Topic 4 of 8 · Grade 7
Materials and their properties
Solids (fixed shape), liquids (take the shape of the container) and gases (fill the space).
Notes for this topic 1 file, in order
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States of matter
Solids (fixed shape), liquids (take the shape of the container) and gases (fill the space).
Changes of state
Melting (ice → water), freezing (water → ice), evaporation (water → water vapour), condensation (water vapour → water).
Properties of materials
Hard or soft, strong or weak, flexible or stiff, waterproof, transparent or opaque, conductors or insulators of heat and electricity, magnetic or non-magnetic. We choose materials for their properties: metal pots conduct heat; plastic handles do not.
Mixtures and separation
- Sieving (sand and stones), filtering (sand and water), evaporation (salt from salt water), magnetism (iron filings from sand), decanting.
Reversible and irreversible changes
Melting chocolate is reversible; burning paper and cooking an egg are irreversible.
Recycling
Reduce, reuse and recycle plastics, paper, glass and metal to protect the environment.
Worked example: How would you separate salt from salty water?
Heat the water until it evaporates; the salt is left behind.
Check yourself
- What is condensation?
- Name a material that is an insulator of electricity.
- Is burning wood a reversible change?
Show answers
- When a gas (water vapour) cools and turns into a liquid.
- Any one: plastic, rubber, wood, glass.
- No – it is irreversible.
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