Acids, Bases and Salts • Topic 3 of 3

Salts — Preparation, Properties & Uses

A salt is the ionic compound formed when the hydrogen of an acid is replaced by a metal (or ammonium) ion during neutralisation. Salts that share a common acid or a common base belong to the same family: NaCl, KCl and CaCl2 are all chlorides; Na2SO4 and CuSO4 are sulphates; Na2CO3 and CaCO3 are carbonates.

The pH of a salt

  • A salt of a strong acid and a strong base (e.g. NaCl) is neutral, pH 7.
  • A salt of a strong acid and a weak base (e.g. NH4Cl) is acidic, pH less than 7.
  • A salt of a weak acid and a strong base (e.g. Na2CO3) is basic, pH greater than 7.

Common salt and the chlor-alkali process

Common salt, sodium chloride (NaCl), is the raw material for many chemicals. Passing electricity through concentrated brine (the chlor-alkali process) gives three useful products: 2NaCl + 2H2O → 2NaOH + Cl2 + H2. The names come from chlor (chlorine) and alkali (sodium hydroxide). NaOH at the cathode, chlorine at the anode and hydrogen are each valuable.

Important salts and their uses

  • Bleaching powder, CaOCl2: made by passing chlorine over dry slaked lime, Ca(OH)2 + Cl2 → CaOCl2 + H2O. Used to bleach cotton and paper and to disinfect drinking water.
  • Baking soda, NaHCO3: a mild, non-corrosive base made from brine: NaCl + H2O + CO2 + NH3 → NaHCO3 + NH4Cl. On heating it gives CO2, which makes cakes rise; it is also used in antacids and fire extinguishers.
  • Washing soda, Na2CO3·10H2O: made by heating baking soda to Na2CO3 and recrystallising. Used in glass, soap and paper industries and to remove the permanent hardness of water.
  • Plaster of Paris (POP), CaSO4·½H2O: made by heating gypsum (CaSO4·2H2O) to about 100°C. Mixed with water it sets into a hard solid; used for plaster casts, moulds and decorative work.

Water of crystallisation

Many salts hold a fixed number of water molecules in their crystals, called the water of crystallisation. Copper sulphate crystals are CuSO4·5H2O (blue); on heating they lose water and turn white, and adding water restores the blue colour. This water explains the formulae of washing soda and gypsum above.

From common salt (NaCl): the chlor-alkali process yields NaOH, chlorine and hydrogen, leading to other saltsCommon saltNaCl + brineelectrolysisNaOH (alkali)Cl₂ (chlorine)H₂ (hydrogen)Washing/baking soda(from NaCl + CO₂)Bleaching powder(Cl₂ + slaked lime)
Solved Examples
1
Worked Example
Predict whether the salt sodium chloride (NaCl) is acidic, basic or neutral, and explain.
Solution
  1. NaCl comes from HCl (strong acid) and NaOH (strong base).
  2. A salt of a strong acid and a strong base does not hydrolyse.
  3. So the solution stays neutral.

Answer: NaCl is a neutral salt with pH 7.

2
Worked Example
Why is an aqueous solution of sodium carbonate (Na2CO3) basic?
Solution
  1. Na2CO3 is the salt of a weak acid (carbonic acid) and a strong base (NaOH).
  2. On dissolving, the carbonate ion reacts with water to release OH− ions.
  3. The extra OH− makes the solution basic.

Answer: It is a salt of a weak acid and strong base, so it gives a basic solution (pH greater than 7).

3
Worked Example
Write the chemical equation for the chlor-alkali process and name the three products.
Solution
  1. Electricity is passed through concentrated brine (NaCl solution).
  2. The equation is 2NaCl + 2H2O → 2NaOH + Cl2 + H2.
  3. NaOH forms at the cathode, Cl2 at the anode and H2 is also released.

Answer: 2NaCl + 2H2O → 2NaOH + Cl2 + H2; the products are sodium hydroxide, chlorine and hydrogen.

4
Worked Example
How is bleaching powder prepared, and give two of its uses?
Solution
  1. Chlorine gas is passed over dry slaked lime, Ca(OH)2.
  2. Ca(OH)2 + Cl2 → CaOCl2 + H2O.
  3. It is used to bleach cotton and paper, and to disinfect drinking water.

Answer: Bleaching powder (CaOCl2) is made by passing Cl2 over slaked lime; it bleaches fabrics/paper and disinfects water.

5
Worked Example
Why does blue copper sulphate turn white on heating, and how can the blue colour be restored?
Solution
  1. Copper sulphate crystals are CuSO4·5H2O; the five water molecules are the water of crystallisation that give the blue colour.
  2. On heating, the crystals lose this water and become white anhydrous CuSO4.
  3. Adding a few drops of water puts the water of crystallisation back, restoring the blue colour.

Answer: Heating drives off the water of crystallisation (blue to white); adding water restores the blue colour.

6
Worked Example
Why must plaster of Paris be stored in a moisture-proof container? Write the equation when it sets.
Solution
  1. POP is CaSO4·½H2O, made by heating gypsum.
  2. If it absorbs moisture from the air, it slowly sets into hard gypsum and becomes useless.
  3. On mixing with water it sets: CaSO4·½H2O + 1½H2O → CaSO4·2H2O.

Answer: Moisture makes POP set into gypsum prematurely; on setting, CaSO4·½H2O + 1½H2O → CaSO4·2H2O.

Key Points

  • A salt forms when the hydrogen of an acid is replaced by a metal/ammonium ion; salts of a common acid or base form a family (chlorides, sulphates, carbonates).
  • Salt pH depends on its parents: strong acid + strong base = neutral; strong acid + weak base = acidic; weak acid + strong base = basic.
  • Chlor-alkali process: 2NaCl + 2H2O gives 2NaOH + Cl2 + H2 (sodium hydroxide, chlorine and hydrogen).
  • Key salts: bleaching powder CaOCl2 (bleaching/disinfection), baking soda NaHCO3 (antacid, baking), washing soda Na2CO3.10H2O (water softening), POP CaSO4.half H2O (casts and moulds).
  • Water of crystallisation is fixed water in a salt's crystals, e.g. CuSO4.5H2O (blue); heating removes it (white), adding water restores it.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.A salt of a strong acid and a weak base is:
Explanation: Such a salt (e.g. NH4Cl) gives an acidic solution with pH less than 7.
Q2.The products of the chlor-alkali process are NaOH, hydrogen and:
Explanation: 2NaCl + 2H2O gives 2NaOH + Cl2 + H2; chlorine is released at the anode.
Q3.The chemical formula of washing soda is:
Explanation: Washing soda is sodium carbonate decahydrate, Na2CO3.10H2O.
Q4.The blue colour of copper sulphate crystals is due to:
Explanation: CuSO4.5H2O owes its blue colour to the five water molecules of crystallisation; heating removes them and the salt turns white.
Practice more MCQs → 📝 Assignment · 30 marks