ChemistryICSE Class 8 · Chapter 5

Language of Chemistry ICSE Class 8: Symbols, Formulae and Balancing Equations

Updated 15 October 2026 · Fact-checked

Chemistry uses symbols for elements (Na, Cl), formulae for compounds (NaCl, H₂O) and chemical equations for reactions. ICSE Class 8 writes formulae using valencies, turns word equations into chemical equations, balances them so that atoms of each element are equal on both sides, as the law of conservation of mass requires, and states what an equation cannot show.

What Language of Chemistry is about

Chemists everywhere use the same short language:

a symbol stands for an element (H for hydrogen, Na for sodium from its Latin name *natrium*);
a formula shows the elements in a compound and the number of atoms of each (H₂O: 2 hydrogen atoms and 1 oxygen atom);
a chemical equation shows a reaction in symbols and formulae.

The CISCE Class VIII curriculum asks you to:

use symbols of elements and write formulae of compounds;
write chemical equations from word equations;
relate the law of conservation of mass to balancing simple equations;
state the information a chemical equation gives;
state its limitations: it does not tell us the catalyst, the conditions, the state of reactants and products, or the nature of the reaction unless these are added.

A symbol written with a number in front (2H) means separate atoms; a subscript (H₂) means atoms joined in one molecule.

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Language of Chemistry sits in the Theme: Language of Chemistry theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Symbols and valencies

A symbol is one or two letters for an element: the first capital, the second small (C, Ca, Cl, Co). Some come from Latin names: Na (sodium), K (potassium), Fe (iron), Cu (copper), Ag (silver), Pb (lead).

Valency is the combining capacity. Common valencies:

Valency 1	Valency 2	Valency 3
H, Na, K, Ag; Cl, Br; OH, NO₃, NH₄	O, Mg, Ca, Zn, Cu, Fe (ferrous); SO₄, CO₃	Al, N (in NH₃), Fe (ferric); PO₄

Groups of atoms such as SO₄, CO₃, NO₃, OH and NH₄ act together as a unit called a radical.

Worked example

Write the symbols for sodium, potassium, iron and chlorine, and give the valency of each in common compounds.

Try it first, then show the solutionSolution

Sodium Na, potassium K: valency 1.
Iron Fe: valency 2 (ferrous) or 3 (ferric); chlorine Cl: valency 1.

Answer: Na 1, K 1, Fe 2 or 3, Cl 1

Writing formulae: the criss-cross method

1. Write the symbols (metal or positive part first).
2. Write the valency above each.
3. Criss-cross: the valency of each becomes the subscript of the other.
4. Reduce to the simplest whole-number ratio, and leave out subscript 1.
5. Put a radical in brackets if more than one is needed: Ca(OH)₂, Al₂(SO₄)₃.

Examples: Al (3) and O (2) → Al₂O₃; Ca (2) and Cl (1) → CaCl₂; Mg (2) and O (2) → Mg₂O₂ → MgO.

Worked example

Write the formula of calcium phosphate (Ca valency 2, PO₄ valency 3).

Try it first, then show the solutionSolution

Criss-cross: Ca gets subscript 3 and PO₄ gets subscript 2.
The phosphate radical needs brackets: $C a_{3} (P O_{4})_{2}$ .

Answer: Ca₃(PO₄)₂

From word equation to chemical equation

A word equation names the reactants and products:
zinc + hydrochloric acid → zinc chloride + hydrogen

To write the chemical (skeleton) equation, replace each name by its correct symbol or formula:
$Z n + H C l \to Z n C l_{2} + H_{2}$ (unbalanced)

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

The arrow means 'gives' or 'forms'; reactants on the left, products on the right; '+' separates substances.

Worked example

Write the skeleton equation for: calcium carbonate → calcium oxide + carbon dioxide. Is it balanced?

Try it first, then show the solutionSolution

$C a C O_{3} \to C a O + C O_{2}$ .
Count: Ca 1 = 1, C 1 = 1, O 3 = 1 + 2. It is already balanced.

Answer: CaCO₃ → CaO + CO₂ (balanced)

Balancing equations

By the law of conservation of mass, atoms are neither created nor destroyed, so a correct equation has the same number of atoms of each element on both sides.

Steps:
1. Write the correct formulae; never change a formula (subscripts) to balance.
2. Count atoms of each element on both sides.
3. Put whole-number coefficients in front of formulae to make the counts equal, starting with the element that appears in the fewest substances (leave free elements such as O₂ or H₂ until last).
4. Recount every element.

Worked example

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

Try it first, then show the solutionSolution

Oxygen: 2 on the left, 1 on the right: put 2 before H₂O: $H_{2} + O_{2} \to 2 H_{2} O$ (still unbalanced).
Hydrogen is now 2 on the left and 4 on the right: put 2 before H₂.
$2 H_{2} + O_{2} \to 2 H_{2} O$ ; check: H 4 = 4, O 2 = 2 ✓.

Answer: 2H₂ + O₂ → 2H₂O

Information from a chemical equation

A balanced equation tells us:
1. the reactants and products (names and formulae);
2. the number of atoms and molecules taking part, and their ratio;
3. the relative masses that react and form (using formula masses), consistent with conservation of mass.

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

Worked example

What information does $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ give? Check mass conservation (C = 12, H = 1, O = 16).

Try it first, then show the solutionSolution

Methane reacts with oxygen to give carbon dioxide and water; 1 molecule of CH₄ needs 2 molecules of O₂.
Masses: $16 + 64 = 80$ on the left; $44 + 36 = 80$ on the right ✓.

Answer: Substances, molecule ratio 1 : 2 : 1 : 2, and masses 80 = 80

Limitations of a chemical equation

An ordinary equation does not tell us:

the physical state of reactants and products (solid, liquid, gas, solution), unless (s), (l), (g), (aq) are added;
the conditions needed: heat, light, pressure, temperature;
the catalyst used, if any;
the speed of the reaction, or whether it happens at all;
the concentration of the reactants (dilute or concentrated);
whether heat is given out or taken in;
the nature of the reaction (for example, whether it is reversible).

Some of this can be added: symbols written above or below the arrow show conditions ( $Δ$ for heat), and state symbols show states.

Worked example

The equation $N_{2} + 3 H_{2} \to 2 N H_{3}$ is balanced. Give three things it does not tell you.

Try it first, then show the solutionSolution

It does not show the temperature and pressure needed, or that a catalyst (iron) is used.
It does not show that the reaction is reversible, or how fast it is.

Answer: Conditions, catalyst, reversibility (also speed and states)

Chemistry in real life: Language of Chemistry

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.

Equation and masses

Suppose a fuel cell in a space vehicle combines hydrogen and oxygen to make drinking water.

Write the balanced equation and the mass of water made from 4 g of hydrogen (H = 1, O = 16).

Try it first, then show the solutionSolution

$2 H_{2} + O_{2} \to 2 H_{2} O$ .
4 g of hydrogen reacts with 32 g of oxygen to give $4 + 32 = 36$ g of water.

Answer: 36 g of water

Memory hook: Masses balance just like atoms.

Cooking gas

Imagine the methane in a gas stove burning in air.

Write the balanced equation and name the products.

Try it first, then show the solutionSolution

Word equation: methane + oxygen → carbon dioxide + water.
$C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ .

Answer: Carbon dioxide and water

Memory hook: Fuels burn to CO₂ and water.

Limitations

Suppose you read $C a C O_{3} \to C a O + C O_{2}$ in a book about making lime.

What important fact about the conditions is missing?

Try it first, then show the solutionSolution

Limestone must be heated strongly in a kiln for this to happen.
The plain equation does not show the heating; it can be shown by writing Δ (heat) over the arrow.

Answer: Strong heating is needed

Memory hook: Equations hide the conditions.

Topics in this chapter

Language of Chemistry formula sheet

Water formation: $2 H_{2} + O_{2} \to 2 H_{2} O$; 4 g + 32 g → 36 g.
Sodium and water: $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$; Balanced example.
Aluminium and oxygen: $4 A l + 3 O_{2} \to 2 A l_{2} O_{3}$; Balance O first, then Al.
Iron and chlorine: $2 F e + 3 C l_{2} \to 2 F e C l_{3}$; Ferric chloride.
Potassium chlorate: $2 K C l O_{3} \to 2 K C l + 3 O_{2}$; Heated with a catalyst (not shown by the equation).

How the school exam asks Language of Chemistry

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Symbols and valency	Write the symbols of iron, sodium, lead; give the valency of the sulphate radical.	Latin-name symbols; learn the valency table.
Writing formulae	Write the formulae of aluminium sulphate, ammonium sulphate, magnesium hydroxide.	Criss-cross valencies; brackets for multiple radicals; simplify.
Word to chemical equation	Convert a word equation into a balanced chemical equation.	Correct formulae first; gases as molecules; then balance.
Balancing	Balance Na + H₂O → NaOH + H₂; Al + O₂ → Al₂O₃.	Coefficients only; leave free elements to last; recount.
Information and limitations	What does this equation tell us, and what does it not tell us?	Substances, ratios, masses; not states, conditions, catalyst, speed, heat, nature.

Tricks to remember Language of Chemistry

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

Criss-cross and cancel
Swap the valencies down as subscripts, then cancel any common factor.
Use it when: Writing formulae.
Coefficients, never subscripts
Change the big numbers in front, never the small ones inside a formula.
Use it when: Balancing.
HONClBrIF
Hydrogen, oxygen, nitrogen, chlorine, bromine, iodine, fluorine exist as two-atom molecules (H₂, O₂, N₂ ...).
Use it when: Writing gases correctly.
SCC-SSHN
An equation does not tell: State, Conditions, Catalyst, Speed, Strength (concentration), Heat, Nature.
Use it when: Limitations.

Language of Chemistry: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Formula of aluminium oxide · 1 mark · MCQExam level

The correct formula of aluminium oxide is:
(a) AlO
(b) Al₂O₃
(c) Al₃O₂
(d) AlO₃

Try it first, then show the solutionSolution

Al has valency 3 and O has valency 2.
Criss-cross: Al₂O₃.

Answer: (b)

Example 2: Two formulae · 2 marks · Short answerExam level

Write the formulae of (i) magnesium hydroxide (ii) ammonium sulphate.

Try it first, then show the solutionSolution

(i) Mg (2), OH (1): $M g (O H)_{2}$ .
(ii) NH₄ (1), SO₄ (2): $(N H_{4})_{2} S O_{4}$ .

Answer: Mg(OH)₂; (NH₄)₂SO₄

Example 3: Word to balanced equation · 3 marks · Short answerExam level

Write a balanced chemical equation for: zinc + hydrochloric acid → zinc chloride + hydrogen.

Try it first, then show the solutionSolution

Skeletal (unbalanced) equation: $Z n + H C l \to Z n C l_{2} + H_{2}$ .
Cl: 1 on the left, 2 on the right; H: 1 on the left, 2 on the right: put 2 before HCl.
$Z n + 2 H C l \to Z n C l_{2} + H_{2}$ (Zn 1, H 2, Cl 2 on each side).

Answer: Zn + 2HCl → ZnCl₂ + H₂

Example 4: Balancing with sodium · 3 marks · Short answerHarder exam level

Balance: $N a + H_{2} O \to N a O H + H_{2}$ .

Try it first, then show the solutionSolution

H: 2 on the left, 3 on the right. Put 2 before H₂O and 2 before NaOH: H is 4 = 2 + 2.
Na: now 2 on the right, so put 2 before Na.
$2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$ ; O 2 = 2 ✓.

Answer: 2Na + 2H₂O → 2NaOH + H₂

Example 5: Balancing with aluminium · 3 marks · Short answerHarder exam level

Balance: $A l + O_{2} \to A l_{2} O_{3}$ .

Try it first, then show the solutionSolution

O: 2 on the left, 3 on the right. The LCM of 2 and 3 is 6: use $3 O_{2}$ and $2 A l_{2} O_{3}$ .
Al: now 4 on the right, so use $4 A l$ .
$4 A l + 3 O_{2} \to 2 A l_{2} O_{3}$ (Al 4, O 6 on each side).

Answer: 4Al + 3O₂ → 2Al₂O₃

Example 6: Information from an equation · 4 marks · Long answerHarder exam level

From $2 H_{2} + O_{2} \to 2 H_{2} O$ , state (i) the reactants and product (ii) the ratio of molecules (iii) the masses that react and form (H = 1, O = 16), and show they obey the law of conservation of mass.

Try it first, then show the solutionSolution

(i) Reactants: hydrogen and oxygen; product: water.
(ii) 2 molecules of H₂ : 1 molecule of O₂ : 2 molecules of H₂O.
(iii) $2 \times 2 = 4$ g H₂ and $32$ g O₂ react; $2 \times 18 = 36$ g H₂O forms.
$4 + 32 = 36$ : total mass is conserved.

Answer: As above

Example 7: Balance and criticise · 4 marks · Long answerHOTS / competency

Potassium chlorate is heated with manganese dioxide to give potassium chloride and oxygen. (i) Write the balanced equation. (ii) A student 'balances' it as $K C l O_{3} \to K C l O + O_{2}$ . What is wrong? (iii) Name two pieces of information missing from your equation.

Try it first, then show the solutionSolution

(i) $2 K C l O_{3} \to 2 K C l + 3 O_{2}$ (K 2, Cl 2, O 6 on each side).
(ii) The student has changed the product's formula: KClO is a different substance, not potassium chloride.
Formulae must never be changed to balance an equation; only coefficients can be changed.
(iii) The need for heating and the catalyst (manganese dioxide) are not shown.

Answer: 2KClO₃ → 2KCl + 3O₂; changing formulae is not allowed; heat and catalyst missing

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Rusting and burning iron

Suppose students study reactions of iron: iron wool burns in oxygen to form magnetic iron oxide, Fe₃O₄; hot iron reacts with chlorine to form iron(III) chloride, FeCl₃; an iron nail in copper sulphate solution gets coated with copper.

(i) · 2 marks
Write the balanced equation for iron burning in oxygen to form Fe₃O₄.
Answer
Answer: 3Fe + 2O₂ → Fe₃O₄
Fe: 3 on the right, so 3Fe. O: 4 on the right, so 2O₂. $3 F e + 2 O_{2} \to F e_{3} O_{4}$ .
(ii) · 3 marks
Write and balance the equation for iron reacting with chlorine.
Answer
Answer: 2Fe + 3Cl₂ → 2FeCl₃
Cl: 2 on the left, 3 on the right; LCM 6: $3 C l_{2}$ and $2 F e C l_{3}$ . Then Fe: 2. $2 F e + 3 C l_{2} \to 2 F e C l_{3}$ .
(iii) · 3 marks
Write the equation for iron with copper sulphate. State two things it does not tell you about this reaction.
Answer
Answer: Fe + CuSO₄ → FeSO₄ + Cu
$F e + C u S O_{4} \to F e S O_{4} + C u$ . It does not show that the reactants are in solution (states), the colour change from blue to green, or how fast the coating forms.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Write the formula of aluminium sulphate. (2 marks)

Step	Marks
Valencies: Al 3, SO₄ 2	1
Criss-cross with brackets: $A l_{2} (S O_{4})_{3}$	1

Balance the unbalanced equation $N_{2} + H_{2} \to N H_{3}$ , and state the molecule ratio. (3 marks)

Step	Marks
N: 2 on the left, so $2 N H_{3}$	1
H: 6 on the right, so $3 H_{2}$	1
$N_{2} + 3 H_{2} \to 2 N H_{3}$ ; ratio 1 : 3 : 2	1

Balance the unbalanced equation $C H_{4} + O_{2} \to C O_{2} + H_{2} O$ and state two limitations of the balanced equation. (4 marks)

Step	Marks
H: 4 on the left, so $2 H_{2} O$	1
O on the right: $2 + 2 = 4$ , so $2 O_{2}$	1
$C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$	1
Limitations: does not show that heat and light are given out, or the states (gas, vapour)	1

Where students lose marks in Language of Chemistry

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

Mistake: Changing subscripts to balance
That changes the substance.
Wrong working
$H_{2} + O_{2} \to H_{2} O_{2}$
Right working
$2 H_{2} + O_{2} \to 2 H_{2} O$
Do this: Only add coefficients in front.
Usually costs: The whole equation.
Mistake: Writing gases as single atoms
Hydrogen, oxygen, nitrogen and chlorine gases are molecules.
Wrong working
$Z n + 2 H C l \to Z n C l_{2} + 2 H$
Right working
$Z n + 2 H C l \to Z n C l_{2} + H_{2}$
Do this: Write H₂, O₂, N₂, Cl₂.
Usually costs: 1 mark.
Mistake: Missing brackets round radicals
CaOH₂ is not the same as Ca(OH)₂.
Do this: Use brackets when a radical has a subscript greater than 1.
Usually costs: 1 mark.
Mistake: Wrong capital letters
Co (cobalt) and CO (carbon monoxide) are different.
Do this: Capital first letter, small second letter.
Usually costs: 1 mark.
Mistake: Not recounting after balancing
Fixing one element often unbalances another.
Do this: Recount every element at the end.
Usually costs: 1 mark.

Practice questions on Language of Chemistry

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 · MCQExam level
The formula of calcium hydroxide is:
(a) CaOH
(b) Ca(OH)₂
(c) Ca₂OH
(d) CaOH₂
Hint
Ca 2, OH 1.
Answer
(b)
Q2 · 1 mark · Very short answerExam level
Write the symbol of lead.
Hint
Latin plumbum.
Answer
Pb.
Q3 · 2 marks · Short answerExam level
Write the formulae of sodium sulphate and aluminium chloride.
Hint
Criss-cross.
Answer
$N a_{2} S O_{4}$ ; $A l C l_{3}$ .
Q4 · 2 marks · Short answerHarder exam level
Balance: $C a + H_{2} O \to C a (O H)_{2} + H_{2}$ .
Hint
Two OH groups need two H₂O.
Answer
$C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$ .
Q5 · 1 mark · Assertion-reasonHarder exam level
Assertion (A): A chemical equation must be balanced.
Reason (R): Atoms are neither created nor destroyed in a chemical reaction.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Hint
Conservation of mass.
Answer
(c)
Q6 · 2 marks · Short answerHarder exam level
Write a balanced equation for: hydrogen peroxide → water + oxygen.
Hint
Balance O last.
Answer
$2 H_{2} O_{2} \to 2 H_{2} O + O_{2}$ .
Q7 · 3 marks · Short answerHarder exam level
Write a balanced equation for: sodium hydroxide + hydrochloric acid → sodium chloride + water. What does it tell you about the numbers of molecules?
Hint
Already balanced.
Answer
$N a O H + H C l \to N a C l + H_{2} O$ ; one molecule of each reactant gives one of each product (ratio 1 : 1 : 1 : 1).
Q8 · 3 marks · Short answerHarder exam level
Write the formula of calcium phosphate and find its formula mass (Ca = 40, P = 31, O = 16).
Hint
Ca₃(PO₄)₂.
Answer
$C a_{3} (P O_{4})_{2}$ : $3 \times 40 + 2 \times (31 + 64) = 120 + 190 = 310$ .
Q9 · 2 marks · Short answerHarder exam level
Balance: $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ and $C + O_{2} \to C O_{2}$ . What do you notice?
Hint
Count atoms.
Answer
Both are already balanced as written: each element has equal atoms on both sides.
Q10 · 2 marks · Short answerHarder exam level
State four limitations of a chemical equation.
Hint
Think of states, conditions, catalyst and speed.
Answer
It does not show the physical states, the conditions (heat, pressure), the catalyst, or the speed of the reaction (also concentration and heat change).
Q11 · 3 marks · Short answerHOTS / competency
Balance $A l + C l_{2} \to A l C l_{3}$ and explain why you cannot simply write $A l + C l_{3} \to A l C l_{3}$ .
Hint
Chlorine gas is Cl₂.
Answer
$2 A l + 3 C l_{2} \to 2 A l C l_{3}$ (Al 2, Cl 6 on each side). Chlorine exists as Cl₂ molecules; 'Cl₃' is not a real substance, so its formula cannot be changed to make the equation balance.
Q12 · 3 marks · Short answerHOTS / competency
In $C a C O_{3} \to C a O + C O_{2}$ , 100 g of calcium carbonate gives 56 g of calcium oxide. How much carbon dioxide forms? Show this agrees with the formula masses (Ca 40, C 12, O 16).
Hint
Conservation of mass.
Answer
$100 - 56 = 44$ g. Formula masses: CaCO₃ $= 100$ , CaO $= 56$ , CO₂ $= 44$ , and $56 + 44 = 100$ ✓.

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.Symbol of sodium?Show answer
Na.
2.Symbol of iron?Show answer
Fe.
3.Valency of the sulphate radical?Show answer
2.
4.Formula of calcium chloride?Show answer
CaCl₂.
5.Which law requires equations to be balanced?Show answer
The law of conservation of mass.
6.Balance H₂ + Cl₂ → HCl.Show answer
H₂ + Cl₂ → 2HCl.
7.What does Δ over the arrow mean?Show answer
Heat is applied.
8.Name two limitations of a chemical equation.Show answer
It does not show the conditions or the catalyst (also states, speed, heat change).

Best order to study it

Learn the symbols of common elements, including Latin-name ones.
Learn the valency table and radicals.
Practise writing formulae by criss-cross.
Turn word equations into skeleton equations and balance them.
Finish with the information from equations and their limitations.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Are your symbols written with the correct capitals?
Did you criss-cross valencies and simplify?
Did you use brackets for multiple radicals?
Are gases written as molecules?
Did you balance with coefficients only and recount?
Can you list five limitations of an equation?

Language of Chemistry: frequently asked questions

Why must chemical equations be balanced?

Atoms are not created or destroyed in a reaction (law of conservation of mass), so each element must have the same number of atoms on both sides.

What is the difference between 2H and H₂?

2H means two separate hydrogen atoms; H₂ means one molecule made of two hydrogen atoms joined together.

Why is MgO written instead of Mg₂O₂?

Formulae are written in the simplest whole-number ratio, so 2 : 2 is reduced to 1 : 1.

How can conditions be shown in an equation?

Write them above or below the arrow, for example Δ for heat or the name of a catalyst, and add state symbols (s), (l), (g), (aq).

Does a balanced equation tell us a reaction will happen?

No. It only shows what happens if the reaction takes place; it does not show whether, or how fast, it occurs.

Based on: CISCE Curriculum for Upper Primary Classes (VI-VIII), Science: Chemistry (Class VIII); CISCE RDCD: Curriculum Pre-School to Class VIII