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.
- 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.
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
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.
- Actual composition of CaOCl2 is 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.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
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:
- It is an ingredient in antacids. Being alkaline, it neutralises excess acid (acidity) in stomach.
- Used in soda-acid fire extinguishers.
CO2 causes bread or cake to rise making soft and spongy.
4. Washing soda (Na2CO3.10H2O)
- A basic salt obtained from NaCl.
- Recrystallisation of sodium carbonate → washing soda.
- 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.
NaCl → NaHCO3 → Na2CO3 (sodium carbonate)
Na2CO3 + 10H2O → Na2CO3.10H2O
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 (CuSO4.5H2O).
- Similarly, the molecule of Na2CO3.10H2O is wet.
- Gypsum is also a salt possessing water of crystallisation (CaSO4.2H2O).
- 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 (CaSO4.½H2O).
- Plaster of Paris on mixing with water, it changes to gypsum again giving a hard solid mass.
Plaster of Paris
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.