Class 7 Science: Physical and Chemical Changes

A change is best classified by asking what materials are present after it and whether the original substance can be recovered by a simple physical method.

What to understand

Describe the central idea in your own words.
Use a model, observation or relationship to explain a changed example.
Notice and correct the likely misconception in the worked reasoning.

Classify with evidence

Melting ice changes state but leaves water as the same substance. Dissolving salt can be reversed by evaporation. Rusting iron involves new substances and is not reversed simply by cooling or filtering.

Colour, gas, temperature change or a solid forming may be clues, but each clue needs context. A bubble can be trapped air; a colour change can come from mixing dyes.

Worked reasoning: Compare tearing paper with burning it. Both change appearance, but burning produces gases and ash with different properties. In a written answer, state the evidence and then name the type of change.

Try, explain and check

Sort

Classify melting wax, cooking an egg and dissolving sugar, explaining one choice.

Show a reasoning prompt

State the evidence, relationship or model that supports your answer, then check whether the units and direction make sense.

Question

Can every chemical change be seen? Explain why not.

Show a reasoning prompt

State the evidence, relationship or model that supports your answer, then check whether the units and direction make sense.

Safety

Observe only approved demonstrations; do not heat or mix unknown substances.

Show a reasoning prompt

State the evidence, relationship or model that supports your answer, then check whether the units and direction make sense.

Next step

Write a three-sentence revision summary, then continue to the related resource. For extra practice, visit the resource library and choose a worksheet that matches the skill you need.

About this resource
Prepared as part of the BIS Quiz learning resource. It is supplementary study material; use your teacher's instructions and prescribed materials when they differ. Report a factual error or unclear explanation.