# Balanced equation

A symbol equation with the same number of each type of atom on both sides. You balance it by changing the big numbers in front of formulae, never the small numbers inside.

Chemistry (school) · Part 04: Chemical reactions · Card 3 of 9 · Last updated 2026-10-19

## Analogy

Like a set of scales, each side must hold the same.

## Example

**Balancing water**

H2 + O2 -> H2O (unbalanced)
2H2 + O2 -> 2H2O
Left: 4 H, 2 O
Right: 4 H, 2 O

## In practice

Hydrogen burning in oxygen needs two H2 for each O2 to make water.

## Practice

### Flashcards

- **What do you change when balancing an equation?** Only the big numbers in front of formulae, never the small ones.

### Interview questions

**Balance the equation Mg + O2 -> MgO and explain what balancing shows.**

- 2Mg + O2 -> 2MgO
- Same number of each atom on both sides
- Atoms are conserved
- Mass is conserved

## Questions people ask

### Why can't you change the small numbers when balancing?

The small numbers are part of the formula and show what the substance is. Changing H2O to H2O2 would make hydrogen peroxide, a different compound. Only the big numbers in front tell you how many of each unit take part, so they are the ones to adjust.

### What are the steps for balancing an equation?

Write the correct formulae. Count the atoms of each element on both sides. Add a big number in front of a formula to fix one element at a time. Recount everything and check both sides match. Leave the element appearing alone, such as oxygen, until last.

## Related concepts

- [Conservation of mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-4/chemistry-school-part-4-concept-2)
- [State symbols](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-4/chemistry-school-part-4-concept-4)

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