ScienceCBSE Class 10 · Acids, Bases and Salts

Baking Soda, Washing Soda and Plaster of Paris

Updated 14 October 2026 · Fact-checked

Baking soda is sodium hydrogencarbonate, made from common salt, and used in baking powder, antacids and fire extinguishers. Washing soda is Na2CO3.10H2O, made by heating baking soda and recrystallising, and used in glass, soap and paper industries and to remove permanent hardness. Plaster of Paris comes from heating gypsum at 373 K.

Notice how water of crystallisation changes: washing soda gains 10H₂O, while gypsum loses water on heating to become plaster of Paris.

Illustration drawn for QuestionBuddy.

Understand Baking Soda, Washing Soda and Plaster of Paris

Common salt (sodium chloride) is the raw material for several everyday chemicals. This topic covers two sodium salts made from it and one calcium salt made from gypsum, plus the idea that links them: water of crystallisation.

**Baking soda, $N a H C O_{3}$ (sodium hydrogencarbonate).** It is made from sodium chloride, water, carbon dioxide and ammonia: $N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$ . It is a mild, non-corrosive basic salt, which is why it can neutralise an acid safely. On heating, as in cooking, it decomposes: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ . Its uses:

Baking powder is baking soda mixed with tartaric acid, a weak edible acid. When heated or wetted, the acid's $H^{+}$ ions react with the hydrogencarbonate to give carbon dioxide, which makes bread or cake rise and turn soft and spongy.
Antacids: being alkaline it neutralises excess acid in the stomach.
Soda-acid fire extinguishers, which rely on the acid + hydrogencarbonate reaction to produce carbon dioxide.

**Washing soda, $N a_{2} C O_{3} \cdot 10 H_{2} O$ .** Heating baking soda gives sodium carbonate; recrystallising sodium carbonate from water gives washing soda, $N a_{2} C O_{3} + 10 H_{2} O \to N a_{2} C O_{3} \cdot 10 H_{2} O$ . It is also a basic salt. Its uses: in the glass, soap and paper industries; to make sodium compounds such as borax; as a cleaning agent at home; and to remove permanent hardness of water.

Water of crystallisation. Crystals that look dry can still contain water locked into them. It is the fixed number of water molecules in one formula unit of a salt. Blue copper sulphate crystals, $C u S O_{4} \cdot 5 H_{2} O$ , turn white when heated as this water is driven off (you see droplets on the cooler part of the tube), and turn blue again when a few drops of water are added. So the ten water molecules in washing soda do not make it wet; they are part of the crystal.

**Plaster of Paris, $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ (calcium sulphate hemihydrate).** Gypsum, $C a S O_{4} \cdot 2 H_{2} O$ , has two water molecules of crystallisation. Heating it at 373 K removes most of them and leaves Plaster of Paris, a white powder. Mix it with water and it turns back into gypsum, setting into a hard solid mass: $C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$ . The 'half water molecule' means two formula units of $C a S O_{4}$ share one water molecule. Doctors use it to hold fractured bones in the right position; it is also used to make toys and decorative materials and to make surfaces smooth. Because it sets on contact with water, it must be stored in a moisture-proof container.

Board questions ask for chemical names and formulae, preparation equations, two uses each, and reasoning such as why baking powder makes a cake fluffy or why Plaster of Paris is kept dry.

Baking Soda, Washing Soda and Plaster of Paris: formulas and key facts

Preparation of baking soda: $N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$; Common salt is a raw material; ammonium chloride is the other product.
Heating baking soda: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$; Happens during cooking; also the first step to washing soda.
Baking powder reaction: $N a H C O_{3} + H^{+} \to C O_{2} + H_{2} O + sodium salt of the acid$; $H^{+}$ comes from tartaric acid; $C O_{2}$ makes cakes rise.
Washing soda: $N a_{2} C O_{3} + 10 H_{2} O \to N a_{2} C O_{3} \cdot 10 H_{2} O$; Recrystallisation of sodium carbonate.
Gypsum to Plaster of Paris: $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$; Gypsum loses water on heating at 373 K.
Setting of Plaster of Paris: $C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$; Turns back into gypsum, a hard solid mass.

How to solve Baking Soda, Washing Soda and Plaster of Paris questions

Questions here are about identification, equations and uses. Use this checklist.

1Fix the identity from the clue: mild base used in cooking or antacids = $N a H C O_{3}$ ; ten water molecules, cleaning, hardness = $N a_{2} C O_{3} \cdot 10 H_{2} O$ ; white powder that sets with water, bones = $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ .
2Write the chemical name and formula together; marks are often split between them.
3Give the preparation equation, balanced, with the condition (heat, 373 K) over the arrow.
4Give uses from the NCERT list, one per line, not vague phrases like 'used in industry'.
5For 'why' questions, link a property to the use: basic and mild, so safe antacid; gives $C O_{2}$ , so cakes rise; sets with water, so kept dry.

Baking Soda, Washing Soda and Plaster of Paris in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Baking powder and carbon dioxide

Suppose you are baking a cake at home and run out of baking powder, but find a box of baking soda. Your friend says, 'They are the same thing; just use the soda.'

Explain how baking powder differs from baking soda, why baking powder makes the cake soft and spongy, and what may go wrong if you heat baking soda alone in the batter.

Try it first, then show the solutionSolution

Baking soda is only sodium hydrogencarbonate, $N a H C O_{3}$ . Baking powder is baking soda mixed with tartaric acid, a weak edible acid.
In the batter, when the powder is wetted or heated, $H^{+}$ from tartaric acid reacts with the hydrogencarbonate: $N a H C O_{3} + H^{+} \to C O_{2} + H_{2} O + a sodium salt$ . The carbon dioxide bubbles make the cake rise and become spongy.
Baking soda alone gives carbon dioxide only by decomposing on heating: $2 N a H C O_{3} \to N a_{2} C O_{3} + H_{2} O + C O_{2}$ .
That leaves sodium carbonate, a basic salt, in the cake, which can give it a bitter taste; the acid in baking powder neutralises the base, avoiding this.

Answer: Baking powder = baking soda + tartaric acid; their reaction releases carbon dioxide that makes the cake rise. Baking soda alone leaves sodium carbonate, which can make the cake taste bitter.

Memory hook: Soda plus a little acid makes the bubbles; soda alone leaves a bitter base behind.

Tricks to remember Baking Soda, Washing Soda and Plaster of Paris

Short memory aids. Say each one aloud once, then use it on a question the same day.

Ten for washing, two for gypsum, half for plaster
Washing soda has 10 $H_{2} O$ , gypsum has 2, Plaster of Paris has ½, copper sulphate has 5.
Use it when: Writing hydrated formulae correctly.
Baking soda's three jobs: BAF
Baking powder, Antacid, Fire extinguisher.
Use it when: Listing uses of baking soda.
Washing soda's jobs: GSP + B + C + H
Glass, Soap, Paper industries; Borax; Cleaning; Hardness removal.
Use it when: Listing uses of washing soda.

Baking Soda, Washing Soda and Plaster of Paris: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Heating baking soda · 2 marks · Very short answerBoard level

What happens when a solution of sodium hydrogencarbonate is heated? Write the equation and name the products. Give one use of sodium hydrogencarbonate that depends on its being basic.

Try it first, then show the solutionSolution

It decomposes: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ ; products are sodium carbonate, water and carbon dioxide (1 mark).
Being a mild base, it is used in antacids to neutralise excess acid in the stomach (1 mark).

Answer: Decomposes to sodium carbonate, water and carbon dioxide; used as an antacid.

Example 2: Gypsum and Plaster of Paris · 4 marks · Short answerHarder board level

(a) How is Plaster of Paris obtained from gypsum? Write the equation with the condition. (b) Write the equation for the reaction of Plaster of Paris with water. (c) Why is it written with half a water molecule? (d) Why must it be stored in a moisture-proof container?

Try it first, then show the solutionSolution

(a) By heating gypsum at 373 K: $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$ (1 mark).
(b) $C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$ , giving a hard solid mass (1 mark).
(c) Two formula units of $C a S O_{4}$ share one water molecule, so per formula unit it is half (1 mark).
(d) In contact with moisture it would slowly turn into gypsum and set hard, becoming useless (1 mark).

Answer: (a) Heat gypsum at 373 K. (b) Plaster + water → gypsum (sets hard). (c) Two units share one water molecule. (d) Moisture would make it set.

Example 3: Water of crystallisation · 4 marks · Short answerHarder board level

A student heats blue copper sulphate crystals in a dry boiling tube. (a) What two observations are made? (b) What happens when a few drops of water are added to the residue after cooling? (c) Define water of crystallisation and give the formula of one other salt from this chapter that has it. (d) Does washing soda feel wet because it contains ten water molecules? Explain.

Try it first, then show the solutionSolution

(a) Blue crystals turn white; water droplets appear on the cooler part of the tube (1 mark).
(b) The blue colour returns (1 mark).
(c) The definite number of water molecules that are part of each formula unit of a salt crystal; for example gypsum $C a S O_{4} \cdot 2 H_{2} O$ or washing soda $N a_{2} C O_{3} \cdot 10 H_{2} O$ (1 mark).
(d) No. The water is part of the crystal structure, not free liquid, so the crystals are dry to touch (1 mark).

Answer: (a) Blue to white; droplets. (b) Blue colour restored. (c) Fixed number of water molecules per formula unit; e.g. $C a S O_{4} \cdot 2 H_{2} O$ . (d) No, the water is part of the crystal.

Example 4: From common salt to washing soda · 5 marks · Long answerHarder board level

Starting from common salt, show with equations how washing soda is obtained in three stages. Then give two uses of washing soda.

Try it first, then show the solutionSolution

Stage 1, baking soda from salt: $N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$ (1 mark).
Stage 2, heat baking soda: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ (1 mark).
Stage 3, recrystallise sodium carbonate: $N a_{2} C O_{3} + 10 H_{2} O \to N a_{2} C O_{3} \cdot 10 H_{2} O$ (1 mark).
Uses: in glass, soap and paper industries; for removing permanent hardness of water (also: making borax, household cleaning) (2 marks).

Answer: Salt → baking soda (with $H_{2} O$ , $C O_{2}$ , $N H_{3}$ ) → sodium carbonate (heat) → washing soda (recrystallisation); used in glass, soap and paper industries and to remove permanent hardness.

Example 5: Identify the compounds · 3 marks · Short answerHOTS / competency

A white powder X is used by doctors to support fractured bones. X is made by heating compound Y, which contains two molecules of water of crystallisation, at 373 K. A sodium compound Z, obtained by heating a substance used in antacids and then recrystallising, is used for removing permanent hardness of water. Identify X, Y and Z with formulae, and write the equation for the formation of X.

Try it first, then show the solutionSolution

X: Plaster of Paris, $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ (1 mark).
Y: gypsum, $C a S O_{4} \cdot 2 H_{2} O$ ; Z: washing soda, $N a_{2} C O_{3} \cdot 10 H_{2} O$ (made from baking soda, the antacid ingredient) (1 mark).
$C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$ (1 mark).

Answer: X = $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ , Y = $C a S O_{4} \cdot 2 H_{2} O$ , Z = $N a_{2} C O_{3} \cdot 10 H_{2} O$ .

Where students lose marks in Baking Soda, Washing Soda and Plaster of Paris

The slips examiners see most often, and what to write instead.

Mistake: Mixing up baking soda and washing soda
Both are sodium salts with 'soda' in the name.
Wrong working
Baking soda is $N a_{2} C O_{3}$ .
Right working
Baking soda is $N a H C O_{3}$ ; washing soda is $N a_{2} C O_{3} \cdot 10 H_{2} O$ .
Do this: Baking = hydrogencarbonate (one Na); washing = carbonate with ten waters.
Usually costs: 1 mark per formula.
Mistake: Wrong water count in Plaster of Paris
Students write the gypsum formula or drop the half.
Wrong working
Plaster of Paris is $C a S O_{4} \cdot 2 H_{2} O$ .
Right working
Plaster of Paris is $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ ; gypsum is $C a S O_{4} \cdot 2 H_{2} O$ .
Do this: Plaster = half, gypsum = two.
Usually costs: 1 mark.
Mistake: Missing the condition for making Plaster of Paris
Students write 'on heating' only.
Wrong working
Gypsum is heated to form Plaster of Paris.
Right working
Gypsum is heated at 373 K to form Plaster of Paris.
Do this: Always write 373 K over the arrow.
Usually costs: ½ mark.
Mistake: Vague uses
Students write 'used in industries' or 'used for cleaning things'.
Wrong working
Washing soda is used in industry.
Right working
Washing soda is used in the glass, soap and paper industries.
Do this: Name the industry or the specific job from the NCERT list.
Usually costs: ½ mark per use.

Exam tips

For each compound, learn four things together: common name, chemical name, formula, preparation equation.
Balance $2 N a H C O_{3}$ on heating; a single $N a H C O_{3}$ cannot give $N a_{2} C O_{3}$ .
When asked 'why is baking powder used', mention tartaric acid and carbon dioxide.
Plaster of Paris questions usually pair the heating equation with the setting equation; write both.
In water-of-crystallisation questions, describe the colour change of copper sulphate both ways.

Practice questions on Baking Soda, Washing Soda and Plaster of Paris

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The chemical formula of washing soda is:
(a) $N a H C O_{3}$ (b) $N a_{2} C O_{3}$ (c) $N a_{2} C O_{3} \cdot 10 H_{2} O$ (d) $C a S O_{4} \cdot 2 H_{2} O$
Hint
Washing soda is a hydrated salt.
Answer
(c) $N a_{2} C O_{3} \cdot 10 H_{2} O$ .
Q2 · 2 marks · Very short answerBoard level
Give two uses each of baking soda and Plaster of Paris.
Hint
Use the NCERT lists.
Answer
Baking soda: in baking powder; as an ingredient of antacids (also in soda-acid fire extinguishers). Plaster of Paris: for supporting fractured bones; for making toys and decoration materials (also making surfaces smooth).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Baking soda is used in antacids.
Reason (R): Baking soda is a mild, non-corrosive basic salt.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Why is a mild base a safe remedy for excess acid?
Answer
(a). Being basic it neutralises excess stomach acid; being mild and non-corrosive it does no harm.
Q4 · 3 marks · Short answerHOTS / competency
A sample of gypsum is heated much more strongly than 373 K for a long time, and a student then tries to use the product as Plaster of Paris for a model. It does not set. Suggest a reason, using what you know about water of crystallisation, and state what the correct heating condition should have been.
Hint
Plaster of Paris still contains half a water molecule per formula unit.
Answer
Plaster of Paris is $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ ; it sets because it recombines with water to form gypsum. Heating too strongly removes all the water of crystallisation, giving a different substance that does not set in the same way. Gypsum should be heated at 373 K so that it loses only 1½ water molecules.
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a school canteen uses three white powders: one to make pakoras crisp, one to scrub greasy steel vessels, and one (kept in a sealed tin) by the school's first-aid room for making temporary splints.
(a) Name the powder used for pakoras and give its formula. (1 mark)
(b) Name the powder used for cleaning and state one more use of it in industry. (1 mark)
(c) Why is the third powder kept in a sealed tin? Write the equation for what happens if it gets damp. (2 marks)
Hint
Match each use to baking soda, washing soda or Plaster of Paris.
Answer
(a) Baking soda, $N a H C O_{3}$ . (b) Washing soda, $N a_{2} C O_{3} \cdot 10 H_{2} O$ ; used in glass, soap or paper industries (or for removing permanent hardness of water). (c) Plaster of Paris sets hard when it takes up water: $C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$ . Kept dry so it does not set before use.
Q6 · 3 marks · Short answerHarder board level
Two students heat equal masses of (i) baking soda and (ii) washing soda crystals in separate dry test tubes and pass any gas through lime water. Predict and explain what each observes, with equations where a reaction occurs.
Hint
Which one decomposes to give a gas? Which one only loses water of crystallisation?
Answer
(i) Baking soda decomposes, $2 N a H C O_{3} \to N a_{2} C O_{3} + H_{2} O + C O_{2}$ ; the carbon dioxide turns lime water milky, $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ . (ii) Washing soda loses its water of crystallisation as droplets, leaving sodium carbonate; no carbon dioxide is given off, so lime water stays clear.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Chemical name of baking soda?Show answer
Sodium hydrogencarbonate, $N a H C O_{3}$ .
2.What is baking powder?Show answer
A mixture of baking soda and tartaric acid, a weak edible acid.
3.How is washing soda obtained from sodium carbonate?Show answer
By recrystallisation.
4.At what temperature is gypsum heated to give Plaster of Paris?Show answer
373 K.
5.Formula of hydrated copper sulphate?Show answer
$C u S O_{4} \cdot 5 H_{2} O$ .
6.Why is washing soda added to hard water?Show answer
It removes permanent hardness.

Baking Soda, Washing Soda and Plaster of Paris: frequently asked questions

What is the difference between baking soda and washing soda?

Baking soda is sodium hydrogencarbonate, NaHCO_3, a mild base used in cooking and antacids. Washing soda is hydrated sodium carbonate, Na_2CO_3 \cdot 10H_2O, used for cleaning, removing permanent hardness and in the glass, soap and paper industries.

Why does a cake become soft and spongy with baking powder?

Tartaric acid in baking powder reacts with sodium hydrogencarbonate when wetted or heated, releasing carbon dioxide gas. The gas bubbles make the cake rise.

What is Plaster of Paris made from?

From gypsum, CaSO_4 \cdot 2H_2O, by heating it at 373 K so it loses part of its water of crystallisation.

Is water of crystallisation the same as moisture?

No. It is a fixed number of water molecules built into each formula unit of the crystal, so the crystals feel dry.

Why is Plaster of Paris stored in a moisture-proof container?

It reacts with water to form gypsum and sets into a hard mass, so damp storage would spoil it.