Solutions
Terms
 Solvent – the substance that is doing the dissolving
 Solute – the substance that is being dissolved
 Solution – the solute and the solvent together
 Precipitate – (can be a verb or a noun) refers to the act of a solute
coming out of solution or to the solute that has fallen to the bottom
 Concentration - a measure of how much solute is dissolved in a
solvent. This is similar to density because it measures how much
mass is in a certain amount of space. However, it is different from
density because it involves the mass of one substance and the volume
of another substance.
 Concentration units are similar to density units. They are g of
solute/100 mL of solvent or simply g/100mL. Concentrations can be
expressed in words as well.
o Unsaturated – able to hold more solute
o Saturated – holding exactly the max solute possible
o Supersaturated – holding more than is usually possible
o Dilute – a relatively small amount of solute in a lot of solvent
 To calculate concentration EXACTLY use this formula:
C = (mass of solute/volume of solvent) x 100
Solubility
 Solubility is a measure of how much substance will dissolve at a
certain temperature in a certain amount of water (or other liquid)
o For example: 12 grams of sugar will dissolve in 100 mL of
water at room temperature. If you heat the water up to 80
degrees now 30 grams of sugar will dissolve. If you increase
the amount of water to 200 mL, twice as much sugar will
dissolve as before.
o When expressing the solubility of something you must give the
amount of water used as well as the temperature.
 Solubility is a characteristic property
 Solubility changes with temperature so the best way to show the
solubility of a substance is on a graph.
Solubility Rules: How solutes behave under certain conditions
Solid solutes:
 Dissolve faster AND more in hotter solvents
 Dissolve faster if stirred or crushed up
 Like Dissolves Like
o Polar molecules dissolve polar molecules – (a polar molecule
has a negative pole on one side and a positive one on the other
this could be because it is ionic or because it has an
asymmetrical covalent bond). The charged particles attract one
another and molecular motion spreads the solute evenly through
the solvent
o Non –polar molecules dissolve non-polar molecules without
charges being involved. A different kind of attractive force
works on non-polar molecules. We won’t go into it here
though.
For gaseous solutes:
 Dissolve faster AND more in colder solvents
 Come out of solution when stirred or shaken
 Like Dissolves Like
Particles in Solution Vocabulary
Polar – Having a positive and a negative side to the molecule (for example,
water is a polar molecule. The hydrogen side carries a positive charge and
the oxygen side carries a negative charge. This causes strong inter-particles
forces between water molecules which can rip apart other charged molecules
that are placed in the water.
Non-polar – Negative and positive charges in the molecule are evenly
distributed so that the molecule has no positive or negative sides. A
different kind of inter-particle force (hard to describe to beginning students)
works on non-polar substances. A non-polar substance can rip apart another
non-polar substance with this force.
Dissociation – The separation of a polar solute into positive and negative
particles (for example, table salt (NaCl)) dissociates into Na+ and ClDissociation can look just like dissolving but technically, it is a chemical
change since bonds between atoms are temporarily broken apart.
Ionization – The process of a particle becoming an ion (a charged particle).
When a substance dissociates it also ionizes. This is because some of the
atoms leave their charges attached to their old partners when they separate.
Electrolyte – a substance that separates into ions in solution. Electrolytes are
found in the body and are important in muscle function. Gatorade is a
beverage designed to replace the electrolytes you lose through sweating in
order to keep your muscles working efficiently during periods of exercise.
Non-electrolyte – a substance that does not separate into ions in solution
Solute and Solvent interactions
 Like dissolves like – meaning polar solvents dissolve polar solutes
and non-polar solvents dissolve non-polar solutes.
 Multiple solutes in solution may interfere with each other’s ability to
dissolve
 Boiling point and freezing point are affected by solutes
o A substance will boil at a higher temperature if a solute is
dissolved in it. (Ex: water that has salt added boils at a higher
temperature)
o A substance will freeze at a lower temperature if a solute is
dissolved in it. (Ex: adding salt to the ice on your sidewalk
will cause the freezing point of the water to become lower …so
your ice will melt).
o In both of the above cases the change in the state change point
is caused by a change in the inter-particle attractions due to the
presence of another chemical.
SOLUBILITY CURVES
1. What explains why solids become
more soluble as temperature
increases and why gasses become
less soluble? (You don't need the
graph for this one.)
2. Which is more soluble NaNO3 or
KCl?
3. How does the line drawn for a
particular substance relate to the
saturation of a solution of that
substance?
4. How many grams of NH4Cl will
dissolve in 100 grams of 90°CWater?
5. If I asked you to make a saturated solution of KCl in 100 grams of water,
what other piece of information would you need before you could start?
6. At what temperature will 10 grams of KClO3 dissolve?
7. At what temperature will equal amounts of KNO3 and NH3 dissolve?
8. Which substance’s solubility is LEAST affected by increasing
temperatures?
9. Which substance’s solubility is MOST affected by increasing
temperatures?
10. Which substance is most soluble at 0 Celsius?
11. If half as much water were used how would the amount of solute
dissolved be affected for all of these substances?
12. A student has a solution of NaNO3 that contains 90 grams of solute in
100 grams of solution at 30  Celsius. Is this solution unsaturated,
saturated or supersatured?
Experimental Work: You are assigned to a group of either three or four
people. Each person in the group will conduct one experiment and share his
or her findings with the group. The group will evaluate the validity of the
experiment and make any suggestions for improvement.
You will design one of these experiments to test the following questions
(two people from the same group cannot do the same experiment):
1.
2.
3.
4.
Is there a limit to the amount of solute that will dissolve in a solvent?
How does temperature affect the solubility of a solid solute?
How does temperature affect the solubility of a gaseous solute?
Do all solutes dissolve the same amount at a given temperature?
Submit a typed report for your experiment that includes all of the following
sections:
Question
Hypothesis (with rationale)
Materials
Procedures
Data
Conclusions
Also attach a well written paragraph in which you summarize the work of
ONE of your partners, evaluate the validity of their results and make
recommendations for improving the experiment if necessary.
See the attached sample report.
Suzy Sample
Per. 1
The Effect of Solutes on Boiling Point
Question: Does a solute added to a solvent change the temperature at which
that solvent boils?
Hypothesis: Adding a solute to a solvent should raise the boiling point
because the particles of the solute are attracted to the solvent.
This means there will be more attractive forces holding the
mixture together than there were when it was just a pure
solvent.
Materials:
Procedures:
1. Add 10 grams of salt to 100 mL of water (the water temperature
should be 20 degrees Celsius)
2. Stir the solute into the solvent until it is completely dissolved.
3. Pour 10 mL of the solution into 3 separate small test tubes.
4. Insert a stoppered thermometer into each test tube so that the bulb of
each is half way into the water. Avoid letting the bulb touch the sides
or bottom of the test tube.
5. Clamp each test tube to a pegboard and heat it over an alcohol burner.
Ensure that each burner has the wick at the same height and that the
flames are all adjusted equally.
6. Record the temperature of each solution every thirty seconds until a
plateau is reached (4 readings of the same temperature).
7. Repeat the experiment using no salt.
8. Repeat the experiment using 20 grams of salt.
Data:
Concentration
of Salt
Solution
0 g/100 mL
10 g/100 mL
20g/100 mL
Plateau temperature
Trial
Trial
One
Two
100 ° C 100 ° C
103 ° C 104 ° C
107° C 108 ° C
Trial
Three
100 ° C
103 ° C
107 ° C
Average plateau
temperature for three
trials
100 ° C
103.3 ° C
107.3 ° C
Conclusions:
The hypothesis was supported. The boiling temperature of the solution was
became higher as more and more solute was added. The experiment was
well run. There was a control group (the solution with no salt added) and all
the trials were treated the same way except for the change in the amount of
salt. For example, all the test tubes were the same size, all the thermometers
were placed the same way and all the flames were adjusted equally. There
was plenty of data; each concentration had three test tubes representing it
and the plateau temperature was determined with four repeated temperature
readings. The thermometers are not very precise yet the data was well
within the range of that precision. Therefore the data is trustworthy and
valid.
Joe’s Experiment:
My lab partner Joe tested whether or not a solute would change the freezing
temperature of a solvent. His experiment had many problems however and
he was unable to draw a definite conclusion. He used paradichlorobenzene
as his solvent and salt as his solute. Unfortunately, the salt did not dissolve
well in his solvent because salt is polar and para is not. He used methods
similar to mine with three test tubes at each concentration. He also used
stoppered thermometers. He started with his test tubes heating in a hot water
bath and then he let the water cool down. He recorded the temperature every
minute and watched for the plateau of freezing. He had nine test tubes in his
water bath at one time and so he had trouble reading all the thermometers
accurately and on time. As a result he does not trust his data and neither do
I. I recommend that Joe repeat the experiment using sugar (which is also
nonpolar like para and also that he do only three test tubes at one time so he
can record his data more accurately.