Can I change a subscript to balance an equation?

No, and this is the mistake that costs the most marks. A subscript is part of the formula and changing it changes the substance — H2O is water and H2O2 is hydrogen peroxide. Only the coefficient in front of a formula may be changed, because that changes how many of the substance there are, not what it is.

What is a coefficient in a chemical equation?

The whole number written in front of a formula, multiplying every atom in it. In 3BaCl2 the coefficient is 3, giving 3 barium atoms and 6 chlorine atoms. A coefficient of 1 is never written.

Is there a fixed order to balance an equation in?

No, but there is a useful habit: take the element that appears in the fewest terms first, and leave an element standing on its own — oxygen or a free metal — until last, because it can be adjusted without disturbing anything else. Different orders reach the same balanced equation.

I balanced one element and now another one is wrong. Have I made a mistake?

Probably not. Changing a coefficient changes every element in that term, so settling one commonly unsettles two. The second worked example on this page does exactly that. Re-count and carry on rather than starting again.

Why does my balanced equation differ from the textbook’s?

If every element matches on both sides, it balances, and there is normally only one answer in lowest terms. Two things cause a genuine difference: coefficients that share a common factor, which should be divided down, and a formula that was wrong before the balancing began — a wrong formula cannot be fixed by any coefficient.

Do all the equations in a school textbook balance?

Not all of them. Two of the equations printed in the CBSE Class 7 chapter on physical and chemical changes are not balanced, and both are shown on this page. That is a reason to check rather than to copy: every equation on this site is atom-counted on every build.

Reference · Step III · ICSE / CBSE / SSLC

Balancing is arithmetic. Writing the formula is the hard part.

An equation is balanced when every element has the same number of atoms on both sides. You change the coefficient in front of a formula and never a subscript inside one — a coefficient changes how many there are, a subscript changes what the substance is. Count, adjust, re-count.

Everything on this page is counted rather than claimed. All 105 equations the three boards set are atom-counted on every build of this site, both sides, element by element — which is how the two that the CBSE Class 7 textbook prints without balancing are identified at the bottom of the page rather than being taken on trust.

Worked example 1 · CBSE, Class 10, Chapter I — Chemical Reactions and Equations

Iron(III) oxide and aluminium

One element at a time, and each one stays settled once it is settled.

Step 1 · Al, Fe still out

Write the skeleton: the correct formulas, and no coefficients yet. Oxygen already matches at three on each side, so it is not the one to start with.

Fe₂O₃ plus Al → Al₂O₃ plus Fe

Atoms on each side after step 1. Al and Fe still differ.

Side Al Fe O

Left 1 2 3

Right 2 1 3

Match no no yes
Step 2 · Al still out

Take iron first — it appears in one term on each side. There are two on the left and one on the right, so the right gets a 2.

Fe₂O₃ plus Al → Al₂O₃ plus 2Fe

Atoms on each side after step 2. Al still differ.

Side Al Fe O

Left 1 2 3

Right 2 2 3

Match no yes yes
Step 3 · balanced

Now aluminium: two on the right and one on the left, so the left gets a 2. Oxygen was never disturbed, and the equation closes.

Fe₂O₃ plus 2Al → Al₂O₃ plus 2Fe

Atoms on each side after step 3. Every element matches, so the equation is balanced.

Side Al Fe O

Left 2 2 3

Right 2 2 3

Match yes yes yes

Atoms on each side after step 1. Al and Fe still differ.
Side	Al	Fe	O
Left	1	2	3
Right	2	1	3
Match	no	no	yes

Atoms on each side after step 2. Al still differ.
Side	Al	Fe	O
Left	1	2	3
Right	2	2	3
Match	no	yes	yes

Atoms on each side after step 3. Every element matches, so the equation is balanced.
Side	Al	Fe	O
Left	2	2	3
Right	2	2	3
Match	yes	yes	yes

Worked example 2 · CBSE, Class 10, Chapter I — Chemical Reactions and Equations

Aluminium and copper(II) chloride

Settling one element unsettles two others. This is normal, and it is not a mistake.

Step 1 · Cl still out

The skeleton. Aluminium and copper each match at one, and only chlorine is out — two on the left against three on the right.

Al plus CuCl₂ → AlCl₃ plus Cu

Atoms on each side after step 1. Cl still differ.

Side Al Cl Cu

Left 1 2 1

Right 1 3 1

Match yes no yes
Step 2 · Al, Cu still out

Chlorine needs a count both sides can reach, and the lowest is six: three lots of two on the left, two lots of three on the right.

Al plus 3CuCl₂ → 2AlCl₃ plus Cu

Atoms on each side after step 2. Al and Cu still differ.

Side Al Cl Cu

Left 1 6 3

Right 2 6 1

Match no yes no
Step 3 · Cu still out

Chlorine is settled and aluminium and copper are now both out — which is the ordinary course of it, not an error. Aluminium is two on the right, so the left gets a 2.

2Al plus 3CuCl₂ → 2AlCl₃ plus Cu

Atoms on each side after step 3. Cu still differ.

Side Al Cl Cu

Left 2 6 3

Right 2 6 1

Match yes yes no
Step 4 · balanced

Copper last, because it stands alone on the right and moving it disturbs nothing else. Three on the left, so three on the right.

2Al plus 3CuCl₂ → 2AlCl₃ plus 3Cu

Atoms on each side after step 4. Every element matches, so the equation is balanced.

Side Al Cl Cu

Left 2 6 3

Right 2 6 3

Match yes yes yes

Atoms on each side after step 1. Cl still differ.
Side	Al	Cl	Cu
Left	1	2	1
Right	1	3	1
Match	yes	no	yes

Atoms on each side after step 2. Al and Cu still differ.
Side	Al	Cl	Cu
Left	1	6	3
Right	2	6	1
Match	no	yes	no

Atoms on each side after step 3. Cu still differ.
Side	Al	Cl	Cu
Left	2	6	3
Right	2	6	1
Match	yes	yes	no

Atoms on each side after step 4. Every element matches, so the equation is balanced.
Side	Al	Cl	Cu
Left	2	6	3
Right	2	6	3
Match	yes	yes	yes

Why the formula has to be right first

No coefficient can rescue a wrong formula. A student who writes calcium hydroxide as CaOH instead of Ca(OH)₂ can balance the equation they have written perfectly and still be marked wrong, because the substance on the page is not the one in the reaction. The same is true of Fe₂O₃ written as FeO₃: it will balance against something, and against nothing the question asked.

That is why this is Step III and not Step I. A formula comes from two valencies crossed over, so the order runs valency, then formula, then equation — and a student who is still guessing at valencies has no business balancing anything yet. The valency chart is the first of those, and the formula index is the second.

What your board sets, and when it explains it

Every board sets equations to balance. Every board also explains how to write a formula after it has already asked for one — and the Kerala SSLC syllabus explains balancing in Class 8, before either has been explained at all.

Equations set by each board's own textbooks, with the chapter in which writing a formula and balancing an equation are first explained. Compiled from the textbooks, chapter by chapter.
Board	Classes	Equations set	Formula writing explained	Balancing explained
ICSE	Classes 6 to 9	0	Class 9, Theme 1 — valency and the formulas of radicals, without which a formula cannot be worked out	Class 9, Theme 1 — valency, the formulas of radicals, and balancing
CBSE	Classes 7 to 10	70	Class 9, Chapter III — Atoms and Molecules	Class 10, Chapter I — Chemical Reactions and Equations
SSLC	Classes 8 to 10	51	Class 9	Class 8, Part I, Chapter IV — Basic Constituents of Matter

The ICSE inventory is recorded by theme rather than enumerated, because the Council publishes themes rather than a compound list — so its count is lower here without its syllabus being lighter. Each board's full chapter-by-chapter list is on its own page.

Two equations the textbook prints unbalanced

These are reproduced exactly as printed, and they do not balance. They are found by counting rather than from a list: this site atom-counts every equation in every inventory on every build, and an equation is allowed to fail only where the transcription records that the textbook itself prints it that way.

CBSE · Class 7 · Chapter VI — Physical and Chemical Changes

Mg plus O₂ → MgO

Counted: O does not match.
CBSE · Class 7 · Chapter VI — Physical and Chemical Changes

Fe plus O₂ plus H₂O → Fe₂O₃

Counted: Fe and H do not match.

The first is short of oxygen and takes a coefficient on each side to close. The second cannot be balanced as written at all — the hydrogen on the left has nowhere to go on the right, so no set of coefficients exists and a product is missing rather than a number. Neither is a reason to distrust a textbook wholesale, and both are a reason to count rather than copy, which is the habit this page is really about.

Questions

Can I change a subscript to balance an equation?: No, and this is the mistake that costs the most marks. A subscript is part of the formula and changing it changes the substance — H₂O is water and H₂O₂ is hydrogen peroxide. Only the coefficient in front of a formula may be changed, because that changes how many of the substance there are, not what it is.
What is a coefficient in a chemical equation?: The whole number written in front of a formula, multiplying every atom in it. In 3BaCl₂ the coefficient is 3, giving 3 barium atoms and 6 chlorine atoms. A coefficient of 1 is never written.
Is there a fixed order to balance an equation in?: No, but there is a useful habit: take the element that appears in the fewest terms first, and leave an element standing on its own — oxygen or a free metal — until last, because it can be adjusted without disturbing anything else. Different orders reach the same balanced equation.
I balanced one element and now another one is wrong. Have I made a mistake?: Probably not. Changing a coefficient changes every element in that term, so settling one commonly unsettles two. The second worked example on this page does exactly that. Re-count and carry on rather than starting again.
Why does my balanced equation differ from the textbook’s?: If every element matches on both sides, it balances, and there is normally only one answer in lowest terms. Two things cause a genuine difference: coefficients that share a common factor, which should be divided down, and a formula that was wrong before the balancing began — a wrong formula cannot be fixed by any coefficient.
Do all the equations in a school textbook balance?: Not all of them. Two of the equations printed in the CBSE Class 7 chapter on physical and chemical changes are not balanced, and both are shown on this page. That is a reason to check rather than to copy: every equation on this site is atom-counted on every build.

This page is Step III. Two come before it.

Alphabets, then words, then sentences. Balancing is the third of those and the one a student cannot reach while the first two are still being guessed at. In The Language of Chemistry, Playbook® that order is enforced rather than suggested: Chapter III does not open until the two chapters before it have been passed to a score.

See how the method works Equations to balance, with answers