Solubility…

A solution is a mixture between a solvent and a solute. The solvent is the liquid in which something dissolves and the solute is the substance dissolved in the solvent. In salt solution, water is the solvent and salt is the solute. Things can be encouraged to dissolve in water by stirring the mixture to distribute the solute amongst the solvent, by grinding the solute up to increase its surface area or by heating the mixture up to give the solvent molecules more kinetic energy.

The more solute dissolved in a solution, the greater the concentration. When the maximum concentration is reached (at a particular temperature) we say that the solution is saturated. Syrup is an example of a saturated solution of sugar in water.

Different substances have different solubilities. That means that a given volume of water can hold a different maximum mass of solute at a particular temperature. For solutes that are solid at room temperature, the solubility usually increases with temperature. Gases tend to get less soluble as the temperature of the solvent increases. Fizzy drinks, which are saturated solutions of carbon dioxide in water, go flat as they warm up because the carbon dioxide comes out of solution and evaporates.

The graph above shows the solubility of some common compounds. The light blue line represents sodium chloride (NaCl or table salt) which does not become much more soluble as the temperature increases. Compare this with potassium nitrate (KNO₃ or saltpetre) which clearly becomes very much more soluble as the temperature increases. At around 24°C the solubility of NaCl and KNO₃ are the same at about 36g of solute per 100g of water.

This variability in the solubility of different compounds allows us to separate mixtures of compounds. Imagine that you had a 50:50 mixture of potassium nitrate and sodium chloride. They are both white crystalline solids which dissolve in water. How could you get almost pure potassium nitrate from the mixture? The secret is to make a saturated solution of the mixture at a high temperature (e.g. 50°C). At this temperature over 80g of potassium nitrate will dissolve in 100g of water and about 38g of sodium chloride will dissolve. As the mixture cools, potassium nitrate will come out of solution much more rapidly than the sodium chloride. At 20°C about 50g of potassium nitrate will have settled out as a solid but only about 1g of sodium chloride. You could filter the mixture to remove this much purified mixture (now about 50:1).

Questions…

1. To which group on the periodic table do sodium and potassium belong? What chemical properties do they share?
2. What compound is represented by…
   • The dark blue line?
   • The red line?
   • The brown line?
3. These compounds are all examples of salts. What chemical reaction produces a salt and water as products?
4. What is the solubility of sodium nitrate at 10°C?
5. CaCl₂ is calcium chloride. Roughly what mass of CaCl₂ would precipitate out if you cooled 100 ml of saturated solution from 25°C to 5°C? How much precipitate would you get if you started with 1 litre of saturated solution?