Conservation of mass: Year 7 introduction

Understand the key idea

In a chemical reaction, atoms are rearranged but never created or destroyed, so the total mass of the products equals the total mass of the reactants. The same is true for changes of state, because the particles are unchanged and only their arrangement differs.

Common mix-up

In a chemical reaction, a measured mass can fall if gas escapes or rise if gas from the air joins the reaction. The total mass is conserved when all reactants and products are included. A closed system keeps matter from entering or leaving; an open container can also show unchanged mass when no matter crosses its boundary.

Try this first

1. Balance a simple equation and count atoms on each side.
2. Explain why a burning candle seems to lose mass.
3. Predict the mass reading if the same reaction runs in a sealed flask.

Ask: If a log burns down to a little ash, where did the rest of it go?