HideShow resource information
  • Created by: chloefaun
  • Created on: 15-04-14 18:18

Hard and soft water

Ca  and Mg  ions make water hard. These will be present in water becasue of the rocks it passes through to reach your area and the minerals dissolve into the water.

Hard water is difficult to get a good lather, it can create a nasty scum instead. This is insoluable and occurs when the Ca or Mg ions form stearates and these create scum. This is a short term problem. Scale is also created, this is calcium hydrogencrbonate, it collects in pipes and decreses efficency of appliances. This is a long term problem.

1 of 9

Advantages and disadvantages of hard water

  • Healthy teeth and bones
  • Less risk of developing heart disease (minerals)
  • Can improve the taste of water
  • More soap needed to produce lather (more expensive in the long run)
  • Scum is unsightly
  • Reduce appliance efficency
2 of 9

Temporary and permanent hard water

Temporary hard water can be softened by boiling it. Permanent hard water stays hard, even when it is boiled.

Temporary hard water contains dissolved hydrogen carbonate ions, HCO3–. When heated, these ions decompose (break down) to form carbonate ions, CO32–. The carbonate ions in the boiled water react with dissolved calcium and magnesium ions to form insoluable precipitates (calcium carbonate and magnesium carbonate).

Permanent hard water contains dissolved sulfate ions, SO42–. These do not decompose when heated. They remain dissolved and do not react with calcium and magnesium ions - so the water stays hard even when boiled.

3 of 9

Softening hard water

Adding sodium carbonate

Sodium carbonate, Na2CO3, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.

The carbonate ions from sodium carbonate react with the calcium and magnesium ions in the water to produce insoluable precipitates. For example:

calcium ions + sodium carbonate → calcium carbonate + sodium ions

Ca2+(aq) + Na2CO3(aq) → CaCO3(s) + 2Na+(aq)

The water is softened because it no longer contains dissolved calcium ions and magnesium ions. It will form lather more easily with soap. However is can only be used in certain circumstances.

4 of 9

Softening hard water

Ion exchange columns

Commercial water softeners often use ion exchange resins. These substances are usually made into beads, which are packed into cylinders called ion exchange columns. These can be built into machines, such as dishwashers, or plumbed into water systems to continuously soften the water.

The resin beads have sodium ions attached to them. As the hard water passes through the column, the calcium and magnesium ions swap places with the sodium ions.

The calcium and magnesium ions are left attached to the beads, while the water leaving the column contains more sodium ions. The hard water is softened because it no longer contains calcium or magnesium ions. Some ion exchange resins use hydrogen ions instead of sodium ions.

5 of 9

Purifying hard water

Sources of water for drinking should be reliable, and they should also be fresh and free of toxic chemicals (such as heavy metals). In the UK, water resources includelakes, rivers, aquifers and reservoirs.

An aquifer is an underground layer of permeable rock, gravel or sand that is soaked with water, while a reservoir is usually an artificial lake, made by building a dam to accumulate and save river water in the valley behind.

In countries where water is scarce, boreholes are drilled to reach water underground.Solids in the water, such as leaves and soil, must be removed. The water is sprayed onto specially-prepared layers of sand and gravel called filter beds.Different-sized insoluble solids are removed as the water trickles through the filter beds. These are cleaned every so often by pumping clean water backwards through the filter.The water is then passed into a sedimentation tank. Aluminium sulfate is added to clump tiny particles together to make larger particles, which settle out more easily. The water is then passed through a fine filter, such as carbon granules, to remove very small particles.

6 of 9

Purifying hard water

Chlorine is added to drinking water to sterilise it. The chlorine kills microbes- including microbes that cause potentially-fatal diseases such as typhoid, cholera and dysentery.

Adding flurine to the water supply happens in some areas but is contraversal. Some people argue that extra fluoride should not be added to water, even if it does improve dental health. They claim that fluoridation:

  • has been linked to tooth mottling (staining), bone disease and pain

  • forces people to consume fluoride when they drink tap water - taking away their personal choice (making it unethical)

7 of 9


Water treatment in the UK means that the water from your tap is safe to drink. However, the water is not pure because it contains dissolved mineral ions and chlorine.

Some people prefer to filter their water rather than use it straight from the tap. Filtering removes impurities and this can improve the taste and quality of the water. Filtering also helps to soften the water.

The filter cartridges can contain:

  • silver to kill bacteria

  • carbon (‘activated charcoal’) to absorb impurities, eg chlorine

  • ion exchange resins to soften the water, and remove heavy metal ions (such as lead ions)

Silver nanoparticles have an antibacterial effect. Their presence in the filter prevents the growth of bacteria within the filter if water is left inside it for long periods. Silver nanoparticles also help break down harmful pesticides which might be in the water.

8 of 9

Testing for pure water

You can test if water is pure by boiling it, pure water boils at 100 degrees, or evaporating it as pure water leaves no solids behind.

9 of 9


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Water hardness and solubility resources »