ChemistryICSE Class 10 · Ammonia

Laboratory Preparation of Ammonia: Reactants, Drying and Collection (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

In the laboratory, ammonia is made by heating ammonium chloride with slaked lime: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃. The gas is dried over quicklime and collected by downward displacement of air, because it is lighter than air and very soluble in water. Nitrides with warm water also give ammonia.

Understand Laboratory Preparation of Ammonia

Reactants and equation. A mixture of ammonium chloride and slaked lime (calcium hydroxide), in the ratio of about 2 : 3 by mass, is heated in a hard-glass round-bottomed flask or boiling tube:

$2 N H_{4} C l + C a (O H)_{2} Δ C a C l_{2} + 2 H_{2} O + 2 N H_{3}$

Any ammonium salt warmed with any alkali gives ammonia in the same way, for example $(N H_{4})_{2} S O_{4} + 2 N a O H Δ N a_{2} S O_{4} + 2 H_{2} O + 2 N H_{3}$ . Ammonium nitrate is avoided because it can decompose explosively on heating.

Apparatus points.

The flask is clamped with its mouth slightly tilted downwards, so that water formed in the reaction does not trickle back on to the hot glass and crack it.
The gas is passed through a drying tower packed with quicklime (CaO). Common drying agents cannot be used: concentrated sulphuric acid and phosphorus pentoxide are acidic and react with the basic gas, and anhydrous calcium chloride forms an addition compound with ammonia.
The gas is collected by downward displacement of air in an inverted gas jar, because ammonia is lighter than air (vapour density 8.5, against about 14.4 for air). It cannot be collected over water because it is extremely soluble in water.
The jar is full when moist red litmus held at its mouth turns blue, or when a glass rod dipped in concentrated hydrochloric acid gives dense white fumes of ammonium chloride.

Ammonia from metal nitrides. Magnesium nitride and aluminium nitride react with warm water to give ammonia and the metal hydroxide:

$M g_{3} N_{2} + 6 H_{2} O \to 3 M g (O H)_{2} + 2 N H_{3}$

$A l N + 3 H_{2} O \to A l (O H)_{3} + N H_{3}$

In the laboratory, warm water is run from a dropping funnel on to the nitride in a flask; the gas is dried and collected as above.

Tests for ammonia. Pungent smell; turns moist red litmus blue (the only common alkaline gas); dense white fumes with concentrated HCl; with copper(II) sulphate solution a pale blue precipitate that dissolves in excess ammonia to give an inky (deep) blue solution.

Laboratory Preparation of Ammonia: formulas and key facts

From ammonium chloride and slaked lime: $2 N H_{4} C l + C a (O H)_{2} Δ C a C l_{2} + 2 H_{2} O + 2 N H_{3}$
From ammonium sulphate and an alkali: $(N H_{4})_{2} S O_{4} + 2 N a O H Δ N a_{2} S O_{4} + 2 H_{2} O + 2 N H_{3}$
From magnesium nitride and warm water: $M g_{3} N_{2} + 6 H_{2} O \to 3 M g (O H)_{2} + 2 N H_{3}$
From aluminium nitride and warm water: $A l N + 3 H_{2} O \to A l (O H)_{3} + N H_{3}$
Preparation at a glance: Reactants ammonium chloride and slaked lime, heated
Flask tilted mouth downwards
Drying agent quicklime, CaO
Collection downward displacement of air
Jar full moist red litmus turns blue at the mouth
Why not these drying agents: Concentrated sulphuric acid reacts to form ammonium sulphate
Phosphorus pentoxide acidic, reacts with ammonia
Anhydrous calcium chloride forms an addition compound with ammonia

Reactants	ammonium chloride and slaked lime, heated
Flask	tilted mouth downwards
Drying agent	quicklime, CaO
Collection	downward displacement of air
Jar full	moist red litmus turns blue at the mouth

Concentrated sulphuric acid	reacts to form ammonium sulphate
Phosphorus pentoxide	acidic, reacts with ammonia
Anhydrous calcium chloride	forms an addition compound with ammonia

How to solve Laboratory Preparation of Ammonia questions

A lab-preparation question asks for six things in some order: reactants, equation, condition, apparatus point, drying, collection and test. Learn them as a checklist.

1Name the reactants (ammonium chloride + slaked lime, or a nitride + warm water) and write the balanced equation.
2Give the condition: heat (ammonium salt + alkali) or warm water (nitride).
3Mention the tilted flask and why.
4Name quicklime as the drying agent and say why the usual drying agents fail.
5State the collection method with both reasons: lighter than air, very soluble in water.
6Give a test to confirm the gas and to know the jar is full.

Laboratory Preparation of Ammonia 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.

Choosing a safe drying agent

Suppose a school laboratory assistant has run out of quicklime and proposes to dry ammonia by passing it through concentrated sulphuric acid, then through anhydrous calcium chloride.

Predict what would happen to the ammonia in each case and suggest what to do instead.

Try it first, then show the solutionSolution

Concentrated sulphuric acid is an acid; ammonia is a base: $2 N H_{3} + H_{2} S O_{4} \to (N H_{4})_{2} S O_{4}$ . The ammonia would be absorbed and lost.
Anhydrous calcium chloride combines with ammonia to form an addition compound, so it would also absorb the gas.
Only a basic drying agent such as quicklime (CaO) removes water without reacting with ammonia.

Answer: Both would absorb the ammonia; buy or prepare quicklime and use that.

Memory hook: A basic gas needs a basic drier: ammonia meets lime.

Tricks to remember Laboratory Preparation of Ammonia

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

Lime in, lime out
Slaked lime makes ammonia; quicklime dries it.
Use it when: Remembering both the reactant and the drying agent.
Light and loves water
Ammonia is Light (collect upside-down jar) and Loves water (never over water).
Use it when: Two reasons for downward displacement of air.
Nitride + water gives hydroxide
N goes to NH₃, the metal goes to its hydroxide, never its oxide.
Use it when: Balancing AlN and Mg₃N₂ equations.

Laboratory Preparation of Ammonia: solved examples

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

Example 1: Drying agent · 1 mark · MCQBoard level

The gas obtained by heating ammonium chloride with slaked lime is dried by passing it through:
(a) concentrated sulphuric acid
(b) quicklime
(c) anhydrous calcium chloride
(d) phosphorus pentoxide

Try it first, then show the solutionSolution

The gas is ammonia, a base.
Acidic drying agents and calcium chloride react with it; quicklime does not.

Answer: (b) quicklime.

Example 2: Two reasons for the collection method · 2 marks · Short answerBoard level

Ammonia is collected by downward displacement of air. Give two reasons, one for using an inverted jar and one for not collecting it over water.

Try it first, then show the solutionSolution

It is lighter than air (vapour density 8.5), so it fills an inverted jar from the top.
It is extremely soluble in water, so it would dissolve instead of collecting.

Answer: Lighter than air; highly soluble in water.

Example 3: Nitride route versus ammonium salt route · 3 marks · Short answerHarder board level

Compare the preparation of ammonia from aluminium nitride with its preparation from ammonium chloride under these heads: (a) the second reagent and the condition, (b) the balanced equation, (c) the solid by-product left in the flask.

Try it first, then show the solutionSolution

(a) Aluminium nitride: warm water, run in from a dropping funnel, no strong heating. Ammonium chloride: slaked lime, and the mixture is heated.
(b) $A l N + 3 H_{2} O \to A l (O H)_{3} + N H_{3}$ ; $2 N H_{4} C l + C a (O H)_{2} Δ C a C l_{2} + 2 H_{2} O + 2 N H_{3}$ .
(c) Aluminium hydroxide (white) from the nitride; calcium chloride from the ammonium chloride.

Answer: (a) Warm water, no heating versus slaked lime with heat (b) as above (c) Al(OH)₃ versus CaCl₂.

Example 4: Reasons in the set-up · 3 marks · Short answerHarder board level

Give reasons: (a) the flask is tilted with its mouth slightly downwards; (b) concentrated sulphuric acid is not used to dry ammonia; (c) a glass rod dipped in concentrated HCl is held at the mouth of the jar.

Try it first, then show the solutionSolution

(a) Water formed in the reaction would otherwise run back on to the hot part of the flask and crack it.
(b) It reacts with ammonia: $2 N H_{3} + H_{2} S O_{4} \to (N H_{4})_{2} S O_{4}$ .
(c) Dense white fumes of ammonium chloride show the jar is full: $N H_{3} + H C l \to N H_{4} C l$ .

Answer: (a) Prevent cracking (b) it reacts with ammonia (c) white fumes show the jar is full.

Example 5: Masses for a given volume · 4 marks · Long answerHOTS / competency

16.05 g of ammonium chloride is heated with excess slaked lime. Calculate (a) the volume of ammonia formed at S.T.P., (b) the mass of calcium hydroxide used, (c) the mass of calcium chloride formed. (N = 14, H = 1, Cl = 35.5, Ca = 40, O = 16)

Try it first, then show the solutionSolution

Molar mass of NH₄Cl $= 53.5$ ; moles $= 16.05/53.5 = 0.3$ .
(a) NH₄Cl : NH₃ = 1 : 1, so 0.3 mol NH₃ $= 0.3 \times 22.4 = 6.72$ L.
(b) NH₄Cl : Ca(OH)₂ = 2 : 1, so 0.15 mol $\times 74 = 11.1$ g.
(c) 0.15 mol CaCl₂ $\times 111 = 16.65$ g.

Answer: (a) 6.72 L (b) 11.1 g (c) 16.65 g.

Example 6: Identifying the salt and the gas · 3 marks · Short answerHOTS / competency

A white solid P, when warmed with sodium hydroxide solution, gives a gas Q that turns moist red litmus blue. A solution of P gives a white precipitate with silver nitrate solution that is insoluble in dilute nitric acid. (a) Identify P and Q. (b) Write the equation for the formation of Q. (c) Why could Q not be dried with anhydrous calcium chloride?

Try it first, then show the solutionSolution

Alkali + warm → alkaline gas: P is an ammonium salt and Q is ammonia.
White precipitate with AgNO₃ (AgCl) shows chloride: P is ammonium chloride.
(b) $N H_{4} C l + N a O H \to N a C l + H_{2} O + N H_{3}$ .
(c) Calcium chloride forms an addition compound with ammonia.

Answer: (a) P: NH₄Cl, Q: NH₃ (b) as above (c) CaCl₂ absorbs ammonia.

Where students lose marks in Laboratory Preparation of Ammonia

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

Mistake: Wrong reagent with the nitride
In 2025 (Q2(i)) examiners noted 'water only' or concentrated sulphuric acid written instead of warm water.
Do this: Write 'warm water' for nitrides.
Usually costs: 1 mark.
Mistake: Oxide instead of hydroxide
The same report found aluminium oxide given as a product instead of aluminium hydroxide.
Wrong working
2AlN + 3H₂O → Al₂O₃ + 2NH₃
Right working
AlN + 3H₂O → Al(OH)₃ + NH₃
Do this: Nitride + water → metal hydroxide + ammonia.
Usually costs: 1 mark.
Mistake: Wrong drying agent, or calling it a dehydrating agent
Candidates wrote concentrated sulphuric acid or calcium hydroxide for the drying tower and confused drying with dehydrating (2025 Q2(i)).
Do this: Quicklime (CaO) is a drying agent: it removes moisture from the gas. A dehydrating agent removes the elements of water from a compound.
Usually costs: 1 to 2 marks.
Mistake: Mixing up NH₃ and HCl collection
In 2024 (Q1(xii)) candidates chose 'heavier than air' because they confused ammonia with HCl.
Do this: NH₃: lighter than air, downward displacement of air. HCl: heavier than air, upward displacement of air.
Usually costs: 1 mark.

Exam tips

Practise drawing the set-up: tilted flask on a burner, delivery tube to a quicklime tower, inverted gas jar.
Always give two reasons for the collection method (light, very soluble) when 2 marks are given.
Write 'moist' red litmus: dry litmus does not change with ammonia.
Learn both nitride equations; the 2025 paper built a 5-mark question around AlN.

Practice questions on Laboratory Preparation of Ammonia

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
Which property of ammonia lets it be collected in an inverted gas jar, mouth downwards?
(a) Its pungent smell
(b) Its vapour density of 8.5, less than that of air
(c) Its high solubility in water
(d) Its basic nature
Hint
Air has a vapour density of about 14.4.
Answer
(b). Ammonia is lighter than air, so it rises and fills the inverted jar from the top. Its solubility explains why it is not collected over water.
Q2 · 1 mark · Very short answerBoard level
Write the balanced equation for the laboratory preparation of ammonia from ammonium chloride.
Hint
Slaked lime is the alkali.
Answer
$2 N H_{4} C l + C a (O H)_{2} \to C a C l_{2} + 2 H_{2} O + 2 N H_{3}$ (on heating).
Q3 · 2 marks · Short answerBoard level
State two tests to confirm that a gas is ammonia.
Hint
Litmus and an acid.
Answer
It turns moist red litmus blue; it gives dense white fumes with a glass rod dipped in concentrated hydrochloric acid. (Also: pale blue precipitate with CuSO₄ solution dissolving in excess to an inky blue solution.)
Q4 · 2 marks · Short answerHarder board level
Write the balanced equation for the action of warm water on magnesium nitride and name the white solid formed.
Hint
Hydroxide, not oxide.
Answer
$M g_{3} N_{2} + 6 H_{2} O \to 3 M g (O H)_{2} + 2 N H_{3}$ ; the white solid is magnesium hydroxide.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Phosphorus pentoxide cannot be used to dry ammonia.
Reason (R): Phosphorus pentoxide is a dehydrating agent.
(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
The real reason is about acid and base.
Answer
(b). Both are true, but the reason it cannot be used is that it is acidic and reacts with basic ammonia.
Q6 · 1 mark · Very short answerHarder board level
Why is ammonium nitrate not used to prepare ammonia in the laboratory?
Hint
Safety.
Answer
It can decompose explosively on heating.
Q7 · 3 marks · Short answerHarder board level
What volume of ammonia at S.T.P. and what mass of aluminium hydroxide are formed when 8.2 g of aluminium nitride reacts completely with warm water? (Al = 27, N = 14, O = 16, H = 1)
Hint
$A l N + 3 H_{2} O \to A l (O H)_{3} + N H_{3}$ .
Answer
Moles of AlN $= 8.2/41 = 0.2$ . NH₃ $= 0.2 \times 22.4 = 4.48$ L. Al(OH)₃ $= 0.2 \times 78 = 15.6$ g.
Q8 · 3 marks · Short answerHOTS / competency
Suppose a student collects ammonia over water in a trough and finds the gas jar never fills, while the water level in the jar rises. Explain the observation and calculate the volume of ammonia at S.T.P. that 5 g of magnesium nitride should have given. (Mg = 24, N = 14)
Hint
Ammonia's solubility; Mg₃N₂ gives 2NH₃.
Answer
Ammonia is extremely soluble in water, so it dissolved and the water rose into the jar. Moles of Mg₃N₂ $= 5/100 = 0.05$ ; NH₃ $= 0.1$ mol $= 2.24$ L at S.T.P.

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.Drying agent for ammonia?Show answer
Quicklime (CaO).
2.Vapour density of ammonia?Show answer
8.5.
3.Products of AlN with warm water?Show answer
Aluminium hydroxide and ammonia.
4.Why is the flask tilted downwards?Show answer
So water formed does not run back and crack the hot flask.
5.How do you know the gas jar is full of ammonia?Show answer
Moist red litmus at the mouth turns blue (or white fumes with a conc. HCl rod).

Laboratory Preparation of Ammonia: frequently asked questions

Why is quicklime used to dry ammonia?

It is a basic drying agent and does not react with ammonia, a basic gas. Acidic drying agents and calcium chloride absorb ammonia.

Why can ammonia not be collected over water?

It is extremely soluble in water, so it would dissolve instead of collecting in the jar.

Can ammonia be prepared from any ammonium salt?

Yes, any ammonium salt warmed with an alkali gives ammonia, but ammonium nitrate is avoided because it can decompose explosively.

What is formed when nitrides react with water?

Ammonia and the metal hydroxide: Mg₃N₂ gives Mg(OH)₂; AlN gives Al(OH)₃.