# Conservation of mass

The total mass of the substances after a reaction equals the total mass before. Atoms are not made or destroyed, only rearranged.

Chemistry (school) · Part 04: Chemical reactions · Card 2 of 9 · Last updated 2026-10-19

## Analogy

Like moving furniture between rooms. The house contains the same furniture afterwards.

## Example

**Mass check**

Magnesium 24 g + oxygen 16 g
= magnesium oxide 40 g
Mass before = mass after

## In practice

If a metal seems to gain mass when it burns, it has joined with oxygen from the air.

## Practice

### Flashcards

- **What is the law of conservation of mass?** The total mass stays the same during a reaction because atoms are not made or destroyed.

### 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 does a reaction in an open flask seem to lose mass?

If a gas is made, it escapes into the air and takes its mass with it, so the flask reads lighter. In a sealed container the mass stays the same. Mass is always conserved. It is just that some of it left the flask.

### Who discovered the conservation of mass?

Antoine Lavoisier, a French chemist in the 1700s, is usually credited. He weighed everything carefully before and after reactions in sealed vessels and found the total mass did not change. This idea became a foundation of modern chemistry. Later work on atoms explained why: atoms are only rearranged in reactions.

## Related concepts

- [Balanced equation](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-4/chemistry-school-part-4-concept-3)
- [Relative atomic mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-1)
- [Relative formula mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-2)

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