GCSE AQA Chemestry unit 3 2.1-2.4 key points

GCSE AQA Chemestry unit 3 2.1-2.4 key points

1. C3 2.1

   • Hard water contains dissolved compounds such as calcium and magnesium salts.
   • The calcium and/or magnesium ions in hard water react with soap producing a precipitate called scum
   • hard water can produce a solid scale when it is heated, reducing the efficiency of heating systems and kettles.
   • Hard water is better then soft water for developing and maintaining bones and teeth. It may also help prevent heart disease.

2. C3 2.3

   • Water for drinking should contain only low levels of dissolved substances and microbes.
   • Water is made fit to drink by filtering it to remove and adding chlorine to reduce the number of microbes
   • We can make pure water by distillation but this requires large amounts of energy which makes it expensive.

3. C3 2.4

   • Chlorine is added to water to sterilise it by killing microbes.
   • Fluoride helps to improve dental health
   • Some argue against the fluoridation of public water supplies. for example, they think that people should have the right to choose if they want to take extra fluoride or not.

4. C3 2.2

   • Soft water does not contain ions that produce scum or scale.
   • Hard water can be softened by removing the ions that produce scum and scale.
   • temporary hardness is removed from water by heating it. permanent hardness is not changed by heating.
   • They hydrogencarbonate ions in temporary hard water decompose on heating. the carbonate ions formed react wit Ca2+ (aq) and Mg2+ (aq) ions, making precipitates
   • Either type of hard water can be softened by adding washing soda or by using an ion-exchange resin to remove to remove calcium and magnesium ions.