Which one of the following is the chemical formula of Washing Soda ?
- (a)NaHCO₃
- (b)Na₂CO₃·10H₂O
- (c)Na₂CO₃·5H₂O
- (d)NaOH
Correct — B, Na₂CO₃·10H₂O. Washing soda is sodium carbonate decahydrate — sodium carbonate that has crystallised with ten molecules of water attached to each formula unit. Two details in the option have to be right together. The salt must be the carbonate, with two sodium atoms, not the hydrogen carbonate with one; and the number of waters of crystallisation must be ten. Only option (b) gets both, which is why it is the answer.
- (a)NaHCO₃ — This is sodium hydrogen carbonate, which is baking soda, not washing soda. It has one sodium rather than two and carries no water of crystallisation, and heating it gives sodium carbonate, carbon dioxide and water — which is why it makes a cake rise.
- (c)Na₂CO₃·5H₂O — The salt is right but the count of waters is not. Five is the figure for copper sulphate crystals, blue vitriol, and it has been borrowed here to make a near miss. Washing soda carries ten.
- (d)NaOH — Sodium hydroxide is caustic soda — a strongly alkaline solid used in soap-making and drain cleaners. It shares the word 'soda' with the answer and nothing else.
Water of crystallisation is the fixed number of water molecules that combine with each formula unit of a salt in its crystalline form. It is written after a dot in the formula, and heating drives it off to leave the anhydrous salt. Washing soda is sodium carbonate decahydrate, obtained by recrystallising the anhydrous sodium carbonate produced in the Solvay process. It is used in laundering, in softening hard water, in the manufacture of glass, soap and paper, and in the making of borax.
The three sodas are the most confused trio in school chemistry and the safest fix is to fasten each name to its formula and its everyday use in one line. Baking soda is sodium hydrogen carbonate, used in cooking and in antacids. Washing soda is sodium carbonate with ten waters, used in laundry and for softening hard water. Caustic soda is sodium hydroxide, used in soap-making. Then learn the small set of hydration numbers alongside, because NDA has asked for them directly — ten for washing soda, five for blue vitriol, two for gypsum. Losing the water changes the substance's appearance and its name but not its chemistry: the anhydrous carbonate that remains after heating is called soda ash.
- Washing soda is Na₂CO₃·10H₂O, sodium carbonate decahydrate.
- Baking soda is NaHCO₃, sodium hydrogen carbonate, and caustic soda is NaOH.
- Waters of crystallisation to remember — washing soda 10, blue vitriol 5, gypsum 2.
- Washing soda is used to soften hard water and in the manufacture of glass, soap and paper.
- Heating washing soda drives off the water of crystallisation and leaves anhydrous sodium carbonate, called soda ash.
- Answering with baking soda because both names contain the word soda.
- Getting the salt right but the number of waters wrong.
- Assuming anhydrous sodium carbonate and washing soda are interchangeable names.
NDA asks for the formula, for the common name given the formula, or for the number of waters of crystallisation across a set of familiar salts.
Consider the following statements : 1. Anhydrous sodium carbonate is commonly known as baking soda 2. Baking soda is used in fire extinguishers 3. Bleaching powder is manufactured in Hasenclever plant Which of the statements given above is/are correct?
- (a) 1, 2 and 3
- (b) 2 and 3
- (c) 3 only
- (d) 1 and 2
Answer(b) 2 and 3
Its first statement fails for exactly the reason this item is set — anhydrous sodium carbonate is washing soda in its dried form, and baking soda is a different compound altogether.
Which one among the following chemicals is used as washing soda ?
- (a) Calcium carbonate
- (b) Calcium bicarbonate
- (c) Sodium carbonate
- (d) Sodium bicarbonate
Answer(c) Sodium carbonate
The name-side version of this item, set a few months earlier, with sodium bicarbonate offered as the same near-miss distractor.
Number of molecules of water of crystallization in copper sulphate, sodium carbonate and Gypsum are
- (a) 5, 10 and 2 respectively
- (b) 10, 2 and 5 respectively
- (c) 5, 2 and 10 respectively
- (d) 2, 5 and 10 respectively
Answer(a) 5, 10 and 2 respectively
The hydration number this item turns on, asked directly and alongside the two other figures examiners borrow to build near-miss options.
- practice — not a real PYQ
The number of water molecules of crystallisation in washing soda is
- (a)2
- (b)5
- (c)7
- (d)10
Answer(d) 10 — washing soda is sodium carbonate decahydrate.
- practice — not a real PYQ
Which of the following is used to remove the permanent hardness of water?
- (a)Baking soda
- (b)Washing soda
- (c)Caustic soda
- (d)Bleaching powder
Answer(b) Washing soda — sodium carbonate precipitates the calcium and magnesium salts responsible for permanent hardness.