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Hardness of Water

Hardness of Water. L.O. I know how to tell if water is hard, what causes it to be hard and how it can be removed. I know the difference between temporary and permanent hard water. What is hard water?. Using hard and soft water. Soft water forms a lather easily with soap.

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Hardness of Water

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  1. Hardness of Water L.O. I know how to tell if water is hard, what causes it to be hard and how it can be removed. I know the difference between temporary and permanent hard water

  2. What is hard water?

  3. Using hard and soft water Soft water forms a lather easily with soap. Hard water needs more soap to form a lather, adding additional cost to cleaning processes. This is because dissolved chemicals in the hard water react with soap to form a scum. How could you test water samples to see if they are hard or soft?

  4. What is hard water? Hard water contains dissolved calcium and magnesium compounds such as: • magnesium hydrogen carbonate [Mg(HCO3)2] • calcium hydrogen carbonate [Ca(HCO3)2] • magnesium sulfate [MgSO4] • calcium sulfate [CaSO4] Hard water can be temporary or permanent. Temporary hard water contains magnesium hydrogen carbonate and calcium hydrogen carbonate. Permanent hard water contains magnesium sulfate and calcium sulfate.

  5. Hard water chemicals

  6. Hard water and soap Rainwater becomes slightly acidic as carbon dioxide from the air dissolves in it, forming carbonic acid. Limestone contains calcium carbonate. As the slightly acidic rainwater trickles through rocks, the calcium carbonate reacts to form soluble calcium hydrogen carbonate: carbonic acid calcium carbonate carbon dioxide calcium hydrogen carbonate +  + + water +  + + H2CO3(aq) CaCO3(s) Ca(HCO3)2(aq) CO2(g) H2O(l) One of the chemicals in soap is sodium stearate. The dissolved calcium and magnesium compounds in hard water react with sodium stearate to form a solid called calcium stearate (or ‘scum’).

  7. How does hard water react with soap? Soap ‘scum’ is formed by the reaction: calcium hydrogen carbonate sodium hydrogen carbonate sodium stearate calcium stearate +  + 2C17H35COONa(s) + Ca(HCO3)2(aq)  (C17H35COO)2Ca(s) + 2NaHCO3(aq) The soap will only form a lather when all the dissolved calcium hydrogen carbonate in the water has reacted. Using hard water can cause problems: • More soap is needed to get a lather. • It can be difficult to clean the scum from bathtubs and sinks. • Hard water can be unsuitable for industrial processes like dying.

  8. Limescale When hard water is heated, the dissolved calcium hydrogen carbonate decomposes to form solid calcium carbonate. These deposits of calcium carbonate are called limescale. calcium hydrogen carbonate calcium carbonate  water + carbon dioxide +  Ca(HCO3)2(aq) CaCO3 (s) H2O(l) + CO2(g) + Limescale can block pipes and coat the heating elements in kettles, washing machines and heaters. Limescale is a poor heat conductor, and reduces the efficiency of appliances.

  9. Removing limescale Weak acids, such as ethanoic acid, can be used as descalers. The acid reacts with the limescale to form soluble compounds, which are then washed away. ethanoic acid calcium carbonate carbon dioxide calcium ethanoate +  + + water +  + + 2CH3COOH(aq) CaCO3(s) (CH3COO)2Ca(aq) CO2(g) H2O(l) What would a person observing this reaction see?

  10. What are the benefits of hard water? • Hard water can be good for health: • calcium is needed for healthy bones and teeth • magnesium is needed for effective metabolism. Some studies have also shown that people living in hard water areas are less likely to suffer from heart disease. The World Health Organisation states that there is not yet enough evidence to confirm a link between hard water and heart disease. What evidence would you gather to look for a link between hard water and heart disease?

  11. Softening temporary hard water Temporary hard water contains calcium hydrogen carbonate. This is relatively easy to remove, because it decomposes on heating to form solid calcium carbonate: calcium hydrogen carbonate calcium carbonate  water + carbon dioxide +  Ca(HCO3)2(aq) CaCO3 (s) H2O(l) + CO2(g) + Knowing this, how would you remove the solid calcium carbonate from the water? What would be left behind after removing temporary hardness using this method?

  12. Softening hard water Both temporary and permanent hard water can be softened by adding sodium carbonate (washing soda). The sodium carbonate reacts with the calcium compounds in the water to form calcium carbonate and soluble sodium compounds, which do not contribute to hardness: calcium hydrogen carbonate sodium hydrogen carbonate sodium carbonate calcium carbonate +  + +  + Ca(HCO3)2(aq) Na2CO3(s) CaCO3(s) 2NaHCO3(aq) What is the word and symbol equation for the reaction between calcium sulfate (which causes permanent hardness) and sodium carbonate?

  13. Softening hard water

  14. Hard water: summary

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