What is a salt? Families of salts
A salt is the compound formed along with water when an acid neutralises a base. Its positive ion (cation) comes from the base and its negative ion (anion) from the acid. NaOH + HCl → NaCl + H₂O.
Salts with the same positive ion or the same negative ion belong to one family. NaCl and Na₂SO₄ are in the sodium family; NaCl and KCl are in the chloride family.
pH of salts
- Strong acid + strong base → neutral salt, pH 7 (NaCl, Na₂SO₄, KNO₃).
- Strong acid + weak base → acidic salt, pH below 7 (NH₄Cl).
- Weak acid + strong base → basic salt, pH above 7 (Na₂CO₃, CH₃COONa, NaHCO₃).
So the parents decide the nature of the salt: the stronger parent wins.
Common salt and rock salt
Common salt is got from sea water by evaporation. Big deposits of solid salt found underground are called rock salt; they are brown because of impurities and are mined like coal.
Sodium hydroxide: the chlor-alkali process
When electricity passes through brine (a strong solution of NaCl in water), it decomposes:
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
Chlorine forms at the anode, hydrogen at the cathode and sodium hydroxide near the cathode. The name comes from the products: chlor (chlorine) + alkali (NaOH).
- NaOH: soaps and detergents, paper making, artificial fibres, degreasing metals.
- Cl₂: water treatment, PVC, disinfectants, pesticides, bleaching powder.
- H₂: fuels, margarine (hydrogenation of oils), ammonia for fertilisers.
- H₂ + Cl₂ → HCl: cleaning steel, medicines, cosmetics.
Bleaching powder, baking soda and washing soda
Bleaching powder, CaOCl₂
Made by passing chlorine over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. (Its exact make-up is complex; CaOCl₂ is the simple formula used.) Uses: bleaching cotton and linen in textile mills, wood pulp in paper factories and washed clothes at laundries; oxidising agent in industry; disinfecting drinking water.
Baking soda, sodium hydrogencarbonate NaHCO₃
Made from NaCl, water, CO₂ and ammonia: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. It is a mild, non-corrosive base. On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ makes cakes and breads rise.
Baking powder = baking soda + a mild edible acid (such as tartaric acid). With water, the acid reacts with NaHCO₃ to give CO₂, and the leftover taste is not bitter. Other uses: antacids, soda-acid fire extinguishers.
Washing soda, Na₂CO₃·10H₂O
Heat baking soda to get sodium carbonate, then recrystallise it from water: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. Uses: making glass, soap and paper; making borax; household cleaning; removing permanent hardness of water.
Water of crystallisation and plaster of Paris
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt crystal. The crystals feel dry, yet the water is there.
- Copper sulphate: CuSO₄·5H₂O (blue). Heating removes the water and leaves white CuSO₄; adding water turns it blue again.
- Washing soda: Na₂CO₃·10H₂O.
- Gypsum: CaSO₄·2H₂O.
Plaster of Paris, CaSO₄·½H₂O
Heat gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Two formula units of CaSO₄ share one water molecule, so we write ½H₂O. Mix plaster of Paris with water and it sets into a hard mass of gypsum again: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
It must be stored in a moisture-proof container, or it sets hard. Heating above 373 K removes all the water and gives dead burnt plaster, which does not set. Uses: plaster for broken bones, toys, decoration, smooth wall surfaces, statues.
Key formulas and definitions
- Acid + base → salt + water
- Chlor-alkali: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
- Bleaching powder: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
- Baking soda: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃; on heating 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
- Washing soda: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
- Plaster of Paris: CaSO₄·2H₂O (373 K) → CaSO₄·½H₂O + 1½H₂O
- Setting: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
- Blue vitriol: CuSO₄·5H₂O (blue) → heat → CuSO₄ (white) + 5H₂O
Worked examples
1. Predict whether a solution of sodium acetate (CH₃COONa) is acidic, basic or neutral.
It comes from acetic acid (weak) and NaOH (strong). The strong base wins, so the solution is basic (pH above 7).
2. In electrolysis of brine, name the gas at each electrode and one use of each.
Anode: chlorine (used for water treatment, PVC). Cathode: hydrogen (fuel, making ammonia). NaOH forms near the cathode (soaps, paper).
3. Why does a cake made with only baking soda taste bitter, and how does baking powder fix this?
Heating baking soda leaves sodium carbonate, which is bitter. Baking powder has an edible acid (tartaric acid) that reacts with the NaHCO₃, so CO₂ still forms but no bitter sodium carbonate remains.
4. What happens when blue copper sulphate crystals are heated in a dry test tube?
Water drops appear on the cooler part of the tube and the crystals turn white: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Adding a few drops of water brings back the blue colour.
5. Why must plaster of Paris be kept in a moisture-proof container?
It reacts with water (even moisture in air) and sets into hard gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Then it is useless for making casts.
6. A doctor uses a white powder X that sets hard when mixed with water to support a fractured bone. X is made by heating Y at 373 K. Identify X and Y.
X is plaster of Paris, CaSO₄·½H₂O. Y is gypsum, CaSO₄·2H₂O.
Common mistakes
- Writing the formula of washing soda as NaHCO₃. Baking soda is NaHCO₃; washing soda is Na₂CO₃·10H₂O.
- Saying plaster of Paris sets by drying. It sets by taking in water and becoming gypsum.
- Thinking every salt is neutral. NH₄Cl is acidic and Na₂CO₃ is basic.
- Writing plaster of Paris as CaSO₄·H₂O. It has half a water molecule per CaSO₄: CaSO₄·½H₂O.