ScienceClass 7Full Content

Physical and Chemical Changes

Difference between physical and chemical changes with examples.

Key Topics

Physical Changes

A change in shape, size, state or appearance WITHOUT forming a new substance. The change is usually REVERSIBLE. Examples: melting ice (water → ice → water), dissolving sugar in water (can evaporate to get sugar back), tearing paper, cutting vegetables, melting wax. NO new substance formed. Same chemical formula before and after.

Chemical Changes

Open topic

A change that produces a NEW substance with different properties. Usually IRREVERSIBLE. Signs of chemical change: new colour, new smell, gas produced, heat/light produced, precipitate formed. Examples: burning paper (ash formed), rusting of iron, curdling of milk, cooking an egg, photosynthesis, ripening of fruit. Cannot get original substance back easily.

Rusting and Crystallisation

Open topic

Rusting: iron reacts with oxygen and water to form rust (Fe₂O₃·xH₂O). Chemical change — reddish-brown, flaky. Prevents with painting, oiling, galvanising (zinc coating). Crystallisation: growing crystals from supersaturated solution — example copper sulphate crystals (blue). This is physical change — can dissolve crystals and get solution back.

Formulas

Rusting (simplified)

Formula detail

Iron + Oxygen + Water → Iron Oxide (rust)

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Fe₂O₃ (rust)

Experiments

Comparing Physical and Chemical Changes

Objective: Classify various changes as physical or chemical

Materials: Ice cube, Paper, Candle, Vinegar, Baking soda, Milk, Lemon

Result: Melting, tearing = physical (reversible). Burning, vinegar+baking soda, curdling milk = chemical (new substance formed)!

Safety: Burning experiment must be done by teacher only.

Video Links

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