Physical and chemical changes
Physical vs chemical change
- A physical change makes no new substance — only the state or shape changes, and it is usually easy to reverse (e.g. melting ice, dissolving sugar).
- A chemical change makes new substances and is usually hard to reverse (e.g. burning, rusting).
Practice
Which is a physical change?
Melting makes no new substance and is easy to reverse — a physical change.
Practice
A chemical change always:
A chemical change produces new substances and is usually hard to reverse.
Signs of a chemical change
- a colour change,
- a gas given off,
- an energy change (heat in or out),
- a precipitate (a solid) forming.
Practice
Which are signs of a chemical change? (Choose all that apply.)
Colour change, gas, energy change and precipitates signal chemistry; melting is physical.
You've got it
Key idea
- physical = no new substance, easy to reverse (melting, dissolving)
- chemical = new substance, hard to reverse (burning, rusting)
- chemical signs: colour change, gas, energy change, precipitate