Why can I not change the small numbers?

The subscripts define the compound. Change them and you have written a different substance, so the equation no longer describes the reaction you were asked about.

Which element should I balance first?

Metals first, then other non-metals, then hydrogen, and oxygen last. Oxygen appears in the most places, so leaving it to the end saves you from going round in circles.

What are state symbols for?

They say what physical form each substance is in: solid, liquid, gas, or dissolved in water. Examiners often award a separate mark for them.

Virtual experiment

Balancing chemical equations

Put a number in front of each formula until both sides have the same atoms. Pick your level.

1/6Equation

0Marks

0Streak

Reaction bench Not balanced

Atom tallyLive

Mass balanceRelative

Left 0 Right 0

Mass not conserved

Method

01Count every element on both sides first.
02Balance metals first, then other non-metals.
03Leave hydrogen second-to-last, oxygen last.
04Treat groups like SO₄ or NO₃ as one unit if they survive the reaction.
05If you end on a half, double every coefficient.

Session complete

0Marks

0%First-try correct

0Hints used

0:00Time taken

Every equation you attempted

Back to the digital lab

What this experiment shows

Balancing an equation is bookkeeping for atoms. Matter is not created or destroyed in a chemical change, so every element must appear the same number of times on both sides. You may change the big numbers in front of a formula - the coefficients - but never the small numbers inside it, because changing those changes the substance itself. This bench gives you equations to balance and tells you which element is still out.

CH₄ + 2O₂ → CO₂ + 2H₂O

What you need

No apparatus - this one is pencil and paper, done on screen
A periodic table for formulae you are unsure of

How it is done

Write the correct formula for every reactant and product first, and then leave them alone.
Count each element on both sides and list the ones that do not match.
Balance metals first, then non-metals, then hydrogen, and leave oxygen until last.
Put coefficients in front of whole formulae only.
Count again from scratch. If the numbers all divide by a common factor, divide them.

What you should see

A balanced equation has the same number of atoms of each element on both sides.
The total charge must balance too, in an ionic equation.
State symbols - (s), (l), (g), (aq) - are usually worth a mark of their own.

Where marks are lost

Changing a subscript to make the count work. H₂O and H₂O₂ are different compounds.
Leaving a coefficient of 1 written in. It is understood, so it is not written.
Finishing with coefficients that share a common factor, such as 2, 4, 2. Divide through.
Forgetting state symbols when the question asks for them.

Questions students ask

Why can I not change the small numbers?: The subscripts define the compound. Change them and you have written a different substance, so the equation no longer describes the reaction you were asked about.
Which element should I balance first?: Metals first, then other non-metals, then hydrogen, and oxygen last. Oxygen appears in the most places, so leaving it to the end saves you from going round in circles.
What are state symbols for?: They say what physical form each substance is in: solid, liquid, gas, or dissolved in water. Examiners often award a separate mark for them.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.