Common salt and the substances made from it

Common salt: a raw material

Common salt, sodium chloride, is much more than a food. It is the starting point for several important substances.

The chlor-alkali process

When electricity passes through a solution of sodium chloride (brine), three products are made. The name chlor-alkali comes from chlorine and the alkali, sodium hydroxide.

• Chlorine gas is given out at the anode (positive).
• Hydrogen gas is given out at the cathode (negative).
• Sodium hydroxide is made near the cathode.

Uses of the products:

• Hydrogen: fuel, margarine, ammonia for fertilisers.
• Chlorine: water treatment, bleach, plastics such as PVC, disinfectants.
• Sodium hydroxide: soaps and detergents, paper, degreasing metals.

You should find that sodium hydroxide is the heaviest product, and hydrogen is less than a fiftieth of the salt's mass.

Bleaching powder

Chlorine reacts with dry slaked lime to make bleaching powder.

Uses: bleaching cotton and linen in textile mills, bleaching wood pulp in paper factories, disinfecting drinking water, and as an oxidising agent in industry.

Baking soda

Baking soda is sodium hydrogencarbonate, $\text{NaHCO}_3$. It is made from common salt.

It is a mild, non-corrosive base.

• On heating, it gives sodium carbonate, water and carbon dioxide: $2\,\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$.
• Baking powder is baking soda with a mild edible acid such as tartaric acid. When water is added, the two react and give out carbon dioxide, and the bubbles make the bread or the cake rise and become soft and spongy.
• It is an antacid, which neutralises extra acid in the stomach.
• It is used in some fire extinguishers (soda-acid type).

Washing soda

When sodium carbonate is crystallised from a solution, it takes up water.

This is washing soda. Its solution is basic.

• It is used in the glass, soap and paper industries.
• It is used to soften hard water.
• It is used to clean clothes and other things at home and in industry.

Practise with new numbers

Try it in Python

This program finds the masses of the products from $175.5\ \text{g}$ of salt.

Remember these

• Brine and electricity give NaOH, $\text{Cl}_2$ and $\text{H}_2$ (chlor-alkali).
• Chlorine + slaked lime gives bleaching powder. Baking soda is $\text{NaHCO}_3$. Washing soda is $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$.
• Baking powder makes a cake rise because it gives out carbon dioxide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name one use of the hydrogen from the chlor-alkali process.

2. What is baking powder made of?

3. Why is baking soda used as an antacid?

4. Give two uses of bleaching powder.