AQA GCSE Chemistry Unit C3 Chapter 3 (Water)

Many ionic compounds are soluble, but many covalent compounds do not dissolve in water:

• Water that has been in contact with the ground may contain dissolved solids
• Solubility is usually measured in grams solute per 100g of solvent at a given temperature
• A saturated solution contains the maximum amount of solute that will dissolve at the temperature of the solution. Cooling a hot saturated solution of a solid causes some of the solute to crystallise

The solubility of a substance at different temperatures can be plotted as a line graph, known as a solubility curve.

• They can be used to find out how much solute will dissolve at a particular temperature
• They can be used to find out how much solute will separate out of a saturated solution for a change in temperature

The solubility of most solids increases as temperature increases but the solubility of gases decreases as temperature increases.

The solubility of gases in water increases as pressure increases. Carbonated water and fizzy drinks are made by dissolving CO2 in them under high pressure

Soft water is water that lathers easily with soap - hard water uses more soap to make a lather and to wash effectively. It's the dissolved salts in hard water that make it hard because they react with soap to form scum.

When water is in contact with rocks, some bits dissolve to produce a solution containing Ca2+ and Mg2+ ions. This means that hard water can be good for us because calcium helps bones and teeth and also reduces the risk of heart disease.

When hard water is heated, it produces scale, which is insoluble. Scale is what furs up boilers and kettles, reducing the efficiency of heating systems and causing blockages.

You can get rid of the hardness (Ca2+ and Mg2+ ions) in water by precipitating them out or by replacing them with ions that don't react with soap.

This can be done by adding washing soda (sodium carbonate). The carbonate ions react with the ions that cause hardness forming precipitates that can't react with soap since they are no longer in solution. The carbonates of metals (except Group 1) are insoluble.

Ion-exchange resins containing sodium ions can also be used. When hard water is passed through, Ca and Mg ions become attached to the resin and sodium ions take their place in the water. The sodium ions can be replaced in the resin by washing it with a concentrated solution of sodium chloride (common salt).

Water supplies need to be fit to drink, wash and to be used as a raw material. Drinking water should only have very low levels of harmful substances in it.

Water from an appropriate source can be treated to make it safe. This is done by sedimentation and filtration to remove solids and by killing any microorganisms in the water. Chlorine is often used to do this. Boiling the water will also destroy most microorganisms.

Water filters can be used to improve the taste of water. They often contain charcoal, an ion-exchange resin and silver to prevent the growth of bacteria.

You can make pure water by distillation or de-ionisation. De-ionising uses an ion-exchange column to remove all ions except H+ and OH-.