Changes of State and Reversible and Irreversible Changes: Year 6 Study Guide
Some changes can be undone and some cannot. Knowing the difference helps in cooking, cleaning and industry.
Matter exists as a solid, liquid or gas. Heating adds energy and particles move faster. Melting changes solid to liquid, boiling or evaporation changes liquid to gas, condensation changes gas to liquid and freezing changes liquid to solid. Water melts and freezes at 0 °C and boils at 100 °C at sea level.
A reversible change can be undone, usually a change of state or dissolving. An irreversible change makes a new substance and cannot easily be reversed, such as burning wood, baking a cake or rusting iron.
Changes of state
| Change | From → to | Energy |
|---|---|---|
| Melting | solid → liquid | heat added |
| Evaporation or boiling | liquid → gas | heat added |
| Condensation | gas → liquid | heat removed |
| Freezing | liquid → solid | heat removed |
Reversible and irreversible
Signs of an irreversible chemical change include a new substance, gas bubbles, a colour change, heat or light, and a smell. Examples: frying an egg, vinegar and baking soda fizzing, rusting, burning. Reversible: ice melting, sugar dissolving (the water can be evaporated to get sugar back), chocolate melting.
What happened?
Colour, gas, heat, smell.
Cool, evaporate, filter.
Reversible or irreversible.
Dissolving and separating
A solute dissolves in a solvent to make a solution. Warm water dissolves most solids faster, and stirring helps. Mixtures can be separated using the properties of their parts: filtering for solids in liquids, evaporation to recover a dissolved solid and magnets for iron. Safety: do not taste experiments and have an adult help with heating.
Worked example and common misconceptions
Example: Salt dissolved in water can be recovered by evaporating the water, so dissolving is reversible. Toast cannot become bread again, so toasting is irreversible. Common misconceptions: thinking 'disappearing' means destroyed, believing steam is the same as the white cloud above a kettle (that is tiny droplets) and thinking melting is always reversible for every material. Test changes in a fair way.
Check your understanding: model answers
1. Irreversible. 2. 100 °C at sea level. 3. Evaporate the water.
✨ Key Takeaways
- Changes of state are usually reversible.
- Irreversible changes make new substances.
- A solute dissolves in a solvent.
- Separate mixtures using different properties.
💬 Questions for Discussion & Reflection
- Is baking a cake reversible or irreversible?
- At what temperature does water boil?
- How could you get salt back from salty water?
Print a science experiment report →