ChemistryICSE Class 8 · Language of Chemistry

Chemical Equations: Word Equations, Balancing, Information and Limitations (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

A balanced chemical equation has the same number of atoms of each element on both sides (law of conservation of mass). Write the correct formulae first, then add whole-number coefficients, never change subscripts. An equation does not show state, conditions, catalyst, speed, concentration, heat change or the nature of the reaction.

Count each atom on both sides: 4 H and 2 O match, so mass is conserved; only coefficients were added, no subscripts changed.

Illustration drawn for QuestionBuddy.

Understand Chemical Equations, Balancing, Information and Limitations

Word equation to skeleton equation. Replace each name by its formula:

zinc + hydrochloric acid → zinc chloride + hydrogen
$Z n + H C l \to Z n C l_{2} + H_{2}$ (unbalanced)

Gases such as hydrogen, oxygen, nitrogen and chlorine are written as molecules: H₂, O₂, N₂, Cl₂.

Balancing (law of conservation of mass). Atoms are neither created nor destroyed, so each element must have the same number of atoms on both sides.
1. Write correct formulae; never change a subscript to balance.
2. Count atoms of each element on both sides.
3. Put whole-number coefficients in front of formulae; start with the element in the fewest substances and leave free elements (O₂, H₂) until last.
4. Recount.

Examples: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ ; $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$ ; $4 A l + 3 O_{2} \to 2 A l_{2} O_{3}$ ; $2 F e + 3 C l_{2} \to 2 F e C l_{3}$ ; $2 K C l O_{3} \to 2 K C l + 3 O_{2}$ .

Information from an equation.
1. The reactants and products.
2. The number of atoms and molecules taking part.
3. The relative masses: for $2 H_{2} + O_{2} \to 2 H_{2} O$ , 4 g of hydrogen reacts with 32 g of oxygen to give 36 g of water (H = 1, O = 16).

Limitations. An ordinary equation does not tell us:

the physical state (unless (s), (l), (g), (aq) are added);
the conditions (heat, light, pressure, temperature);
the catalyst, if any;
the speed of the reaction;
the concentration of the reactants;
whether heat is given out or taken in;
the nature of the reaction, e.g. reversible.

Some can be added: Δ above the arrow for heat, state symbols after the formulae.

Chemical Equations, Balancing, Information and Limitations: formulas and key facts

Balanced examples: $Z n + 2 H C l \to Z n C l_{2} + H_{2} 2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$; Count each atom.
More examples: $4 A l + 3 O_{2} \to 2 A l_{2} O_{3} 2 F e + 3 C l_{2} \to 2 F e C l_{3} 2 K C l O_{3} \to 2 K C l + 3 O_{2}$; Whole-number coefficients.
Masses from an equation: $2 H_{2} + O_{2} \to 2 H_{2} O : 4 g + 32 g \to 36 g$; H = 1, O = 16.

How to solve Chemical Equations, Balancing, Information and Limitations questions

Write correct formulae, list atoms on both sides, change coefficients only.

1Write the skeleton equation with correct formulae.
2Make a table of atoms on the left and right.
3Balance the element in the fewest substances first, leave O₂ and H₂ last.
4Recount every element.
5Add state symbols or conditions if asked.

Chemical Equations, Balancing, Information and Limitations 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 cooking gas

Suppose the main gas in a cooking cylinder is methane burning in air.

Balance $C H_{4} + O_{2} \to C O_{2} + H_{2} O$ .

Try it first, then show the solutionSolution

C: 1 and 1. H: 4 on the left needs $2 H_{2} O$ .
O: right side $2 + 2 = 4$ , so $2 O_{2}$ on the left.

Answer: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$

Memory hook: Carbon first, then hydrogen, oxygen last.

Water from hydrogen

Imagine 8 g of hydrogen burns completely in oxygen (use $2 H_{2} + O_{2} \to 2 H_{2} O$ , 4 g : 32 g : 36 g).

Find the oxygen needed and the water formed.

Try it first, then show the solutionSolution

The mass ratio is 4 : 32 : 36 so 8 g (twice 4) needs $2 \times 32 = 64$ g of oxygen.
Water $= 2 \times 36 = 72$ g.

Answer: 64 g oxygen; 72 g water.

Memory hook: Scale the ratio.

Tricks to remember Chemical Equations, Balancing, Information and Limitations

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

Count, change, recount
Count atoms, change coefficients, recount.
Use it when: Balancing.
Never touch subscripts
Changing a subscript changes the substance.
Use it when: Rule.
O₂ and H₂ last
Balance the free elements at the end.
Use it when: Order.

Chemical Equations, Balancing, Information and Limitations: solved examples

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

Example 1: Coefficient · 1 mark · MCQBoard level

In $4 A l + x O_{2} \to 2 A l_{2} O_{3}$ , the value of $x$ is:
(a) 1
(b) 2
(c) 3
(d) 4

Try it first, then show the solutionSolution

O on the right $= 2 \times 3 = 6$ , so $x = 3$ .

Answer: (c) 3

Example 2: Balance · 2 marks · Short answerBoard level

Balance: (a) $H_{2} + C l_{2} \to H C l$ , (b) $M g + O_{2} \to M g O$ .

Try it first, then show the solutionSolution

(a) $H_{2} + C l_{2} \to 2 H C l$ . (1)
(b) $2 M g + O_{2} \to 2 M g O$ . (1)

Answer: (a) H₂ + Cl₂ → 2HCl (b) 2Mg + O₂ → 2MgO

Example 3: From words to symbols · 3 marks · Short answerHarder board level

Write the balanced equation for: aluminium + hydrochloric acid → aluminium chloride + hydrogen. Show the atom count.

Try it first, then show the solutionSolution

Skeleton: $A l + H C l \to A l C l_{3} + H_{2}$ . (1)
Balanced: $2 A l + 6 H C l \to 2 A l C l_{3} + 3 H_{2}$ . (1)
Atoms: Al 2 = 2; H 6 = 6; Cl 6 = 6. (1)

Answer: 2Al + 6HCl → 2AlCl₃ + 3H₂

Example 4: Balance three · 4 marks · Long answerHarder board level

Balance: (a) $F e_{2} O_{3} + C O \to F e + C O_{2}$ , (b) $C H_{4} + O_{2} \to C O_{2} + H_{2} O$ , (c) $N a_{2} C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ .

Try it first, then show the solutionSolution

(a) $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$ . (1½)
(b) $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ . (1)
(c) $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$ . (1½)

Answer: See the working.

Example 5: What an equation says · 4 marks · Case-basedHOTS / competency

For $2 H_{2} + O_{2} \to 2 H_{2} O$ (H = 1, O = 16): (a) state three pieces of information, (b) state three things it does not tell, (c) find the oxygen needed for 8 g of hydrogen and the water formed.

Try it first, then show the solutionSolution

(a) Reactants and products; 2 molecules of H₂ react with 1 molecule of O₂ to give 2 molecules of water; 4 g + 32 g gives 36 g. (1)
(b) State of substances, conditions such as a spark or heat, heat change/speed. (1)
(c) 8 g H₂ needs $2 \times 32 = 64$ g O₂ and gives $2 \times 36 = 72$ g water. (2)

Answer: (c) 64 g oxygen, 72 g water

Where students lose marks in Chemical Equations, Balancing, Information and Limitations

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

Mistake: Changing subscripts
Subscripts define the substance.
Wrong working
H₂ + O₂ → H₂O₂
Right working
2H₂ + O₂ → 2H₂O
Do this: Change only coefficients.
Mistake: Not recounting
A single balance may upset another element.
Wrong working
"Al + O₂ → Al₂O₃ with 3 O₂ only."
Right working
"4Al + 3O₂ → 2Al₂O₃."
Do this: Recount all elements.
Mistake: Claiming the equation shows speed
It does not.
Wrong working
"The equation tells how fast."
Right working
"It does not show speed, conditions or catalyst."
Do this: Learn the limitation list.

Exam tips

Make a two-column atom table for every equation.
Use whole-number coefficients only.
Quote the law of conservation of mass as the reason.
For limitations, give at least four points.

Practice questions on Chemical Equations, Balancing, Information and Limitations

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
Which law decides that an equation must be balanced?
Hint
Mass.
Answer
The law of conservation of mass.
Q2 · 2 marks · Short answerHarder board level
Balance: $C a (O H)_{2} + H C l \to C a C l_{2} + H_{2} O$ .
Hint
Start with Cl.
Answer
$C a (O H)_{2} + 2 H C l \to C a C l_{2} + 2 H_{2} O$ .
Q3 · 3 marks · Short answerHarder board level
A student balances $H_{2} + O_{2} \to H_{2} O$ by writing $H_{2} + O_{2} \to H_{2} O_{2}$ . What is wrong? Give the correct equation.
Hint
Subscripts.
Answer
Changing a subscript changes the substance (hydrogen peroxide instead of water). Only coefficients may be changed: $2 H_{2} + O_{2} \to 2 H_{2} O$ .
Q4 · 2 marks · Short answerHarder board level
Rewrite $2 K C l O_{3} \to 2 K C l + 3 O_{2}$ with the conditions: heated, manganese dioxide as catalyst. Which limitation of the plain equation does this overcome?
Hint
Conditions on the arrow.
Answer
$2 K C l O_{3} M n O_{2} Δ 2 K C l + 3 O_{2}$ . It shows the heat and the catalyst, which a plain equation does not.
Q5 · 4 marks · Long answerHOTS / competency
Balance $A l + F e_{2} O_{3} \to A l_{2} O_{3} + F e$ . State the type of reaction, and which metal is more reactive.
Hint
Balance oxygen first.
Answer
$2 A l + F e_{2} O_{3} \to A l_{2} O_{3} + 2 F e$ (Al 2, Fe 2, O 3 on both sides). It is a displacement reaction; aluminium is more reactive than iron because it displaces iron from its oxide.

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.What law gives balancing?Show answer
Conservation of mass.
2.Can subscripts be changed to balance?Show answer
No.
3.Balanced equation of burning hydrogen?Show answer
2H₂ + O₂ → 2H₂O.
4.Does an equation show the catalyst?Show answer
No, unless added.
5.What does Δ mean over an arrow?Show answer
Heat.

Chemical Equations, Balancing, Information and Limitations: frequently asked questions

How do you balance a chemical equation?

Write correct formulae, count atoms of each element, add whole-number coefficients until the counts are equal on both sides, and recount.

What information does a chemical equation give?

The reactants and products, the number of molecules taking part and the relative masses reacting and formed.

What are the limitations of a chemical equation?

It does not show the physical state, conditions, catalyst, speed, concentration, heat change or the nature of the reaction unless these are added.