ACIDS, BASES AND SALTS

MORE ABOUT SALTS

Family of Salts

Salts having same positive or negative radicals belong to a family. E.g. NaCl & Na₂SO₄ belong to family of sodium salts. NaCl & KCl belong to the family of chloride salts.

Salts & their Chemical formulae	Formed from which Acids & bases?
Potassium sulphate (K₂SO₄)	H₂SO₄& KOH
Sodium sulphate (Na₂SO₄)	H₂SO₄& NaOH
Calcium sulphate (CaSO₄)	H₂SO₄ & CaCO₃
Magnesium sulphate (MgSO₄)	H₂SO₄ & Mg(OH)₂
Copper sulphate (CuSO₄)	H₂SO₄ & Cu(OH)₂
Sodium chloride (NaCl)	HCl & NaOH
Sodium nitrate (NaNO₃)	HNO₃ & NaOH
Sodium carbonate (Na₂CO₃)	H₂CO₃ & NaOH
Ammonium chloride (NH₄Cl)	HCl + NH₄OH

pH of Salts

Salts of a strong acid & a strong base are neutral (pH = 7).

Salts of a strong acid and weak base are acidic (pH < 7).

Salts of a strong base and weak acid are basic (pH > 7).

pH of some salt samples (soluble in distilled water) and the acid & base used to form the salts are given below:

Salt	pH	Acid used	Base used
Sodium chloride	7	HCl	NaOH
Potassium nitrate	7	HNO₃	KOH
Aluminium chloride	7	HCl	Al(OH)₃
Zinc sulphate	7	H₂SO₄	Zn(OH)₂
Copper sulphate	< 7	H₂SO₄	Cu(OH)₂
Sodium acetate	> 7	CH₃COOH	NaOH
Sodium carbonate	> 7	H₂CO₃	NaOH
Sodium hydrogen carbonate	> 7	H₂CO₃	NaOH

Chemicals from Common Salt

Common salt (sodium chloride, NaCl) is a neutral salt formed by the reaction of HCl & NaOH solution.

NaCl is separated from seawater containing many salts.

Deposits of large crystals of solid salt are also found in several parts. These are often brown due to impurities. This is called rock salt.

There are beds of rock salt formed when seas of bygone ages dried up. Rock salt is mined like coal.

Common salt is used in food. It is also a raw material to make sodium hydroxide, baking soda, washing soda, bleaching powder etc.

1. Sodium hydroxide (NaOH)

When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide. It is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Chlorine gas is given off at the anode, and hydrogen gas at the cathode. NaOH solution is formed near the cathode.

All three products are useful.

Uses of NaOH: De-greasing metals, soaps & detergents, paper making, artificial fibres etc.

Uses of Cl₂: For water treatment, in swimming pools, PVC, disinfectants, CFCs, pesticides etc.

Uses of H₂: Fuels, margarine, ammonia for fertilisers.

Important products from the chlor-alkali process

2. Bleaching powder (CaOCl₂)

It is manufactured by using chlorine produced during the electrolysis of brine.

Chlorine acts on dry slaked lime to give bleaching powder.

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Actual composition of CaOCl₂is quite complex.

Uses of Bleaching powder:

For bleaching cotton & linen in the textile industry.
For bleaching wood pulp in paper factories.
For bleaching washed clothes in laundry.
As an oxidising agent in many chemical industries.
To make drinking water free from germs.

3. Baking soda

Chemical name: Sodium hydrogencarbonate (NaHCO₃).

It is produced using sodium chloride.

It is a mild non-corrosive basic salt.

When it is heated for cooking, the following reaction occurs.

Uses of Baking soda:

To make tasty crispy pakoras, etc.
It is added for faster cooking.
To make baking powder (baking soda + mild edible acid such as tartaric acid). When baking powder is heated or mixed in water, the following reaction occurs:

NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of acid

(From any acid)

CO₂ causes bread or cake to rise making soft and spongy.

It is an ingredient in antacids. Being alkaline, it neutralises excess acid (acidity) in stomach.
Used in soda-acid fire extinguishers.

4. Washing soda (Na₂CO₃.10H₂O)

A basic salt obtained from NaCl.

NaCl → NaHCO₃→ Na₂CO₃ (sodium carbonate).

Recrystallisation of sodium carbonate → washing soda.

Na₂CO₃ + 10H₂O→ Na₂CO₃.10H₂O

Uses of washing soda:

Used in glass, soap and paper industries.
To manufacture sodium compounds such as borax.
Used as a cleaning agent for domestic purposes.
For removing permanent hardness of water.

Are the Crystals of Salts really Dry?

Heat few copper sulphate crystals in a dry boiling tube.

Copper sulphate becomes white and some water droplets appear in the boiling tube.

When 2-3 drops of water are added, copper sulphate crystals restore its blue colour.

Copper sulphate crystals contain water of crystallisation. When heated the crystals, this water is removed and the salt turns white.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.

Five water molecules are present in one formula unit of hydrated copper sulphate (Cu SO₄. 5H₂O).

Similarly, the molecule of Na₂CO₃.10H₂O is wet.

Gypsum is also a salt possessing water of crystallisation (CaSO₄.2H₂O).

Plaster of Paris

On heating gypsum at 373 K, it loses water molecules and becomes a white powder. This is called Plaster of Paris or Calcium sulphate hemihydrate (CaSO₄. ½ H₂O).

Plaster of Paris on mixing with water, it changes to gypsum again giving a hard solid mass.

CaSO₄. ½ H₂O +1½ H₂O→ CaSO₄.2H₂O

(Plaster of Paris) (Gypsum)

Only half a water molecule is shown to be attached as water of crystallisation. It is written in this form because two formula units of CaSO₄ share one molecule of water.

Uses of Plaster of Paris:

Used as plaster to support fractured bones.
To make toys and materials for decoration.
To make surfaces smooth.

Plaster of Paris gets its name from large gypsum deposits in Montmartre in Paris.

Acids, Bases and Salts | Class 10 CBSE | Web Notes | Part 4 | More about Salts

ACIDS, BASES AND SALTS

Family of Salts

pH of Salts

Chemicals from Common Salt

Are the Crystals of Salts really Dry?

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