SOLUBILITY AND SOLUBILITY CURVES

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SOLUBILITY AND SOLUBILITY CURVES

• OBJECTIVE
• To investigate and explain the effect of
  structure and temperature on the solubility of
  solids in water

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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 its 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.

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Solubility Curves

• When solubility is plotted against temperature, a
  solubility curve is obtained.
• The solubility at 58 0C
• is 20 g per 100 g H2O
• The solubility at 82 0C
• is 40 g per 100g H2O
• It will therefore take 40 -20 20 g
• of the salt to re-saturate the solution
• if the temperature is raised from 58 0C to 82 0C.

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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
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• Use the graph that to answer the following
  questions.
• How many grams of potassium nitrate would
  dissolve in 100g of water at 50C?
• 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
  60C. You find that you can dissolve 110 g of
  potassium nitrate in it. If you now cool the
  water to 20C, 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 80C, how much
  potassium nitrate could you dissolve in it?
• 5. If you now cooled the solution to 40C, how
  much potassium nitrate would crystallize out?

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• 6. How many grams potassium nitrate would
  dissolve in 42 g of water at 20C?
• 7. How many grams of water will it take to
  dissolve 75 g of KNO3 at 65C?
• 8. What is the percent KNO3 in solution that is
  saturated at 70C?

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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.

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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.

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• 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'.

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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.

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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.
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Procedures which affect the rate at which
substances dissolve

• Crushing
• Stirring
• Heating
• Why?

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• Activity 2
• Fill in the formula for each chemical, then
  calculate the difference column and answer the
  questions.

Name of chemical Formula solubility at 15C solubility at 80C Difference in solubility
Copper (II) sulfate 18.8 55.0
Potassium chloride 32.8 51.3
Potassium nitrate 25.8 169.0
Sodium carbonate 16.4 45.8
Sodium chloride 35.9 38.4
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• 1. Which is the least soluble at 15C?
• 2. Which is most soluble at 15C?
• 3. Which one is most soluble at 80C?
• 4. Which one increases in solubility by the
  least?
• 5. Which one increases in solubility by the most?

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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. 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?
10 0C 20 0C 30 0C
NH3 8700 6800 5300
CO2 116 84.8 65.2
O2 3.7 3.0 2.6