3.1 Solubility

Solubility

You may have noticed sometimes that, if you try to dissolve salt (or some other solute) in a small amount of water, it will initially dissolve, but then appears not to be able to dissolve any further when you keep adding more solute to the solvent. This is called the solubility of the solution. Solubility refers to the maximum amount of solute that will dissolve in a solvent under certain conditions.

Solubility

Solubility is the ability of a given substance, the solute, to dissolve in a solvent. If a substance has a high solubility, it means that lots of the solute is able to dissolve in the solvent.

So what factors affect solubility? Below are some of the factors that affect solubility:

• the quantity of solute and solvent in the solution
• the temperature of the solution
• other compounds in the solvent affect solubility because they take up some of the spaces between molecules of the solvent, that could otherwise be taken by the solute itself
• the strength of the forces between particles of the solute, and the strength of forces between particles of the solvent

Experiment : factors affecting solubility

Aim:

To determine the effect of temperature on solubility

Method:

1. Measure 100cm ${}^3$ of water into a beaker
2. Measure 100 g of salt and place into another beaker
3. Slowly pour the salt into the beaker with the water, stirring it as you add. Keep adding salt until you notice that the salt is not dissolving anymore.
4. Record the amount of salt that has been added to the water and the temperature of the solution.
5. Now increase the temperature of the water by heating it over a bunsen burner.
6. Repeat the steps above so that you obtain the solubility limit of salt at this higher temperature. You will need to start again with new salt and water!
7. Continue to increase the temperature as many times as possible and record your results.

Results:

Record your results in the table below:

As you increase the temperature of the water, are you able to dissolve more or less salt?

Conclusions:

As the temperature of the solution increases, so does the amount of salt that will dissolve. The solubility of sodium chloride increases as the temperature increases.

Investigating the solubility of salts

The data table below gives the solubility (measured in grams of salt per 100 g water) of a number of different salts at various temperatures. Look at the data and then answer the questions that follow.

1. On the same set of axes, draw line graphs to show how the solubility of the three salts changes with an increase in temperature.
2. Describe what happens to salt solubility as temperature increases. Suggest a reason why this happens.
3. Write an equation to show how each of the following salts ionises in water:
   1. KNO ${}_3$
   2. K ${}_2$ SO ${}_4$
4. You are given three beakers, each containing the same amount of water. 5 g KNO ${}_3$ is added to beaker 1, 5 g K ${}_2$ SO ${}_4$ is added to beaker 2 and 5 g NaCl is added to beaker 3. The beakers are heated over a bunsen burner until the temperature of their solutions is 60 ${}^0$ C.
   1. Which salt solution will have the highest conductivity under these conditions? (Hint: Think of the number of solute ions in solution)
   2. Explain your answer.