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Water

What you need to know

Reflections and Exam tips

 

Water

Water is the most abundant substance on the Earth’s surface and moves in a continuous cycle (water cycle).

The importance of water

  1. Water is essential to life on Earth
  2. Water is used widely as a coolant in industrial processes
  3. 75% of the world’s surface is covered by water
  4. Water is used widely as a solvent in industrial processes

Drinking water

Drinking water is treated by passing water through filter beds to remove solid particles and then adding small amounts of chlorine to kill bacteria.

Domestic water filters contain carbon, silver and ion exchange resins to remove dissolved solids and improve taste and quality.

Some water authorities add other chemicals to drinking water e.g. many add fluoride ions which prevent tooth decay at controlled levels.

Hard water

Hardness in water is caused by dissolved calcium and/or magnesium ions which enter the water as it passes over or percolates through rocks containing soluble compounds of calcium or magnesium.

Soft water (which does not contain these ions) forms a lather with soap.

Hard water does not form a lather with soap but forms scum which is an insoluble salt of calcium with the anions in soap (stearate).

Types of hardness

Temporary hardness

Permanent hardness

Reason: Presence of calcium or magnesium carbonate in the water

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(HCO2)2 ——> Ca CO3  +  CO2  + H2O

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

Advantages of hard water

Hard water helps prevent heart disease, provides calcium for healthy bones and teeth, is necessary for
brewing and tastes better than soft water.

Disadvantages of hard water

Wastes soap (due to scum formation) and forms scale in boilers, pipes and kettles leading to increased costs (reduced efficiency).