Do you live in an area with hard water? If you do, you will be used to the scum that forms when you use soap in the water.
In this unit you can find out what hard water is, how it forms, its effects, and how we can remove hardness from water.
The effects of hard water and how it forms
There are advantages and disadvantages for people who live in hard water areas. Look at the table below:
|Disadvantages of hard water
||Advantages of hard water
|It is more difficult to form a lather with soap.
||Some people prefer the taste.
|Scum may form in a reaction with soap, wasting the soap.
||Calcium ions in the water are good for children's teeth and bones.
|Limescale (a hard crust) forms inside kettles. This wastes energy whenever you boil a kettle.
||It helps to reduce heart disease.
|Hot water pipes 'fur up'. Limescale starts to coat the inside of the pipes which can eventually get blocked up.
||Some brewers prefer using hard water for making beer.
||A coating of limescale inside copper pipes, or especially old lead pipes, stops poisonous salts dissolving into water.
|Figure 1. This pipe has been partially blocked by limescale.
People whose water comes from places with lots of limestone or chalk have hard water. Remember that limestone and chalk are
both made up mainly from calcium carbonate. So perhaps hard water is produced when calcium carbonate dissolves in the water.
In fact, calcium carbonate is almost insoluble in pure water but it does react with
An acid is a substance that forms a solution with a pH value of less than 7. Acidic solutions contain an excess of hydrogen
. Even the cleanest rainwater is slightly acidic because some carbon dioxide gas dissolves in it as it falls through the air.
This forms a weakly acidic solution:
carbon dioxide + water
(g) + H2
The weak carbonic acid slowly reacts with and dissolves away the calcium carbonate in the rock it passes over.
calcium carbonate + carbonic acid
(s) + H2
Notice that the calcium hydrogencarbonate formed in the reaction is soluble in water. It dissolves and its calcium ions,
(aq), get into the water. These calcium ions are what cause the hardness in water.
(aq) + HCO3−
Sometimes water flows over gypsum rock (calcium sulfate) which does dissolve slightly in water, making hard water directly.
You can add different metal ions in solution to water to see which ones make the water hard. You simply shake the solution
of metal ions with some water which has a little soap solution added. If the water is hard, scum will form in the water and
it is difficult to get a lather. Try this below:
If we remove calcium (or magnesium) ions from hard water, the water is said to be softened. It will then form a lather with
soap more easily and no scum will be formed. In the experiment in Fig.3 above, we saw how calcium reacts with soap solution:
If we add enough soap, all the calcium ions are removed from solution as a
A precipitate is an insoluble solid formed when two solutions react together.
of scum, and the water becomes soft. However, this would waste soap. Also, if you are washing clothes, the scum would stick
to them. So we need other ways to take the calcium ions out of solution. Here are some methods we use: 1. Add washing soda (sodium carbonate)
Washing soda is added to hard water before washing clothes in it. The calcium ions are removed in a precipitation reaction:
An ionic equation is one which shows the atoms and ions that actually change in a reaction, e.g.
Zn(s) + Cu2+
(aq) + Cu(s).
for the reaction is:
(aq) + CO32−
(s) 2. Ion-exchange column
We can also remove the calcium ions from hard water by passing it through an ion-exchange column. The column contains a special
resin. As the water passes through the resin, calcium ions replace sodium ions in the resin and the water is softened. You
can see this happening in Fig.4 below.
After a column has been used for a while, you can recharge the resin with sodium ions by passing sodium chloride (common
salt) solution through it.
Temporary harness is caused by calcium hydrogencarbonate dissolved in the water. We saw how this forms in the first section
in this unit.
We can remove temporary hardness by boiling the water. This is the reaction that causes limescale to form inside a kettle.
When we heat the water, the calcium hydrogencarbonate breaks down:
(s) (limescale) + CO2
(g) + H2
Removing hardness in this way only works for temporarily hard water and also wastes energy, so the method is not generally
used. Permanently hard water (e.g. water that has passed over gypsum) is softened using the methods in the previous section
(washing soda and ion-exchange resins). These methods will also remove the calcium ions from temporary hard water.
Hard water contains calcium ions, Ca2+(aq), or magnesium ions, Mg2+(aq).
These ions react with soap, making it difficult to form a lather and producing scum.
Hard water can be softened by adding washing soda (sodium carbonate) which removes the calcium ions in a precipitation reaction.
Alternatively, the hard water can be passed through an ion-exchange resin in a column. Sodium ions replace the calcium ions
in the water as it passes through the column.
Temporary hardness can be removed by boiling the water. The calcium hydrogencarbonate dissolved in the water breaks down
when heated and forms limescale.
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