C3: Water

Water of the correct quality is essential for life. Water naturally contains microorganisms and dissolved salts, which should be sufficientley low levels to be safe for humans to drink.

How good quality/safe to drink water is produced...

1. The water is passed through a filter bed to remove any solid particles.

2. Chlorine gas is then added to kill any harmful microorganisms.

3. Fluoride is added to drinking water in order to reduce tooth decay (although too much fluoride can cause discolourisation of the teeth.

To improve the taste and quality of tap water, more dissolved substances can be removed by passing the the water through a filter containing carbon, silver and ion exchange resins.

Any water can be distilled to produce pure water, i.e. water that contains no dissolved substances. The water is boiled to produce steam , which is condensed by cooling it to produce pure liquid water. This process uses a lot of energy, which makes distallation an exspensive process.

The amount of compunds present in tap water determines whether it's described as hard or soft.

Soft Water doesn't contain many dissolved compunds so it readily forms lather with soap.

Most hard water contains calcium or magnesium compounds that dissolve in natural water as it flows over ground or rocks containing compunds of these elements. These dissolved substances react with soap to form scum, which makes it harder to form a lather. Soapless detergents do not form scum.

Permanently hard water remains hard upon boiling.

Temporary hard water is softend upon boiling.

Temporary hard water contains hydrogencarbonate ions that decompose upon heating to produce carbonate ions. These carbonate ions react with calcium and/or magnesium ions to form precipitates (insoluble solids).

Advantages of hard water:

• The dissolved compounds in water are good for your health, e.g. calcium compunds help the development of strong bones and teeth, and also help to reduce the risk of heart diseases.

Disadvantages of hard water:

• More soap is needed to form a lather because before the soap gets anywhere near the dirt it has to react with all the dissolved calcium and magnesium ions in the water. This increases costs.
• Using hard water often leads to deposits (called scale) forming in heating systems and appliaces like kettles, which reduces their efficiency.

Ideally, we should wash with soft water, drink hard water.

To make hard water soft, the dissolved calcium and magnesium ions need to be removed. This can be done in one of two ways:

Method 1

Add sodium carbonate solution (washing soda) to it. The carbonate ions react with calcium and magnesium ions to form calcium carbonate and magnesium carbonate (respectively), which precipitate out of the solution as they are both insoluble.

Equation:

Method 2

Pass the hard water through an ion-exchange column, which contains hydrogen ions or sodium ions. As the hard water passes through the column, the calcium and magnesium ions contained in it are replaced by hydrogen or sodium ions. This is why water softeners need to be topped up with salt - to replace the sodium ions that have been exchanged.