AQA GCSE Science CHEMISTRY Unit C3.2 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.

• You should be able to use your skills, knowledge and understanding to:

  • evaluate the use of commercial water softeners,

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

    • you may be asked to evaluate different methods of softening water, or of providing drinking water of sufficient quality,

  • consider the advantages and disadvantages of adding chlorine and fluoride to drinking water,

    • You will be expected to interpret and evaluate information and data that is provided in questions set within these contexts.

Summary of syllabus-specification statements expressed as learning objectives for

AQA GCSE Science CHEMISTRY Unit C3.2.1 Hard and soft water

• a) Know that soft water readily forms lather with soap.

  • Know that hard water reacts with soap to form scum and so more soap is needed to form lather.

  • Know that soapless detergents do not form scum.

  • You should know how to measure the hardness of water by titration with soap solution and be able to do it.

•  b) Know that hard water contains dissolved compounds, usually of calcium or magnesium.

  • Know that the compounds are dissolved when water comes into contact with rocks.

• c) Know there are two types of hard water.

  • Permanent hard water remains hard when it is boiled.

  • Temporary hard water is softened by boiling.

  • You should be able to distinguish between temporary hard water and permanent hard water.

• d) HT only: Know that 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) Appreciate that using hard water can increase costs because more soap is needed.

  • Know that when temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles.

• f) Know that 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) Know and understand that hard water can be made soft by removing the dissolved calcium and magnesium ions.

  • Know this can be done by:

    • (i) adding sodium carbonate, which reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate

    • (ii) 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.

AQA GCSE Science CHEMISTRY Unit C3.2.2 Purifying water

• a) Appreciate that water of the correct quality is essential for life.

  • Appreciate that for humans, drinking water should have sufficiently low levels of dissolved salts and microbes.

  • Know that 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.

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

  • 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.

• c) Know and understand that chlorine may be added to drinking water to reduce microbes and fluoride may be added to improve dental health.

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

• d) Pure water can be produced by distillation.

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

• Revise demonstrations you saw or practical work you did to develop skills and understanding which may have included the following:

  • 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:

    • (i) temporary hardness: test before and after boiling, with soap

    • (ii) 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).

• AQA GCSE Separate Science CHEMISTRY for triple award

• When revising, these pages provide you with a summary of what you need to know and be able to do.

• BUT remember, your primary source of revision are your class notes, investigations and AQA GCSE science textbooks.

• Revision notes and quizzes to help you revise

  • Water Chemistry Notes - water treatment, hard and soft water etc.

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  • AQA GCSE Chemistry Unit C3.2 Water

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