cl”. Place 1.0 grams of NaCl into the “NaCl” tube and 1.0 grams of
NH4Cl into the “NH4Cl” tube. Add 5 mL of room temperature water to
each tube. Stopper, and shake until both salts are dissolved.
Add 1.4 grams more NaCl into the “NaCl” tube and 1.4 grams more
NH4Cl into the “NH4Cl” tube. Stopper, and shake for about 5 minutes
and note the results. Un-stopper, and place both tubes into a beaker of
boiling water. Periodically stir each test tube. After 5 minutes of heating,
note the results.
Place the warm tubes in a test tube rack and allow them to cool
(about 10 minutes). Note any changes. If there has been no change in the
“NH4Cl” tube, run cold tap water over the outside of the test tube.
3
Name
Date
Report for Solutions Experiment:
Concentration of Saturated K2Cr2O7:
1.
Weight of empty evaporating dish
g
2.
Weight of dish and solution
g
3.
Weight of dish and dry K2Cr2O7
g
4.
Percentage of K2Cr2O7 (show calculations below)
%
Miscibility of Liquids:
5.
Which compounds are miscible?
6.
Which compounds are immiscible?
7.
Arrange the compounds in decreasing polarity:
Hi
Lo
Relative Solubility of a Solute:
8.
Which compound was the bottom layer?
9.
Which compound is the most dense?
10.
The two liquids are:
miscible
immiscible
(circle the correct statement)
11.
In which substance is iodine most soluble?
(circle the correct solvent)
water
cyclohexane
4
12.
What would you guess about the polarity of iodine from your
observations?
Solubility and Particle Size:
13.
Time to dissolve the fine crystals
min
14.
Time to dissolve the coarse crystals
min
15.
How is the rate of dissolving related to the particle size?
Solubility and Temperature:
16.
Are the tubes containing 1 gram NaCl in 5 mL of water and
1 gram NH4Cl in 5 mL of water, saturated or unsaturated?
17.
Are the tubes containing a total of 2.4 grams NaCl in 5 mL
of water and 2.4 grams NH4Cl in 5 mL of water, saturated
or unsaturated?
18.
Which of the mixtures was unsaturated and which was
saturated upon heating?
19.
On cooling, which mixture became supersaturated? How
could you tell?