A physical change alters the form or appearance of a substance without changing what it's made of — melting ice, dissolving sugar, or tearing paper are all physical changes, and they're generally reversible. A chemical change (chemical reaction) transforms a substance into one or more entirely new substances with different properties, and is usually difficult or impossible to reverse. Common indicators that a chemical change has occurred include a colour change, the production of gas (bubbles), a temperature change (energy released or absorbed), a new smell, or the formation of a precipitate (solid forming from two liquids mixing).
Example
Melting chocolate is a physical change — it's still chocolate, just in a different state, and it can be re-solidified. Burning wood is a chemical change — the wood transforms into ash, smoke and gases, releasing heat and light, and can't be turned back into wood.
Key terms
Physical change:
A change in form or appearance that doesn't create a new substance.
Chemical change:
A change that transforms a substance into one or more new substances.
Precipitate:
A solid that forms when two liquids are mixed together in a chemical reaction.
Questions
1. A physical change alters:
The form or appearance of a substance, not what it is made of
The substance into something entirely new
Nothing at all
Only the colour, always
2. A chemical change creates:
One or more new substances
No change at all
Only a change in shape
Only a change in temperature
3. Melting ice is an example of a:
Physical change
Chemical change
Neither type of change
An irreversible change only
4. Burning wood is an example of a:
Chemical change
Physical change
A completely reversible change
No change at all
5. Physical changes are generally:
Reversible
Always permanent
Always dangerous
Impossible to observe
6. A precipitate is:
A solid forming when two liquids mix in a reaction
A type of gas only
A colour with no substance
A physical change only
7. Indicators of a chemical change include:
Colour change, gas production, temperature change
Only a change in shape
Nothing observable
Only a change in size
8. Why is dissolving sugar in water considered a physical change, not a chemical one?
The sugar molecules are still sugar, just spread through the water — no new substance is formed
Dissolving always creates an entirely new substance
Dissolving is impossible to reverse
Sugar disappears completely and stops existing
9. Why is rusting iron considered a chemical change?
Iron reacts with oxygen to form a new substance (iron oxide) with different properties
Rust is just iron in a different physical shape
Rusting can be easily reversed by melting the metal
Rusting involves no chemical reaction at all
10. Bubbles forming when two substances are mixed is a common sign of:
A chemical change
A physical change only
No change occurring
A reversible melting process
11. Why can a temperature change during a reaction indicate a chemical change has occurred?
Chemical reactions often release or absorb energy as bonds break and reform
Temperature never changes during any chemical process
Temperature change only ever indicates a physical change
Chemical reactions cannot involve any energy change
12. Tearing a piece of paper is an example of a:
Physical change
Chemical change
Both a physical and chemical change equally
Neither type of change
13. Why might a physical change (like freezing water) be reversible, while a chemical change (like burning paper) usually isn't?
A physical change doesn't alter the substance's identity, while a chemical change creates fundamentally new substances
Physical and chemical changes are always equally reversible
Reversibility has no connection to the type of change
Chemical changes are always more easily reversed than physical ones
14. A new smell appearing during a reaction between two substances most likely indicates:
A chemical change has occurred
A purely physical change
No change of any kind
An error in the experiment only
15. Why might it sometimes be difficult to tell whether a change is physical or chemical just by looking at it?
Some changes (like dissolving) can visually resemble a chemical change without actually forming a new substance
Physical and chemical changes always look completely different
Chemical changes never have any visible indicators
This distinction is always obvious with no ambiguity
16. Why is cooking an egg considered a chemical change rather than simply a physical one, even though no new substance seems added?
Heat causes the egg proteins to permanently change structure, forming a new substance that cannot revert to raw egg
Cooking an egg is always classified as a purely physical change
The egg remains chemically identical after cooking
Cooked and raw egg have identical chemical properties
17. Why might firefighters need to understand the difference between a physical and chemical change when responding to different types of fires?
Chemical reactions during a fire (like burning fuel) behave very differently from simply melting or evaporating substances, requiring different responses
Physical and chemical changes require an identical firefighting response
This distinction is irrelevant to fire safety
Fires never involve any chemical changes
18. Why can observing multiple indicators together (e.g. gas bubbles AND a temperature change) give stronger evidence of a chemical change than observing just one?
Multiple simultaneous indicators reduce the chance the observation is explained by an unrelated physical process
A single indicator is always just as reliable as multiple combined indicators
Multiple indicators never provide any additional certainty
Indicators of chemical change should never be considered together
19. A student mixes two clear liquids and a cloudy solid forms at the bottom. What does this observation most strongly suggest, and why?
A chemical change, since a precipitate (new solid substance) has formed from the reaction
A physical change, since liquids were simply combined
No change has occurred at all
This observation provides no useful evidence either way
20. A student heats a metal until it glows, then lets it cool back to its original solid form with no other visible change. Is this most likely a physical or chemical change, and why?
Physical — the metal returned to its original state and properties with nothing new formed
Chemical — glowing always means a new substance has been created
This cannot be classified without first weighing the metal
Heating always causes an irreversible chemical change
21. Understanding physical and chemical changes mainly helps you to:
Identify and explain whether a substance's identity has fundamentally changed or just its form
Assume every observable change is identical in nature
Ignore evidence like colour, temperature or gas production
Treat all changes in matter as permanently irreversible
Answer key (parent copy)
1. The form or appearance of a substance, not what it is made of
2. One or more new substances
3. Physical change
4. Chemical change
5. Reversible
6. A solid forming when two liquids mix in a reaction
7. Colour change, gas production, temperature change
8. The sugar molecules are still sugar, just spread through the water — no new substance is formed
9. Iron reacts with oxygen to form a new substance (iron oxide) with different properties
10. A chemical change
11. Chemical reactions often release or absorb energy as bonds break and reform
12. Physical change
13. A physical change doesn't alter the substance's identity, while a chemical change creates fundamentally new substances
14. A chemical change has occurred
15. Some changes (like dissolving) can visually resemble a chemical change without actually forming a new substance
16. Heat causes the egg proteins to permanently change structure, forming a new substance that cannot revert to raw egg
17. Chemical reactions during a fire (like burning fuel) behave very differently from simply melting or evaporating substances, requiring different responses
18. Multiple simultaneous indicators reduce the chance the observation is explained by an unrelated physical process
19. A chemical change, since a precipitate (new solid substance) has formed from the reaction
20. Physical — the metal returned to its original state and properties with nothing new formed
21. Identify and explain whether a substance's identity has fundamentally changed or just its form