Baking soda, washing soda, plaster of Paris and bleaching powder

Making baking soda and washing soda (Solvay process)

Raw materials: **limestone** (CaCO₃), **brine** (a strong solution of common salt, NaCl) and **ammonia** (NH₃). Your NIOS lesson spells it Solvey.

1. 1
   CaCO₃(s) → CaO(s) + CO₂(g), on strong heating

   Limestone gives the carbon dioxide
2. 2★
   NaCl(aq) + NH₃(g) + CO₂(g) + H₂O(l) → NaHCO₃(s) + NH₄Cl(aq)

   CO₂ is passed into cold brine already saturated with ammonia. Baking soda dissolves only slightly, so it settles out as white crystals
3. 3
   2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g), on heating

   Strong heating of baking soda gives sodium carbonate
4. 4★
   Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O

   Recrystallising sodium carbonate from water gives washing soda crystals

★ key step

Uses of baking soda and washing soda

Baking soda or baking powder?

On heating, baking soda gives carbon dioxide: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The gas makes the dough rise. But the sodium carbonate left behind tastes bitter. Baking powder is baking soda mixed with a mild edible acid, tartaric acid. The acid neutralises the sodium carbonate. So cakes and bread rise soft and fluffy, with no bitter taste.

Plaster of Paris from gypsum

Setting: mixed with water, plaster of Paris turns back into hard gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Water of crystallisation is water held inside a salt crystal.

Making bleaching powder (Hasenclever method)

Raw materials: dry **slaked lime**, Ca(OH)₂, and **chlorine** gas, Cl₂.

1. 1
   Dry slaked lime is fed in at the top of a set of cast iron cylinders.
2. 2
   Chlorine gas enters near the bottom and rises against the falling lime.
3. 3★
   Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

   Bleaching powder collects at the bottom
4. 4
   Uses

   Bleaching cotton, linen and wood pulp; making wool unshrinkable; disinfecting drinking water; making chloroform; an oxidising agent in industry

★ key step

Activity: tell washing soda from baking soda (practical)

1. 1
   Dissolve each white powder in water in two test tubes, A and B. Add a drop of phenolphthalein to each.

   Washing soda turns deep pink, as it is strongly basic. Baking soda stays colourless or turns only faint pink, as it is only mildly basic
2. 2
   Dip pH paper in each solution.

   Washing soda shows the higher pH
3. 3
   Add a few drops of vinegar (acetic acid) to each.

   Both fizz and give CO₂; baking soda fizzes faster
4. 4★
   Heat a little of each solid in a dry boiling tube, and pass any gas into lime water.

   Only baking soda gives CO₂ that turns lime water milky. Washing soda gives off only its water of crystallisation

★ key step