Changes Around Us Physical and Chemical
Ice melting, paper burning, iron rusting — changes are happening all around us. This Class 7 Curiosity chapter separates physical changes from chemical changes, identifies the signs of a chemical change, and looks at rusting and crystallisation as everyday examples.
Learning objectives
- Distinguish physical and chemical changes.
- Recognise the signs of a chemical change.
- Explain rusting and how to prevent it.
- Describe crystallisation.
Key concepts
Physical changes
In a physical change, no new substance is formed and the change is usually reversible. The substance may change in size, shape or state, but it stays the same material. Melting ice, dissolving sugar in water, and tearing paper are physical changes — the water, sugar and paper are still chemically the same.
Chemical changes
In a chemical change, one or more new substances are formed, and the change is usually difficult to reverse. Burning paper, cooking food, souring of milk and rusting of iron are chemical changes, because the original substances turn into different ones with new properties.
Signs of a chemical change
Several clues suggest a chemical change: a new substance forms, there may be a change in colour, a gas may be given off, heat or light may be produced or absorbed, and sometimes a solid (precipitate) settles. Noticing these signs helps tell a chemical change from a physical one.
Rusting and crystallisation
Rusting is a chemical change in which iron reacts with oxygen and moisture to form reddish-brown rust; it can be prevented by painting, greasing or galvanising (coating with zinc). Crystallisation is a physical process used to obtain pure crystals of a substance, such as salt or copper sulphate, from its solution.
Important formulas
Rusting
iron + oxygen + water → rust
Key definitions
- Physical change
- A change with no new substance, usually reversible.
- Chemical change
- A change in which new substances form, usually irreversible.
- Rusting
- The slow chemical change of iron into rust in air and moisture.
- Crystallisation
- Obtaining pure crystals of a substance from its solution.
Solved examples
Q1. Is melting of ice a physical or chemical change?
Solution: Physical — only the state changes, no new substance forms.
Q2. Name a chemical change in the kitchen.
Solution: Cooking food (or souring of milk).
Q3. How can rusting be prevented?
Solution: By painting, greasing or galvanising the iron.
Common mistakes to avoid
- Thinking all changes that look big are chemical.
- Believing dissolving sugar is a chemical change (it is physical).
- Assuming chemical changes are always easily reversible.
- Confusing rust (chemical) with simple wetting of iron.
Changes Around Us Physical and Chemical — MCQ Quiz
10 questions with instant feedback. Use number keys 1–4 to answer.
In a physical change, a new substance is:
Practice questions
Short answer
What happens to the substance in a physical change?
It stays the same material; only size, shape or state may change.
Give one sign of a chemical change.
Formation of a new substance, a gas, heat, light, or a colour change.
What is galvanising?
Coating iron with a layer of zinc to prevent rusting.
Long answer
Distinguish between physical and chemical changes, giving examples and signs.
A physical change is one in which no new substance is formed and which is usually reversible; the material may change in size, shape or state but remains the same substance. Examples include melting ice, dissolving sugar in water and tearing paper. A chemical change produces one or more new substances and is usually difficult to reverse; examples include burning paper, cooking food, souring of milk and rusting of iron. Several signs point to a chemical change: a new substance with different properties, a change in colour, a gas being given off, heat or light produced or absorbed, or a solid settling out. Recognising these signs helps tell the two kinds of change apart.
Explain rusting, the conditions needed for it, and how it can be prevented.
Rusting is a chemical change in which iron slowly reacts with oxygen in the presence of moisture to form a reddish-brown substance called rust, which flakes away and weakens the metal. Both air (oxygen) and water are needed for rusting to occur, which is why iron rusts faster in damp conditions. Rusting can be prevented by keeping these away from the iron: painting or oiling/greasing forms a protective layer, and galvanising coats the iron with a layer of zinc that protects it. Using rust-resistant alloys such as stainless steel is another way to avoid the problem.
HOTS (Higher Order Thinking)
Why does cut fruit turning brown count as a chemical change?
Because the fruit reacts with oxygen in the air to form a new, differently coloured substance — a sign of a chemical change.
Iron gates are painted regularly. How does this prevent rust?
Paint forms a barrier that keeps oxygen and moisture away from the iron, so the rusting reaction cannot occur.
Quick revision
Revision notes
- Physical change: no new substance, usually reversible.
- Chemical change: new substance, usually irreversible.
- Signs: new substance, colour change, gas, heat/light, precipitate.
- Rust = iron + oxygen + moisture; prevent by painting/greasing/galvanising.
Key takeaways
- New substance = chemical change.
- Rusting needs oxygen and moisture.
- Crystallisation is a physical purification.
Frequently asked questions
Is boiling water a physical or chemical change?
Physical — it changes state but stays water.
Why is rusting a chemical change?
Iron turns into a new substance, rust, with different properties.
What is crystallisation used for?
To obtain pure crystals of a substance from its solution.