Salts, their pH, and chemicals from common salt
A salt forms when an acid and a base react; its pH depends on their strength. Common salt is the raw material for sodium hydroxide and bleaching powder.
Families of salts
| Family | Common ion | Examples |
|---|---|---|
| Sodium salts | Na⁺ | NaCl, Na₂SO₄, Na₂CO₃ |
| Chloride salts | Cl⁻ | NaCl, KCl, CaCl₂ |
| Sulphate salts | SO₄²⁻ | Na₂SO₄, K₂SO₄, CuSO₄ |
pH of salts
| Made from | Nature of the salt | Examples |
|---|---|---|
| Strong acid + strong base | Neutral, pH 7 | NaCl (from HCl and NaOH), Na₂SO₄ |
| Strong acid + weak base | Acidic, pH below 7 | NH₄Cl (from HCl and NH₄OH) |
| Weak acid + strong base | Basic, pH above 7 | Na₂CO₃, NaHCO₃, CH₃COONa |
Common salt
Sodium chloride is made by evaporating sea water in salt pans, as along the coasts of Gujarat and Tamil Nadu. Large crystals of rock salt, often brown because of impurities, are mined from beds left behind when ancient seas dried up. Common salt is the raw material for sodium hydroxide, bleaching powder, baking soda and washing soda.
The chlor-alkali process (making sodium hydroxide)
Electricity is passed through **brine**, a strong solution of sodium chloride in water. It is called chlor-alkali because it gives chlorine and an alkali.
- 1★2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
- 2Chlorine gas is given off at the anode (positive electrode).Uses: water treatment, swimming pools, PVC, disinfectants, pesticides
- 3Hydrogen gas is given off at the cathode (negative electrode).Uses: fuels, margarine (hydrogenation of oils), ammonia for fertilisers
- 4Sodium hydroxide solution forms near the cathode.Uses: de-greasing metals, soaps and detergents, paper making, artificial fibres
★ key step
Bleaching powder
- Made by the action of chlorine on dry slaked lime: 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O.
- It is written as Ca(ClO)₂, though its real composition is quite complex.
- Used for bleaching cotton and linen in the textile industry, wood pulp in paper factories, and washed clothes in laundries.
- Used as an oxidising agent in many chemical industries.
- Used to disinfect drinking water, making it free of germs.
Bleaching powder is added to village wells and water tanks to kill germs, especially in the rainy season. Much of India's common salt is made by drying sea water in salt pans along the coast.
1 marks: “Is a solution of sodium carbonate acidic, basic or neutral?” Basic, because it is the salt of a weak acid (carbonic acid) and a strong base (NaOH).
2 marks: “How is bleaching powder made? Give two uses.” By the action of chlorine on dry slaked lime: 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O. Uses: disinfecting drinking water; bleaching cotton, linen and wood pulp.
3 marks: “What is the chlor-alkali process? Write the equation and one use of each product.” Electrolysis of brine. 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. NaOH: soaps; Cl₂: water treatment, PVC; H₂: fuel, margarine.
Chlor-alkali gives three: Cl₂ at the Anode, H₂ at the Cathode, NaOH near the Cathode. For salts: strong + strong = neutral, otherwise the stronger parent wins.