C3.2 Water


Hard water

Hard water contains Ca 2+ or Mg 2+ ions and it is difficult to form a lather with. The use of hard water can result in the formation of limescale and more soap is needed to form a lather.

Carbon dioxide dissolves in water to produce a weak acid called carbonic acid H2CO3;

CO2 + H2O => H2CO3

Water containing carbonic acid reacts with calcium carbonate (chalk, limestone, marble) to form calcium hydrogen carbonate;

Calcium carbonate + water + carbon dioxide => calcium hydrogen carbonate

Ca(HCO3) is soluble in water and causes temporary hardness. The water can get to chalk, limestone and marble by traveling through the soil and layers to get to the rocks to form this chemical reaction

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Hard water wastes soap as the magnesium and calcium ions in the water will react. This reaction reduces the effectiveness of soap and produces 'scum' a precipitate. Only when all the magnesium and calium ion have reacted with the soap the soap can lather;

sodium stearate (soap) + Ca 2+ ions => Calcium stearate (scum) + Na + ions

Soapless deterrgents however do not form scum;

Both soap and detergents can cause environmental damage and contain long molecules. Both products are made from oils, however soapless detergents do not form scum after use with hard water. Moreover, soaps are made from animal fats and vegetable oils reacted with sodium hydroxide. Soap is biodegradable and soapless detergents are not.

Detergents contain long and thin molecules, one end is hydrophilic and the other hydrophobic. The side that does not like water (hydrophobic) sticks to the side that has the oily dirty surface. The water loving molecules however (hydrophilic) covers the dirt so it can easily be washed away.

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Softening hard water

Temporary hardness is caused by the presence of calcium hydrogen carbonate and can be easily removed by boiling the water. The hydrogen carbonate ions (HCO3) undergo thermal decomposition forming carbonate ions which react with calcium or magnesium to form precipitates.

2HCO3 => CO3 + CO2 + H2O

Permenant hard water is caused by the presence of calcium and magnesium ions in water. Permenant hardness is more difficult to remove than temporary hardness. There are 2 methods that can be used to remove permenant hardness from water ion exchange resin and adding sodium carbonate.

Ion exchange resin exchanges the Ca and Mg ions with Na or H ions as there are charged resin beads which attract the Ca and Mg ions.

Washing soda, also known as sodium carbonate, reacts with calcium sulfate and magnesium sulfate in the water. A precipitate of calcium carbonate forms and this removes the calcium and magnesium ions from the solution

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Advantages and disadvantages of hard water

Using hard water can increase costs due to the use of soap, when temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles. The calcium compounds in hard water are good for the development of teeth and bones and this can help to reduce heart disease.

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Purifying water

When water is taken from the river, it is often cloudy and dirty. During sedimentation any larger pieces of solid sink to the bottom. When the water is filtered, small pieces of solid are trapped on the filter. In some cases this process can also remove microbes, such as bacteria too. Chlorine is then added to kill any harmful microbes that might still be in the water. Sometimes flouride is also added. Many scientists think flouride helps to keep our teeth healthy. Water then flows down pipes that lead into taps in our homes.

People sometimes use further water purification methods in their home, an example of this is a filter jug which passes tap water through a filter cartridge. This contains carbon and ion exchange resin and in some cases silver. These remove dissolved substances from tap water and improve the taste and quality.

Pure water can be obtained by distillation, this is when water evaporates and condenses in a different tube and the other substances will be left behind. However, this is not done on a vast scale because it is expensive.

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