3A - WATER

Hard water makes scum and scale

- with soft water you get a lather with soap

- with hard water you get a scum with soap

- in hard water the dissolved calciuma and magnesium ions react with the soap to make scum (insoluble)

- when heated hard water forms a scale on the insides of appliances like kettles

- this reduces the efficiency of heating systems and they may need to be replaced.

- scale is also a bit of a thermal insulator

Hardness is caused by Ca   and Mg   ions

- rain falling on some types of rocks can dissolve compounds like magnesium sulfate and calcium sulfate which are soluble

Hard water isn't all bad

- Ca    ions are good for healthy teeth and bones also, people in areas of hard water are less likely to developing heart disease

Remove the dissolved Ca    and Mg   ions to make hard water soft

- there are two types of hardness - temporary and pernament

temporary hardness - caused by the hydrogencarbonate ion

pernament hardness - caused by dissolved calcium sulfate

1) temporary hardness is removed by boiling. The calcium hudrogencarbonate decomposes to form calcium carbonate which is insoluble. (limescale)

2) both types of hardness can be softened by adding washing soda (sodium carbonate)

3) both types of hardness can also be removed by running the water through an ion exchange column

- an ion exchange column has lots of sodium ions (or hydrogen ions) and exchange them for calcium or magnesium ions in the water that runs through them. 

You can use titration to compare the hardness of water samples

results from titration can tell you the follwowing things about the water

- distilled water contains little or no hardness

- whether the water is hard - local water tends to be hard

- imported water contains both temporary and pernament hardness

drinking water needs to be of good quality

- must be free from poisonous salts and harmful microbes

- most of our drinking water comes from resevoirs

water from resevoirs goes to the water treatment works for treatment

- the water passes through a mesh screen to remove twigs and lagre particles

- chemicals are added to make solids and microbes stick together and fall to the bottom

- the water is filtered through gravel beds to remove all solids

- water is chlorinated to kill off any harmful microbes left

Adding flouride and chlorine to water has disadvantages

- flouride is added to water in some parts of the country as it helps to reduce tooth decay

- chlorine is added to prevent disease

HOWEVER

- chlorine can react with other natural substances to produce toxic by-products

- in high doses flouride can cause cancer and bone problems

- some people believe that they should be able to choose what is being put into their water

- levels of chemicals added to drinking water needs to be carefully moderated so as not to cause in serious cases cancer.