When we mix two or more materials, sometimes nothing new is made. Sand and pebbles, or salt and water, are mixtures: each material is still there and can be separated again. These are reversible changes.
Sometimes mixing makes a brand-new material. When vinegar is added to bicarbonate of soda, it fizzes and a gas called carbon dioxide is made. When iron is left in wet air it rusts. Burning wood makes ash and smoke. These changes are usually irreversible: we cannot get the starting materials back. Signs that a new material has formed include bubbles of gas, a colour change, a smell, light or heat, and a new solid appearing.
Worked example
Is rusting a reversible or an irreversible change?
- Iron joins with oxygen and water to make rust, a new material.
- We cannot turn rust back into shiny iron easily.
Answer:So rusting is an irreversible change.
Key words
- mixture
- two or more materials mixed but not joined
- reversible
- can be undone
- irreversible
- cannot be undone; a new material is made
- rust
- a brown material made when iron reacts with air and water
New questions every time.
NGSS-referenced: 5-PS1-4