Lesson focus: A chemical reactionA change where substances turn into new substances. rearranges atoms into new substances, while observationSomething you notice using your senses or measure with equipment, such as a colour change or a temperature. provide evidence that this may have happened.
Distinguish chemical from physical change and evaluate several kinds of reaction evidence.
Build on earlier learning
Chemical formulae and balancingYear 7 distinguished elements, compounds and mixtures; reactions now rearrange atoms between substances.
You will be successful when you can…
- Define reactant and product.
- Identify evidence of new substances.
- Separate observation from inference.
- Explain why one sign may be inconclusive.
Words that carry the science
- chemical reaction
- A process that rearranges atoms into new substances.
- reactant
- A substance present at the start of a reaction.
- product
- A substance formed by a reaction.
- observation
- Something directly detected or measured.
- inference
- An explanation drawn from observations and prior knowledge.
Reactions form new substances
01Reactions form new substances
During a chemical reactionA change where substances turn into new substances., bonds and particle groupings change as atoms rearrange. The atoms themselves are not created, destroyed or changed into other elements.
A physical change alters state, shape or mixing without producing a new substance. Many physical changes are reversible, but reversibility alone does not define the distinction.
02Observations become evidence
Gas production, a precipitate, persistent colour change, light or an unexpected temperature change can support the inferenceAn idea you work out from what you observed, rather than something you saw directly. that productA new substance made in a chemical reaction. formed.
Record what happened before explaining it: “bubbles formed” is an observationSomething you notice using your senses or measure with equipment, such as a colour change or a temperature.; “a gas product formed” is an inference.
03Control experiments make evidence stronger
Boiling also produces bubbles, dissolving can change temperature and dilution can change colour. Compare with control experiments and identify products where possible.
Strong evidence combines several observations with a plausible particle explanation or a product test.
Signs of a chemical reaction
In a chemical reaction new substances are made. Signs that a reaction is happening include a gas being given off (fizzing), a colour change, a solid (precipitate) forming and a temperature change. The animation runs six times faster than real time.
White circles: gas bubbles. Blue specks: a precipitate.
Controls
Temperature changes are illustrative. Dissolving can also change the temperature slightly, so look for more than one sign.
Classify an unfamiliar change
Two colourless solutions form a cloudy solid and the mixture warms.
- Observe: cloudiness appears and temperature rises.
- Infer that an insoluble solid may have formed.
- The new solid and energy transfer support chemical change.
- Confirm using control experiments, repeat observations or separation and identification of the solid.
How to make the reasoning visible: The evidence strongly supports a reaction, but product identification makes the claim more secure.
Evidence or alternative explanation?
Judge each observation cautiously.
| Change | Observation | Possible interpretation | Alternative |
|---|---|---|---|
| Tablet in water | bubbles | gas product | trapped air |
| Ice warming | temperature rises | energy transfer | physical heating |
| Two solutions | solid appears | precipitate product | undissolved contamination |
| Copper heating | surface turns black | new oxide | surface coating |
Which row gives the strongest initial evidence and why?
Why should observations and inferences be written separately?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a reactant?
What is a product?
Name one reaction observation.
A puddle disappears on a warm day. Is this evidence of a reaction?
Why is reversibility an unreliable single test?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student says fizzing proves that a chemical reaction has happened.
What does fizzing show?
(2)Evaluate the claim. Suggest stronger evidence.
(2)Total for this question: 4 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., weigh evidence and alternatives before concluding.