SOLUBILITY AND SOLUBILITY CURVES

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```							                   OBJECTIVE

To investigate and explain the effect of structure
and temperature on the solubility of solids in
water
What is Solubility?
 Solubility: the mass of solute which will saturate 100 g of solvent at a
given temperature.

 Unit: - g per 100 g solvent (e.g.. Water, ethanol)

 That means, if it takes 32 g of NaCl to saturate 100 g of what at 25 0C,
then it’s solubility is 32 g per 100 g water at 25 0C.

 The solubility of a solid in a liquid generally increases as temperature
increases: the hotter the liquid, the easier it is to dissolve the solid.
Thought: Ever tried making juice
with cold water?
Solubility Curves
 When solubility is plotted against temperature, a solubility curve is
obtained.                                  60

 The solubility at 58 0C
50
is 20 g per 100 g H2O
40

 The solubility at 82 0C        Solubility
30
g per 100 g H2O
is 40 g per 100g H2O
20

10

It will therefore take 40 -20 = 20 g
0
of the salt to re-saturate the solution 0      20   40         60           80   100   120
Temperature / 0C
if the temperature is raised from 58 0C to 82 0C.
Reminder: Some solids dissolve only slightly. Other solids are very soluble - a
lot can be dissolved in a small volume of solvent. The solubility of solids also
depends on the temperature of the solvent.

Activity 1

Draw a graph to show the solubility of potassium nitrate, the figures are given below.

Temperature goes along the X-axis and solubility goes on the Y-axis. Be sure to label

your axes. Draw a smooth curve through the points on the graph.

Temperature in °C                     0    20    40    60     80       100
Solubility in g/100g water            13   32    64    110    169      246
Use the graph that to answer the following
questions.

1.   How many grams of potassium nitrate would dissolve in
100g of water at 50°C?

2. At what temperature is the solubility of potassium nitrate
200g per 100g of water?

3. Imagine that you have heated 100 g of water to 60°C. You
find that you can dissolve 110 g of potassium nitrate in it. If you
now cool the water to 20°C, only 32 g can stay dissolved in the
water. What will happen to the other 78 g?

4. If you heated 100 g of water to 80°C, how much potassium
nitrate could you dissolve in it?

5. If you now cooled the solution to 40°C, how much potassium
nitrate would crystallize out?
 6. How many grams potassium nitrate would dissolve in 42 g of water
at 20°C?

 7. How many grams of water will it take to dissolve 75 g of KNO3 at
65°C?

 8. What is the percent KNO3 in solution that is saturated at 70°C?
Factors affecting solubility:
 1. Temperature: Solutes dissolve more easily with hot
solvents than cold solvents.
Temperature is a measure of the kinetic energy of the atoms and
molecules of a substance. The more heat energy a body
possesses the greater the speed with which its’ molecules are
moving and colliding, because of this the atoms of hot
solvents are able to break apart the bonds holding solute
molecules together more easily.
2. Pressure:

 Pressure is a factor to gases dissolved with liquids. The
presence of gases with high pressure above a mixture
containing dissolved gases prevents these gases from
escaping.
 E.g Opening a coke bottle relieves pressure and allows
dissolved CO2 gases to escape from the liquid.
 Fortunately for fish, oxygen is slightly soluble in water. A
fish takes water in through its mouth. The water passes
over the fish's gills, which are able to remove the dissolved
oxygen. The water, minus the dissolved oxygen, then passes
out through the gill slits.

 Fizzy drinks contain a dissolved gas - carbon dioxide. It
isn't a very soluble gas, so it is forced into the water under
pressure. When you unscrew the top of a bottle of fizzy
drink, the pressure is released and the carbon dioxide
comes back out of solution. You see bubbles of gas
streaming to the surface. Carbon dioxide is added to drinks
to improve the taste. If you leave the top off a bottle, most
of the carbon dioxide will be released and the drink will
taste 'flat'.
3. Nature of the solvent
There are 2 main types of solvents:
1. Polar or charged
2. non-polar or uncharged.

Polar/charged solvents dissolve solutes with charged
particles. E.g H2O and NaCl. Non-Polar solvents
dissolve uncharged substances. E.g. I2 is dissolved by
ethanol, hexane or other uncharged substances.
Concentration of solutions
 A dilute solution contains more solvent than solute.
 E.g. Sea water
 A concentrated solution contains relatively more solute than
solvent.
 E.g. 95% HCl
 A saturated solution contains as much solute as it can possibly
hold at that temperature and pressure.
E.g. a Thoroughly stirred glass of juice will have some sugar left on the
bottom of the glass. this indicates that the solvent water) is holding
as much solute as it can hold.

 A supersaturated solution contains more solute than it is
supposed to hold at a given temperature and pressure.

* Supersaturated solutions are rare and are unstable, physically disturbing
them can cause excess dissolved solute to precipitate and fall to the bottom.
Procedures which affect the rate
at which substances dissolve:
 Crushing

 Stirring

 Heating

Why?
 Activity 2:

 Fill in the formula for each chemical, then calculate
the difference column and answer the questions.
Name of       chemical   solubility at   solubility at   Difference in
Formula    15°C            80°C            solubility
Copper (II)              18.8            55.0
sulfate

Potassium                32.8            51.3
chloride
Potassium                25.8            169.0
nitrate
Sodium                   16.4            45.8
carbonate
Sodium                   35.9            38.4
chloride
 1. Which is the least soluble at 15°C?

 2. Which is most soluble at 15°C?

 3. Which one is most soluble at 80°C?

 4. Which one increases in solubility by the least?

 5. Which one increases in solubility by the most?
Activity 3: This table shows the solubility of some gases in water.
The solubility of a gas is the mass in g that will dissolve in 1oo g of
water. The solubility is shown at three different temperatures.

10 0C              20 0C              30 0C
NH3               8700               6800               5300

CO2               116                84.8               65.2

O2                3.7                3.0                2.6

1. Which of the gases is the most soluble?
2. Which of the gases is the least soluble?
3. In what way is the effect of temperature on the
solubility of gases different from its effect on the
solubility of solids?

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