Materials (Classes VI–VIII) • Topic 5 of 5

Physical & Chemical Changes

NCERT Class 7 'Physical and Chemical Changes' draws the central line of this topic. In a PHYSICAL CHANGE, only the physical state, shape, size or appearance changes — NO new substance forms and the change is usually REVERSIBLE. Examples: melting of ice (and freezing water), boiling/evaporating water and condensing steam, dissolving sugar/salt in water, folding or tearing paper, breaking a glass, stretching a rubber band, making a salt solution. In a CHEMICAL CHANGE (a chemical reaction), a NEW substance with new properties forms; it is usually IRREVERSIBLE and is often accompanied by heat/light, a colour change, evolution of a gas, a smell, or sound. Examples CTET uses again and again: RUSTING of iron (iron + oxygen + water → rust, iron oxide — a CHEMICAL change, and a clear exam favourite), burning of a candle/paper/fuel, cooking of food, digestion, souring of milk (curdling), photosynthesis, ripening of fruit, and the reaction of baking soda with vinegar (gas evolved). CONDITIONS for rusting: the presence of BOTH oxygen (air) and moisture (water); preventing one (oil/paint/grease coating, galvanising with zinc) prevents rust. Note CRYSTALLISATION is a physical process used to obtain pure crystals from a solution. TRAP CASES: melting wax is physical, but BURNING the candle is chemical; dissolving sugar is physical, but burning/caramelising it is chemical; the SAME substance can undergo either kind of change. PEDAGOGY/MISCONCEPTION: children call every change that 'can't be undone easily' chemical and every reversible one physical — but the real test is whether a NEW substance forms; a teacher uses paired demos (melt ice vs burn paper; dissolve sugar vs rust a nail) to build the distinction. HOW TESTED: classify a given change, identify rusting as chemical, or state the conditions/new-substance criterion.

✅ Solved examples

1. Is the RUSTING of an iron gate a physical change or a chemical change, and why?
A CHEMICAL change. Iron reacts with oxygen and moisture to form a new substance, rust (iron oxide), which has different properties from iron and cannot easily be turned back. It is the classic exam example of a chemical change.
2. Melting of ice into water is which kind of change?
A physical change — only the state changes (solid to liquid), no new substance forms, and it is reversible by freezing.
3. A burning candle shows BOTH kinds of change at once. Identify each.
The MELTING of the wax near the flame is a physical change (it can resolidify); the BURNING of the wax/vapour is a chemical change (new substances — carbon dioxide, water, heat and light — form). A favourite CTET 'both' question.
4. Two conditions are necessary for iron to rust. They are:
Presence of oxygen (air) AND moisture (water). Removing either — for example by oiling, painting or galvanising the iron — prevents rusting.

✏️ Practice — try these, take hints as needed

1. Souring (curdling) of milk into curd is classified as which kind of change?
A new substance with new taste forms.
You cannot turn curd back into milk.
Chemical change
2. What single criterion best decides whether a change is chemical rather than physical?
Not just whether it is reversible.
Look for something newly made.
Whether a new substance (with new properties) is formed
3. Dissolving common salt in water to make a solution is a physical change. Give one reason.
No new substance is formed.
The salt can be recovered by evaporation.
No new substance forms and the change is reversible (salt is recovered by evaporation)
4. Coating an iron pipe with zinc to stop it rusting is called:
Named after the zinc layer.
Galvan____.
Galvanising (galvanisation)

📝 Topic test — 8 questions

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