Removing hardness

 

Removing hardness

You have now seen how hard water is formed.

The calcium or magnesium in the water is present as charged particles called ions.

The most common cause of hardness is the calcium ion, Ca⁺(aq).

 

These calcium ions react with ions from soap (sodium stearate) to form scum

calcium ions (aq) + stearate ions (aq) hard water calcium stearate (s)

soap scum

If we can remove these Ca* ions from the water we will get rid of the hardness.

 

Temporary hardness

Do you remember how calcium ions from insoluble calcium carbonate get into hard water?

Limestone reacts with acidic rainwater. It forms a solution of calcium ions and hydrogencarbonate ions. This is called temporary hardness.

When this solution is boiled, the calcium and hydrogencarbonate ions react.

They turn back into the insoluble calcium carbonate.

Therefore calcium ions are removed from the water.

 

 

calcium ions + hydrogencarbonate ions

Ca2(aq) + 2HCO3 (aq)

calcium carbonate + carbon dioxide + CaCO3ls)

(limescale)

water

CO2lg) +H2O()

 

 

The calcium carbonate formed is the limescale you get inside kettles and hot-water pipes.

Temporarily hard water can be softened by boiling.

Do you think this is a cheap way to get rid of hardness?

Imagine your heating bills if you had to boil all your water before

you could use your washing machine.

However, not all hard water can be softened Other calcium by boiling. compounds, such as calcium sulfate from gypsum, are not removed by boiling.

These form permanently hard water.

 

Removing all types of hardness

We have seen how we can soften temporarily hard water by boiling. But permanent hardness can also be removed permanent means forever, but all types of hardness can be removed from water.

 

1. Washing soda (sodium carbonate)

When you add washing soda to hard water, the calcium ions are removed. They react with the carbonate ions from the washing soda.

This forms insoluble calcium carbonate:

calcium ions + carbonate ions

Ca(aq) + COg(aq)

(in hard water) (from washing

soda)

calcium carbonate

CaCO3ls)

precipitate

(insoluble)

 

This is called an ionic equation.

It only shows the ions which are affected in the reaction.

 

If a solid forms when two solutions are mixed, it is called a precipitation reaction.

 

Most modern washing powders have their own water softeners added.

 

2. Jon-exchange column

This method is more suitable for large-scale treatment of hard water.

Look at the diagram opposite:

The column is filled with a resin which holds plenty of sodium ions (Nat ions).

Hard water goes in at the top. On the way down, the calcium ions are swapped for sodium ions.

The calcium ions get stuck on the resin.

Sodium ions, which don't cause hardness, come out in the water at the bottom.

 

Experiment 22.3 Removing hardness

Look back to Experiment 22.2 on page 291

Use that method to test the hardness of your water samples before and after treatment

1. Test some temporarily hard water.

Now boil a fresh sample of the water and re-test it.

2. Test some permanently hard water.

Now adda spatula of washing soda to a fresh sample.

Make sure it dissolves, then re-test.

3Test your samples of the hard water after they have passed through an ion-exchange column.

 

 

 

Class Assessment

 

1.   The calcium or magnesium in the water is present as charged particles called --------------.

A.  atom

B.  ion

C. molecule  

 

2.   Calcium ions react with ions from soap (sodium stearate) to form -------.

A.  methane

B.  soap

C. scum

 

3.   Temporarily hard water can be softened by---------------------.

A. boiling

B.  filtering

C. Cooling    

 

4.   When you add -------------- to hard water, the calcium ions are removed.

A.  washing soda   

B.  sol

C. alcohol

 

D. calcium carbonate is not soluble in -----------------.

A. water  

B.  sol

C. alcohol

 

E.          A --------------------------- is an area from which water is drained.

A.  Evaporation 

B.  watershed

C. Precipitation

 

 

1.   Most modern washing powders have their own water softeners.

A. True

B.  False

2.   The column is filled with a resin which holds plenty of carbon ions.

A.  True

B. False

3.   A solid forms when two solutions are mixed, it is called a precipitation reaction.

A. True

B.  False

4.   The calcium ions get stuck on the resin.

A. True

B.  False

5.    A coating of scale (limescale) inside copper or lead pipes stops poisonous salts dissolving into our water.

A. True

B.  False