Chemical changes and ways to slow them
Distinguish chemical reactions from physical changes, and explain how to slow decay, fuel-fired cooking, burning and rusting.
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Food Chemistry · about 25–35 min
What you need to understand
State the new evidence and explain why it suggests that new products formed.
Definitions
- chemical reaction
a change that forms one or more new products
For example: Gas production can be evidence of reaction when interpreted in context.
Key idea
- LookAn iron food can is coated with unbroken tin.
- ThinkState the new evidence and explain why it suggests that new products formed.
- DoUse evidence of new products and match each unwanted reaction with the factor that slows it.
Review the full food chemistry topic picture
Explanation
A chemical reaction forms one or more new substances, called products. A physical change changes a substance’s state, shape or size without forming a new substance. Melting ice and boiling water are physical changes: both still involve water.
Gas, a temperature change, a new solid or a colour change can suggest a reaction, but each observation needs context. Boiling produces water vapour without forming a new substance. Crystallising salt from solution produces solid salt that was already present. Heating alone can raise temperature without a chemical reaction. Ask whether the evidence points to a new substance, rather than treating one visible sign as proof.
| Change | How it can be slowed | Reason |
|---|---|---|
| Food decay | Use suitable preservation, such as refrigeration. | Lower temperature slows much microbial activity and the changes it causes. It does not guarantee that every microbe is killed. |
| Cooking on a fuel burner | Reduce fuel flow using the burner’s normal control. | A smaller flame transfers less heat to the food, so cooking changes can take longer. This does not mean every cooking reaction needs oxygen supplied to the food. |
| Burning fuel | Reduce the available fuel or oxygen. | Combustion requires fuel and oxygen. Limiting either can slow the burning. |
| Rusting of an iron can | Keep the iron covered by an intact tin coating. | The coating forms a barrier to water and oxygen, which are needed for rusting. Scratches expose the iron. |
Cooking can include both physical and chemical changes. Water evaporating is a
physical change; browning of food can involve reactions forming new
substances. The relevant question is which change is being described. Explore food preservation methods
Pause and say it: State the new evidence and explain why it suggests that new products formed.
Common mistake
Tempting wrong idea: Every change of state forms a new product.
Why it fails: Melting ice gives liquid water, which is still the same substance. A change of state changes how the particles are arranged, not what the substance is, so no new product forms.
Use this instead: Melting, freezing and boiling do not by themselves form new products.
Practical work
What to show: Use evidence of new products and match each unwanted reaction with the factor that slows it.
Before you finish: State the changed condition, not just ‘make it slower’.
Practical link: The topic investigation is taught in “Separating Mixtures”
6. Worked Examples
Modelled example 1
Rust prevention
Problem
An iron food can is coated with unbroken tin. Explain how the coating protects the iron and why a scratch matters.
Study the worked solution
Reason from the evidence
Method
Identify how the unbroken coating protects the iron underneath.
Reason
An intact tin barrier keeps the iron covered and slows its rusting.
Working
- The iron is completely covered by tin.
- The coating forms a barrier.
If the coating is badly scratched, the barrier no longer fully covers the iron.
State the conclusion
Working
Tin plating slows rusting by keeping the iron covered.
Common misconception 2
Does difficult-to-reverse mean chemical?
Learner response
“Cutting food is hard to reverse, so it must be a chemical reaction.” Correct the test being used.
Return to the definition of reaction
View solution step by step
Use product formation
Method
Ask whether new products formed rather than whether the change is easy to reverse.
Reason
Some physical changes are difficult to reverse, so reversibility alone is unreliable.
Working
Cutting changes size and shape; evidence of new products is needed before calling it a chemical reaction.
Challenge 3
Slow three everyday reactions
Problem
A caterer wants to slow food decay, slow cooking on a gas burner and protect an iron can from rusting. Give one action for each.
Match each action to its mechanism
Hints
Hint 1: treat each reaction separately
Preservation, the gas burner’s fuel flow, and tin-plating control different conditions or pathways.
Hint 2: state the matched action
Do not write only “slow it down”.
View solution step by step
Control decay and cooking
Method
Use a suitable food-preservation method, and reduce the gas burner’s fuel flow using its normal control.
Reason
Preservation changes conditions for microbes. A lower fuel flow makes a smaller flame, transferring less heat to the food.
Working
food: preserve it; gas-burner cooking: reduce fuel flow and heating
Control rusting
Method
Keep an intact tin plating over the iron.Reason
Tin-plating the iron can slows rusting.Working
Inspect damage because a scratch exposes the iron beneath.
Guided practice
Try it with support
A substance gives off gas and leaves a new solid. Explain why this supports a chemical reaction, then state how to slow rusting of an iron can.
Evidence must point to new products.
Rusting needs contact with the surroundings.
Check the guided answer
Answer: Gas and a new solid can support the formation of new products. An intact tin coating slows rusting by keeping oxygen and water away from the iron.
Check: One observation should be interpreted in context rather than treated as automatic proof.
Practise and continue
Practise this
Match each unwanted change to a way of slowing it: food decay, a cooking reaction, a fire and rusting iron.
Need a hint?
Change a condition needed by each reaction.
Check your answer
Answer: Food decay can be slowed by food preservation methods. On a gas burner, reducing fuel flow reduces heating and can slow cooking. Burning fuel can be slowed by limiting available fuel or oxygen. An intact tin coating forms a barrier that slows rusting of the iron can.
Check: State the changed condition, not just ‘make it slower’.
Think like a scientist
Why is reversibility alone an unreliable test for a chemical reaction?
Check the reasoning
Some physical changes are hard to reverse and some chemical processes can be reversed under suitable conditions; the key question is whether new products formed.
Remember: Use reaction evidence together with the substance context.
One-minute check
How does a chemical reaction differ from melting or boiling water?
Explain how reducing a gas burner’s fuel flow can slow cooking without claiming that every cooking reaction needs oxygen.
Correct this common mistake: “Every change of state forms a new product.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: