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 CurvesWhen 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 gof the salt to re-saturate the solution 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

100Solubility 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 solventThere are 2 main types of solvents:

1. Polar or charged2. 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 solutionsA dilute solution contains more solvent than solute.

E.g. Sea waterA 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 Formula

solubility at 15°C

solubility at 80°C

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

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.

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