Chemistry AQA GCSE Core Revision Notes on Water

Water

The water we drink is not pure water because it contains dissolved substances. It should be safe to drink water that has been treated. This means that the water does not contain anything that could cause us harm. Some of the dissolved substances are beneficial to our health but some cause hard water.

What you need to know:

to:

■ evaluate the use of commercial water softeners

:

You should be able to distinguish between temporary hard water and permanent hard water; to measure the hardness of water by titration with soap solution; to evaluate different methods of softening water, or of providing drinking water of sufficient quality; to interpret and evaluate information and data that is provided in questions set within these contexts.

■ consider and evaluate the environmental, social and economic aspects of water quality and hardness

■ consider the advantages and disadvantages of adding chlorine and fluoride to drinking water.

Hard and soft water

a) Soft water readily forms lather with soap. Hard water reacts with soap to form scum and so more soap is needed to form lather. Soapless detergents do not form scum.

b) Hard water contains dissolved compounds, usually of calcium or magnesium. The compounds are dissolved when water comes into contact with rocks.

c) There are two types of hard water. Permanent hard water remains hard when it is boiled. Temporary hard water is softened by boiling.

d) Temporary hard water contains hydrogencarbonate ions (HCO3 ––) that decompose on heating to produce carbonate ions which react with calcium and magnesium ions to form precipitates.

e) Using hard water can increase costs because more soap is needed. When temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles.

f) Hard water has some benefits because calcium compounds are good for the development and maintenance of bones and teeth and also help to reduce heart disease.

g) Hard water can be made soft by removing the dissolved calcium and magnesium ions. This can be done by:

■ adding sodium carbonate, which reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate

■ using commercial water softeners such as ion exchange columns containing hydrogen ions or sodium ions, which replace the calcium and magnesium ions when hard water passes through the column.

Additional guidance:

Water of the correct quality is produced by:

■ choosing an appropriate source

■ passing the water through filter beds to remove any solids

■ sterilising with chlorine.

TIPS: 

• Detailed knowledge of specific water filters is not required.

•  Examination questions may give information about water filters so that comparisons can be made.

• You should understand the principles of how ion exchange resins work but do not need detailed knowledge of the structure or chemical nature of specific resins.

• You should be aware of the arguments for and against the addition of fluoride to drinking water.

• You should be aware of the large amount of energy needed for distillation and, as a consequence, of the high costs involved.

Purifying water

a) Water of the correct quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes.

b) Water filters containing carbon, silver and ion exchange resins can remove some dissolved substances from tap water to improve the taste and quality.

c) Chlorine may be added to drinking water to reduce microbes and fluoride may be added to improve dental health.

d) Pure water can be produced by distillation.

You need to know the steps to the following procedures:

■ investigation of which ions cause hard water, eg adding soap solution to solutions of NaCl, CaCl2, KCl, and MgCl2

■ making temporary hard water by adding excess carbon dioxide to limewater

■ determining hardness of samples of water – shake with soap solution – measuring cm3 of soap to get permanent lather

■ the removal of hardness:

– temporary hardness: test before and after boiling, with soap

– permanent hardness: test before and after addition of sodium carbonate

■ testing hard water before and after passing through an ion exchange column

■ using conductivity sensors to analyse different samples of hard and soft water

■ design and carry out an investigation to compare the effectiveness of commercial water softeners using soap titration

■ investigating the various types of water ‘filters’ that are commercially available

■ distillation of seawater – design a simple apparatus to do the distillation and check the quality of the distillate (boiling point and evaporation to dryness of a sample on a watch glass).

Additional guidance:

You should be able to calculate and compare the amount of energy released by different fuels given the equation:

Q = mc ΔT

For comparison purposes, energy values could be given in kJ or calories for a given mass or amount of substance, eg calories per gram, kJ per mole or kJ per gram. If candidates are required to convert from calories to joules, the conversion factor will be given in questions. You may be provided with information for comparison and evaluation. For example, you may be given information about the ingredients of a particular food or the components of a fuel, but will not be expected to have knowledge of the constituents of commercial products beyond that specified in the subject content for this unit.