Unit 1, Module 2 Booklet 2 obtaining resources from our planet Lesson 4: Water hardness

Lesson aims The causes of hardness in water and distinguish between hard and soft waters by their action with soap.

Different types of water In South Wales we have soft water. It readily forms a lather with soap and does not cause problems with limescale in kettles, washing machines and dishwashers. However in other parts of the country this is not the case and hard water can cause problems in appliances.

Different types of water

Key facts about hard and soft water 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

Hard water What causes hard water? As water passes through rocks containing calcium and magnesium compounds these dissolve in the water causing hardness. Fact: ions in hard water are Mg2+ and Ca2+

Hard water Hard water doesn’t produce soap suds easily. As rain water flows through rocks, it picks up calcium and magnesium ions. The result is - hard water. Hard water forms scale (calcium carbonate) in pipes, and restricts water flow. Hard water mixes with soap to form calcium or magnesium stearate which is insoluble – this is why soap scum forms. Hard water doesn’t produce soap suds easily. Additional Science Chapter 8 This fact can be used to prove for water hardness, and to compare the hardness of different samples.

Equations for forming temporary hard water Rain water is slightly acidic. This means when it falls on the rocks the following reaction happens. Forming the acid H2O + CO2  H2CO3 (carbonic acid) Reaction with chalk/limestone CaCO3 + H2CO3  Ca(HCO3)2

How scum wastes soap It is the dissolved Ca2+ and Mg2+ that react with soap to form scum. Sodium stearate + calcium ions  calcium stearate (scum) + sodium ions This means more soap is needed because of the soap wasted as scum. A lather is only formed when the calcium/magnesium ions have been used up.

How scale damages appliances Temporary hard water contains hydrogen carbonate ions (HCO3-) that decompose on heating to produce carbonate ions These react with calcium and magnesium ions to form precipitates of calcium carbonate (limestone) and magnesium carbonate. These are solid scale

Damaging appliances When temporary hard water is heated in a kettle, washing machine, dishwasher or central heating this leads to deposits of these metal carbonates which we call limescale. It can significantly reduce the efficiency of these systems and in some cases prevents them from working at all.

Types of hardness Additional Science Chapter 8 Temporary hardness Reason:- Presence of calcium or magnesium carbonate in the water Temporary hardness Solution:- Boiling the water. The hydrocarbonates decompose, leaving insoluble calcium carbonate or magnesium carbonate – this is the scale found in kettles and boilers. Equation:- Ca(HCO3)2  Ca CO3 + CO2 + H2O Permanent hardness Reason:- Presence of calcium or magnesium sulphate in the water. Solution:- Soften the water by adding sodium carbonate or by ion exchange. Equation:- Na2CO3 + CaSO4  Ca CO3 + Na2SO4 Additional Science Chapter 8

Water hardness experiment Use soap solution to test whether the water samples are: a) Soft b) Temporary hard (boiling reduces the soap needed) or c) Permanent hard (uses a lot of soap to make a lather and this isn’t reduced by boiling).

Basic soap titration – Overview You can use soap solution to investigate hardness in water The more soap solution needed the ‘harder’ the water. Soft water requires the least amount of soap solution to produce a lather. Permanent hard water needs the highest volume of soap solution before and after boiling. Temporary hard water needs less soap to form a lather after it is boiled, compared to what it needed before.

Measuring hardness- Results table Water Sample A A boiled B B boiled C C boiled D D boiled Number of additions of soap/cm3

Measuring hardness- Results table Water Sample A A boiled B B boiled C C boiled D D boiled Number of additions of soap/cm3

Measuring hardness- Results table Water Sample A A boiled B B boiled C C boiled D D boiled Number of additions of soap/cm3 10 3 20 5 18 22

Measuring hardness- Type of water? Water Sample A A boiled B B boiled C C boiled D D boiled Number of additions of soap/cm3 10 3 20 5 18 22 Temporary hard Permanent hard Soft Permanent hard

Evaluation (i) which sample is soft water? . Referring to samples A, B, C and D and giving reasons: (i) which sample is soft water? C, because it took the least volume of soap to produce a lather. (ii) which sample is the hardest water? B & D, because both required the most soap solution to produce a lather. (iii) which sample is temporary hard water? A, because following boiling the volume of soap solution required to produce a lather decreased (iv) which samples are permanent hard water? B & D Because before and after boiling, they both required a lot of soap to produce a lather. 7. How did you make the experiment a fair test? [1] We used the same volume of water sample each time.

Page 14 evaluation 8. Underline the correct statement/s also related to the validity of this experiment.   (a) The results would be repeatable if we did the experiment carefully (b) The repeated results are not similar unless we re-do the experiment many times (c) We cannot tell if the results are repeatable unless we re-do the experiment many times (d) If another group had similar results to ours then we can say our results are reproducible. This in turn would contribute to our strength of evidence (validity)

Evaluation (vii). What other dependent variable could be used as a measure (instead of volume of soap added to create a lather)? Height of lather.

Lesson checkpoint The causes of hardness in water and distinguish between hard and soft waters by their action with soap. What ions make water hard? How are appliances affected by hard water? How is soap solution used to test for water hardness?