A chemical reaction turns one or more starting substances (reactants) into new substances (products) with different properties — this is different from a physical change like melting, where no new substance forms. A word equation shows this: Reactants → Products. Signs a chemical reaction has happened include a colour change, gas bubbles, a temperature change, or a new solid forming (precipitate).
Example
Burning methane gas reacts with oxygen to form carbon dioxide and water: Methane + Oxygen → Carbon Dioxide + Water. The methane and oxygen are used up and turned into completely different substances.
Key terms
Reactant:
A starting substance in a chemical reaction.
Product:
A new substance formed by a chemical reaction.
Precipitate:
A solid that forms out of a solution during a chemical reaction.
Questions
1. A chemical reaction turns reactants into:
Products
Nothing at all
The exact same substance
A physical change only
2. Reactants are:
The starting substances in a reaction
The final substances
A type of gas only
A colour
3. Products are:
The new substances formed
The starting substances
A type of container
A measuring tool
4. A word equation is written as:
Reactants → Products
Products → Reactants
Reactants = Products always identical
A single word
5. A sign that a chemical reaction has occurred includes:
Gas bubbles forming
The substance staying exactly the same
Nothing changing at all
A book being read
6. Melting ice into water is an example of a:
Physical change, not a chemical reaction
Chemical reaction
Precipitate forming
New product forming
7. A precipitate is:
A solid that forms out of a solution
A type of gas only
A colour change only
A temperature reading
8. In "Methane + Oxygen → Carbon Dioxide + Water", the reactants are:
Methane and Oxygen
Carbon Dioxide and Water
Only Methane
Only Water
9. In "Methane + Oxygen → Carbon Dioxide + Water", the products are:
Carbon Dioxide and Water
Methane and Oxygen
Only Oxygen
Only Methane
10. Rusting of iron is an example of a:
Chemical reaction, since iron becomes a new substance (iron oxide)
Physical change only
A reversible process with no new substance
Neither a physical nor chemical change
11. Which is a physical change, not a chemical reaction?
Freezing water into ice
Burning wood
Rusting metal
Baking a cake
12. A temperature change during a reaction (like a hot pack warming up) suggests:
A chemical reaction is likely occurring
Nothing is happening
Only a physical change occurred
The reaction has failed
13. Why can't a physical change (like cutting paper) be reversed into a chemical reaction?
No new substance is formed in a physical change, so there's nothing chemical to reverse
All physical changes are actually chemical reactions
Physical changes always create new substances
There is no difference between the two
14. Baking a cake (chemical reaction) is different from melting butter (physical change) because baking:
Creates new substances that can't easily be reversed
Doesn't involve heat at all
Is identical to melting butter
Never changes any properties
15. Why is a colour change alone not always definitive proof of a chemical reaction?
Colour can sometimes change due to physical factors like dilution, without a new substance forming
Colour changes are always chemical reactions with no exceptions
Colour never changes in any process
Physical changes never involve colour
16. Which best explains why burning (combustion) is considered a chemical reaction rather than a physical change?
The original fuel is converted into new substances like carbon dioxide and water, with different properties
Burning doesn't actually change the substance at all
The ash is chemically identical to the original fuel
No new substances are ever formed in burning
17. A student mixes two clear liquids and a yellow solid forms and settles at the bottom. This is most likely evidence of:
A precipitate forming from a chemical reaction
A physical change only
Nothing significant happening
An error in the experiment with no meaning
18. Why do scientists represent chemical reactions with symbolic equations (like CH₄ + 2O₂ → CO₂ + 2H₂O) rather than only word equations?
Symbols precisely show the exact atoms and quantities involved
Symbols have no advantage over words
Word equations are always more accurate
Symbolic equations remove the need for reactants
19. Digesting food in your stomach involves chemical reactions because:
Enzymes break food down into new, different molecules your body can use
Food stays chemically identical after digestion
Digestion is purely a physical process with no new substances
No reactants or products are involved
20. Which observation would be the strongest evidence that a chemical reaction (not just a physical change) has occurred?
A new substance forms with completely different properties from the originals
The substance changes shape only
The substance changes temperature slightly with no other change
Nothing observable changes at all
21. Why is it important in chemistry to distinguish reversible physical changes from often-irreversible chemical reactions?
It helps predict whether a process can be undone and what safety precautions may be needed
The distinction has no practical importance
All changes are always reversible regardless of type
Chemical reactions are always identical to physical changes
Answer key (parent copy)
1. Products
2. The starting substances in a reaction
3. The new substances formed
4. Reactants → Products
5. Gas bubbles forming
6. Physical change, not a chemical reaction
7. A solid that forms out of a solution
8. Methane and Oxygen
9. Carbon Dioxide and Water
10. Chemical reaction, since iron becomes a new substance (iron oxide)
11. Freezing water into ice
12. A chemical reaction is likely occurring
13. No new substance is formed in a physical change, so there's nothing chemical to reverse
14. Creates new substances that can't easily be reversed
15. Colour can sometimes change due to physical factors like dilution, without a new substance forming
16. The original fuel is converted into new substances like carbon dioxide and water, with different properties
17. A precipitate forming from a chemical reaction
18. Symbols precisely show the exact atoms and quantities involved
19. Enzymes break food down into new, different molecules your body can use
20. A new substance forms with completely different properties from the originals
21. It helps predict whether a process can be undone and what safety precautions may be needed