Physical and Chemical Changes

What you'll learn

Define and distinguish physical and chemical changes.
Recognize signs of a chemical change.
Everyday examples of each type.
Reversible vs irreversible changes.

Key concepts

Physical changes

A physical change alters the form or appearance of a substance but does NOT change its chemical composition.

Key features:

No new substance is formed.
Original substance can usually be recovered (reversible).
Chemical properties remain the same.

Examples of physical changes:

Change	Original substance	After change
Ice melting	Water (solid)	Water (liquid) — same H₂O
Water boiling	Water (liquid)	Water vapour (gas) — same H₂O
Tearing paper	Paper	Smaller pieces of paper
Dissolving salt in water	Salt (NaCl)	Salt solution — salt can be recovered by evaporation
Stretching a rubber band	Rubber	Stretched rubber — same material
Crushing a can	Aluminium	Crushed aluminium — same metal
Melting wax	Solid wax	Liquid wax — cools back to solid
Magnetising iron	Iron	Magnetised iron — can be demagnetised
Cutting wood	Wood	Smaller pieces of wood

Chemical changes

A chemical change produces one or more new substances with different chemical properties.

Key features:

New substance(s) formed.
Usually irreversible (cannot easily get back original).
Chemical composition changes.
Often accompanied by observable signs.

Signs of a chemical change:

Change in colour (e.g., apple turning brown when cut).
Production of gas (bubbles — e.g., vinegar + baking soda → CO₂).
Change in temperature (heat given out or absorbed).
Formation of precipitate (insoluble solid forms in a liquid).
Change in smell (new odour — burning, fermentation).
Light or flame produced (burning).

Examples of chemical changes:

Change	New substance(s) formed
Burning wood	Carbon dioxide, water, ash, smoke
Rusting of iron	Iron oxide (Fe₂O₃) — different colour, texture, mass
Digestion of food	Simpler molecules (glucose, amino acids)
Cooking an egg	Denatured protein — egg can't be uncooked
Milk turning sour	Lactic acid (curd/yoghurt)
Photosynthesis	Glucose and oxygen from CO₂ and water
Respiration	CO₂ and water from glucose and oxygen
Baking a cake	New compounds from flour, egg, sugar reaction
Neutralisation	Salt and water from acid and base

Comparison table

Feature	Physical change	Chemical change
New substance?	No	Yes
Reversible?	Usually yes	Usually no
Chemical composition	Same	Different
Energy change	Small	Often significant
Examples	Melting, dissolving, tearing	Burning, rusting, digestion

Reversible vs irreversible

Reversible changes: original substance can be recovered easily — most physical changes.
- Examples: melting ice → refreeze; dissolving sugar → evaporate to get sugar back.
Irreversible changes: cannot easily recover original substance — most chemical changes.
- Examples: burning paper → cannot un-burn; cooking egg → cannot un-cook.

Note: Some chemical changes are reversible in special conditions (e.g., electrolysis of water back to hydrogen and oxygen), but they require significant energy input.

Rusting — a detailed chemical change

Rusting: iron + oxygen + water → iron oxide (rust)
4Fe + 3O₂ + nH₂O → 2Fe₂O₃·nH₂O (rust)
Rust is reddish-brown; flakes off; weakens iron.
Prevention: painting, greasing/oiling, galvanisation (coating with zinc), using stainless steel.

Crystallisation

Crystallisation: a physical process by which dissolved substance forms crystals as solution cools or evaporates.
Used to: purify substances, grow large crystals of alum/salt/copper sulphate.
New substance: NOT formed (same compound, different form) → physical change.

Quick check

Define physical change. Give three examples.
Define chemical change. List three signs that a chemical change has occurred.
Is dissolving salt in water a physical or chemical change? Explain.
Is burning wood reversible? Why or why not?
How can you prevent iron from rusting? Name two methods.

Open the Practice tab for graded questions on Physical and Chemical Changes.

Physical & Chemical Changes