Spot the New Substance
Tell a chemical change from a physical one by its signs.
What this lesson covers
Why it matters
Tear a sheet of paper and it is still paper. Set it alight and you get ash, smoke and heat — something new. Certain tell-tale signs reveal when a brand-new substance has formed.
Predict first
Which of these is a sure sign that a NEW substance has formed (a chemical change)?
Signs of a chemical change: a colour change, a gas given off, heat or light released, an insoluble solid (precipitate) forming, or a new smell. Just changing shape or state is only a physical change.
- A gas bubbles off, or a new colour appears — correct
- The shape or size changes
- It melts or freezes
What you do
Sort each observation: is it a sign of a chemical change, or just a physical change?
Check yourself
Which observation signals a chemical change?
A new gas means a new substance — a chemical change; melting and bending are physical.
Milk turning sour is a chemical change because…
Souring forms new substances and cannot be reversed — a chemical change.
- Bubbles of a new gas appear — correct
- An ice cube melts
- A wire is bent
- a new substance with a new taste and smell forms — correct
- it only changes shape
- it can easily be turned back to fresh milk