Chemical vs Physical Changes
Understand the difference between physical and chemical changes, learn to identify evidence of chemical reactions, and explore everyday examples from Australian life.
Physical Changes
A physical change alters the form or appearance of a substance but does not produce a new substance. The original substance can usually be recovered. Physical changes are typically reversible.
Examples of Physical Changes
- Ice melting into water (state change)
- Water evaporating into water vapour
- Cutting paper into smaller pieces
- Dissolving salt in water (salt can be recovered by evaporation)
- Bending a piece of aluminium foil
- Crushing a can
Key Features
- No new substance is formed
- Often reversible
- Change in shape, size, or state
- Chemical composition stays the same
- Mass is conserved
Australian Curriculum Connection
This lesson aligns with AC9S7U03: "The properties of substances are determined by their structure; physical and chemical changes involve different types of rearrangements of particles."
Chemical Changes
A chemical change (or chemical reaction) produces one or more new substances with different properties to the original materials. Chemical changes are usually irreversible — you cannot easily get the original materials back.
Examples of Chemical Changes
- Burning wood (combustion)
- Iron rusting (oxidation)
- Cooking an egg
- Baking a cake
- Vinegar reacting with bicarbonate of soda
- Digesting food in your stomach
Evidence of a Chemical Change
- Gas produced (bubbling/fizzing)
- Colour change
- Light or heat given off (or absorbed)
- Precipitate formed (solid appears in liquid)
- New smell produced
- Irreversible change observed
Comparing Physical and Chemical Changes
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversible? | Usually yes | Usually no |
| Chemical composition changes? | No | Yes |
| Energy change (big)? | Small | Often large |
| Example | Melting ice | Burning wood |
Identifying a Chemical Change: Decision Guide
Observe: Did the substance change colour unexpectedly?
Chemical clueObserve: Did bubbles of gas form (not from boiling)?
Chemical clueObserve: Was heat or light produced?
Chemical clueObserve: Did a new solid (precipitate) appear in a liquid?
Chemical clueTest: Can you reverse the change easily?
If no = ChemicalKey Vocabulary
Physical Change
A change in the form or appearance of a substance that does not produce a new substance and is usually reversible.
Chemical Change
A change that produces one or more new substances with different properties; usually irreversible.
Precipitate
A solid that forms and settles out when two liquids are mixed during a chemical reaction.
Combustion
A chemical reaction in which a substance reacts with oxygen to produce heat and light (burning).
Worked Examples
Is dissolving sugar in tea a physical or chemical change?
Step 1: Is a new substance formed? No — the sugar molecules are still sugar; they have simply spread through the water.
Step 2: Can it be reversed? Yes — if you evaporate the water, the sugar crystals reappear.
Answer: This is a physical change. The sugar dissolves but is not chemically altered.
Is burning a match a physical or chemical change? Give evidence.
Step 1: Identify evidence: heat and light are produced; the match head turns to black ash (colour change); smoke (gas) is released; the match cannot be unburned.
Step 2: New substances (carbon dioxide, water vapour, ash) are formed.
Answer: This is a chemical change (combustion). Evidence: heat and light produced, colour change, gas released, irreversible.
Vinegar and bicarbonate of soda: classify and explain
Observation: When vinegar (acetic acid) is added to bicarbonate of soda, vigorous bubbling occurs and the container feels slightly cold.
Evidence of chemical change: Gas is produced (CO₂ bubbles), the mixture feels cold (energy absorbed from surroundings — endothermic), and new substances are formed.
Answer: This is a chemical change. The evidence is gas production and temperature change, and the original vinegar and bicarbonate cannot be recovered.
Knowledge Check
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Key Concepts Summary
- ✓ Physical changes alter form or state but do not create new substances; they are usually reversible.
- ✓ Chemical changes create new substances and are usually irreversible.
- ✓ Evidence for a chemical change: gas produced, colour change, heat/light released, precipitate formed, new smell, irreversible.
- ✓ Examples: melting (physical), rusting (chemical), dissolving salt (physical), burning wood (chemical).
- ✓ The chemical composition of a substance does not change in a physical change.