ChemistryICSE Class 8 · Matter

Law of Conservation of Mass: Statement, Experiment and Gases from the Air (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

In a chemical reaction mass is neither created nor destroyed: the total mass of the reactants equals the total mass of the products, because atoms are only rearranged. Reactions in air must include the gases taken in (burning magnesium gains oxygen) or given out (burning candle loses CO₂ and water vapour).

Both pans weigh the same: the 4 + 2 atoms in the reactants are only regrouped into water, so total mass is unchanged.

Illustration drawn for QuestionBuddy.

Understand Law of Conservation of Mass

Statement. In a chemical reaction, mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products.

Explanation. In a reaction atoms are only rearranged into new combinations. No atom is lost or gained, so the total mass is unchanged.

Experiment: barium chloride and sodium sulphate. Put the two solutions in separate tubes in a flask; weigh the whole set-up; tip the tubes so the solutions mix (a white precipitate of barium sulphate forms); weigh again. The mass is unchanged.

$B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$

Gases from or to the air. Some reactions seem to break the law until the gases are counted.

Burning magnesium: the ash (magnesium oxide) is heavier than the metal because oxygen from the air combined with it: $2 M g + O_{2} \to 2 M g O$ .
Burning a candle or wood: the remains are lighter because carbon dioxide and water vapour escape.

In a closed container nothing enters or leaves, so the total mass stays the same. The law applies to the whole system, including any atmospheric gases.

Using the law.

Mass of a product $=$ total mass of reactants $-$ mass of the other products.
Mass of oxygen absorbed $=$ mass of oxide $-$ mass of metal.
Mass of a gas lost $=$ initial mass $-$ final mass of the open vessel.

For $2 H_{2} + O_{2} \to 2 H_{2} O$ (H = 1, O = 16), 4 g of hydrogen reacts with 32 g of oxygen to give 36 g of water.

Law of Conservation of Mass: formulas and key facts

Conservation of mass: $mass of reactants = mass of products$; Include gases from or to the air.
Precipitation experiment: $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$; Total mass unchanged.
Magnesium: $2 M g + O_{2} \to 2 M g O$; Product heavier by the oxygen added.
Water: $2 H_{2} + O_{2} \to 2 H_{2} O : 4 + 32 = 36$; g.

How to solve Law of Conservation of Mass questions

Write the reaction, list everything that goes in and out, and equate the totals.

1Write the word or balanced equation.
2Identify gases that enter (oxygen) or leave (CO₂, water vapour).
3Total mass in $=$ total mass out; solve for the unknown.
4For an open vessel, the difference in mass is the gas lost or gained.

Law of Conservation of Mass 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.

Burning a piece of magnesium ribbon

Suppose 4.8 g of magnesium ribbon is burnt in air and the white ash weighs 8.0 g.

Find the mass of oxygen that combined.

Try it first, then show the solutionSolution

Mass of oxide = mass of magnesium + mass of oxygen.
Oxygen $= 8.0 - 4.8 = 3.2$ g.

Answer: 3.2 g of oxygen.

Memory hook: The ash gained air.

Candle in a box

Imagine a 50 g candle with its stand burns down to 46.5 g in an open dish.

What happened to the missing mass?

Try it first, then show the solutionSolution

The mass lost $= 50 - 46.5 = 3.5$ g.
It left as carbon dioxide and water vapour; in a closed box the total would stay 50 g.

Answer: 3.5 g escaped as gases.

Memory hook: It floated away, not vanished.

Tricks to remember Law of Conservation of Mass

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

Atoms just rearrange
No atom is lost, so no mass is lost.
Use it when: Explanation.
Open vessel, count the air
Heavier ash: gas absorbed. Lighter ash: gas lost.
Use it when: Air reactions.
In = out
Total mass in $=$ total mass out.
Use it when: Numericals.

Law of Conservation of Mass: solved examples

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

Example 1: Statement · 1 mark · MCQBoard level

The law of conservation of mass states that in a chemical reaction:
(a) mass is created
(b) mass is destroyed
(c) total mass of reactants equals total mass of products
(d) mass of products is always less

Try it first, then show the solutionSolution

This is the statement of the law.

Answer: (c)

Example 2: Heavier ash · 2 marks · Short answerBoard level

Why is the ash formed on burning magnesium in air heavier than the magnesium?

Try it first, then show the solutionSolution

Magnesium combines with oxygen from the air, $2 M g + O_{2} \to 2 M g O$ . (1)
The oxide includes the mass of the oxygen added, so it is heavier. (1)

Answer: Oxygen from the air is added.

Example 3: Oxygen absorbed · 3 marks · Short answerHarder board level

4.8 g of magnesium burns in air to give 8.0 g of magnesium oxide. (a) Find the mass of oxygen that combined. (b) State the law you used. (c) If 2.4 g of magnesium had been used, what mass of oxide would form, assuming the same ratio?

Try it first, then show the solutionSolution

(a) $8.0 - 4.8 = 3.2$ g. (1)
(b) Conservation of mass. (1)
(c) Half the quantity gives half the oxide: $\frac{8.0}{2} = 4.0$ g. (1)

Answer: (a) 3.2 g (b) conservation of mass (c) 4.0 g

Example 4: The candle · 3 marks · Short answerHarder board level

A candle with a holder has a mass of 50.0 g. After burning in an open dish it weighs 46.5 g. (a) What mass was lost and in what forms? (b) What can you say about the total mass if the burning took place inside a closed, weighed box of air? (c) What are the products?

Try it first, then show the solutionSolution

(a) $50.0 - 46.5 = 3.5$ g, as carbon dioxide and water vapour. (1)
(b) The total mass of the closed box and its contents would be the same before and after burning, because the gases stay inside and are included. (1)
(c) Carbon dioxide and water (vapour). (1)

Answer: (a) 3.5 g as gases (b) unchanged (c) CO₂ and H₂O

Example 5: The precipitation experiment · 4 marks · Case-basedHOTS / competency

20.8 g of barium chloride is mixed with 14.2 g of sodium sulphate in solution and gives 11.7 g of sodium chloride and a precipitate of barium sulphate. (a) Find the mass of barium sulphate. (b) Write the balanced equation. (c) Would the mass in a sealed flask change? (d) Would it change in an open flask? Explain.

Try it first, then show the solutionSolution

(a) Reactants $= 20.8 + 14.2 = 35.0$ g; barium sulphate $= 35.0 - 11.7 = 23.3$ g. (1)
(b) $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$ . (1)
(c) No, the mass is conserved. (1)
(d) No; no gas is produced or absorbed, so the mass is the same. (1)

Answer: (a) 23.3 g (b) as above (c) no (d) no change

Where students lose marks in Law of Conservation of Mass

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

Mistake: Ignoring the air
Gases count in the total.
Wrong working
"4.8 g Mg gives 4.8 g MgO."
Right working
"4.8 g Mg + 3.2 g O₂ gives 8.0 g MgO."
Do this: Add oxygen to the reactants.
Mistake: Saying mass is destroyed in burning
Mass leaves as gas.
Wrong working
"A candle burns away."
Right working
"It forms CO₂ and water vapour that escape."
Do this: Account for the gases.
Mistake: Forgetting the unit
Mass in grams.
Wrong working
"Mass is 3.2."
Right working
"Mass is 3.2 g."
Do this: State the unit.

Exam tips

Name the law and give both reactants and products masses.
Mention the gas whenever a mass appears to change.
Quote the barium chloride experiment briefly.
Show subtraction steps clearly.

Practice questions on Law of Conservation of Mass

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 · Very short answerBoard level
State the law of conservation of mass.
Hint
Total mass.
Answer
In a chemical reaction mass is neither created nor destroyed; the total mass of the reactants equals that of the products.
Q2 · 2 marks · Short answerHarder board level
18 g of water is split into hydrogen and oxygen by electricity (4 g H₂ : 32 g O₂ from 36 g H₂O). Find the masses of the two gases.
Hint
Half of 36 g.
Answer
18 g is half of 36 g, so hydrogen $= 2$ g and oxygen $= 16$ g; total 18 g.
Q3 · 3 marks · Short answerHarder board level
10 g of limestone is heated strongly in an open crucible and the residue (calcium oxide) weighs 5.6 g. Where did the missing mass go? How much was it?
Hint
Gas is released.
Answer
Mass lost $= 10 - 5.6 = 4.4$ g; it escaped as carbon dioxide: $C a C O_{3} \to C a O + C O_{2}$ .
Q4 · 2 marks · Short answerHarder board level
4.8 g of magnesium is burnt in a sealed flask of air; the flask and contents weigh 105.0 g before burning. What do they weigh after? Why?
Hint
Closed system.
Answer
105.0 g. Nothing enters or leaves the sealed flask, so the total mass is conserved even though the magnesium gained oxygen from the air inside.
Q5 · 4 marks · Long answerHOTS / competency
A student says: "Burning destroys wood because the ash is lighter." Correct her, and suggest an experiment to prove it.
Hint
Include the gases.
Answer
The wood is not destroyed: its carbon and hydrogen combine with oxygen from the air to form carbon dioxide and water vapour, which escape, so the ash is lighter. Experiment: burn it in a closed, weighed container of air; the total mass before and after is the same.

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.Statement of the law?Show answer
Total mass of reactants = total mass of products.
2.Why is Mg ash heavier?Show answer
It has combined with oxygen.
3.Why is candle residue lighter?Show answer
CO₂ and water vapour escape.
4.Experiment with a precipitate?Show answer
BaCl₂ and Na₂SO₄.
5.Where does the law apply fully?Show answer
In a closed system.

Law of Conservation of Mass: frequently asked questions

What is the law of conservation of mass?

It states that in a chemical reaction mass is neither created nor destroyed, so the total mass of the reactants equals that of the products.

Why is magnesium oxide heavier than magnesium?

Because oxygen from the air combines with the magnesium; the mass of the oxygen is added to the oxide.

Why does a burning candle appear to lose mass?

The products, carbon dioxide and water vapour, escape into the air; in a closed container the total mass would stay the same.