10 min readConcept sheet1-mark MCQ and assertion-reason; 2 or 3-mark questions on preparation and uses; sometimes a 5-mark answer

Baking soda, washing soda, plaster of Paris and water of crystallisation

Baking soda (NaHCO₃) and washing soda (Na₂CO₃·10H₂O) come from common salt. Plaster of Paris comes from gypsum. Water of crystallisation is water held inside a salt crystal.

Baking sodaNaHCO₃, sodium hydrogencarbonate

Washing sodaNa₂CO₃·10H₂O, sodium carbonate decahydrate

Plaster of ParisCaSO₄·½H₂O, calcium sulphate hemihydrate

GypsumCaSO₄·2H₂O, calcium sulphate dihydrate

Three useful salts

Salt	How it is made	Uses
Baking soda, NaHCO₃	NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃	Baking powder; antacid; soda-acid fire extinguishers
Washing soda, Na₂CO₃·10H₂O	Heat baking soda to get sodium carbonate, then recrystallise it: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O	Glass, soap and paper industries; making borax; cleaning at home; removing permanent hardness of water
Plaster of Paris, CaSO₄·½H₂O	Heat gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O	Plaster for supporting fractured bones; toys; decoration; making surfaces smooth

Key reactions of baking soda

1★
On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
The CO₂ makes cakes and bread rise
2
Problem: the sodium carbonate left behind tastes bitter.
3
So baking powder = baking soda + a mild edible acid, such as tartaric acid.
4
When the powder is heated or mixed with water, the acid reacts with baking soda and gives CO₂.
The CO₂ makes the dough soft and spongy, with no bitter taste
5
As an antacid, baking soda neutralises extra acid in the stomach. In soda-acid fire extinguishers, it reacts with an acid to give CO₂, which puts out the fire.

★ key step

Water of crystallisation

Water of crystallisation is the fixed number of water molecules in one formula unit of a salt. Blue copper sulphate crystals, CuSO₄·5H₂O, have five. Washing soda has ten and gypsum has two. The crystals feel dry, even though they hold water.

Activity: heating copper sulphate crystals

1
Heat a few blue copper sulphate crystals in a dry boiling tube.
2
Observe: water drops form on the cooler part of the tube, and the crystals turn white.
The salt loses its water of crystallisation: CuSO₄·5H₂O → CuSO₄ + 5H₂O
3
Let it cool, then add 2 to 3 drops of water to the white powder.
4★
Observe: the blue colour comes back.
The salt takes back its water of crystallisation

★ key step

Gypsum vs plaster of Paris

Gypsum, CaSO₄·2H₂OHas two molecules of water of crystallisation. Heating it at 373 K removes some of the water and gives plaster of Paris.

Plaster of Paris, CaSO₄·½H₂OHas half a water molecule per CaSO₄, because two CaSO₄ units share one water molecule. Mixed with water, it sets into hard gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.

Plaster of Paris is stored in a moisture-proof container. Otherwise it takes water from the air and sets hard.

In real life

A pinch of baking soda makes pakoras and bhaturas light and fluffy, and doctors use plaster of Paris casts to hold broken bones in place. Washing soda is used at home to clean greasy vessels.

Exam angle

2 marks: “A bag of plaster of Paris left open during the monsoon turns into hard lumps. Explain why, with an equation, and say how it should be stored.” It takes up moisture from the air and sets into hard gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. It should be kept in a moisture-proof container.

3 marks: “How is washing soda made from baking soda? Write equations and give two uses.” Heat baking soda: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Recrystallise: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. Uses: removing permanent hardness of water; glass and soap industries.

5 marks: “What is baking powder? Why does it make a cake soft and spongy, and why is baking soda alone not used?” Baking soda + a mild edible acid (tartaric acid). On heating or with water, CO₂ is given off and puffs up the dough. Baking soda alone leaves bitter sodium carbonate (2NaHCO₃ → Na₂CO₃ + H₂O + CO₂); the acid prevents this.

Remember it

Baking = Bicarbonate (HCO₃), Washing = Ten waters, Plaster = Half a water, Gypsum = Two waters.