Physical and Chemical Change: Looking for Evidence — Answers
Answer sheet with model answers and success criteria for the Year 8 mixtures & chemical reactions worksheet.
Success TutoringWorksheetsAnswer sheetYear 8 · Science · Mixtures & Chemical ReactionsPhysical and Chemical Change: Looking for Evidence — Answers
Part A: Physical or chemical?
Decide whether each change is a physical change or a chemical change.
In a physical change, no new substance is made. The substance may change state, shape or size, or it may dissolve, but its particles stay the same. Physical changes are usually easy to reverse.
In a chemical change (a chemical reaction), one or more new substances are made. The atoms in the starting substances (reactants) are rearranged to form different substances (products) with different properties. Chemical changes are usually difficult to reverse.
- APhysical change
- BChemical change
Answer: Physical change
The water particles are the same before and after; only their arrangement changes. Freezing the water turns it back into ice.
- APhysical change
- BChemical change
Answer: Chemical change
Burning produces new substances (carbon dioxide, water vapour, ash and smoke) and releases heat and light. It cannot be reversed.
- APhysical change
- BChemical change
Answer: Chemical change
Iron reacts with oxygen and water to form rust (a hydrated iron oxide), a new substance with different properties: it is reddish-brown and flaky.
- APhysical change
- BChemical change
Answer: Physical change
The sugar is still there (the water tastes sweet) and it can be recovered by evaporating the water. No new substance forms.
- APhysical change
- BChemical change
Answer: Chemical change
The ingredients react to form new substances, and gas bubbles make the cake rise. A baked cake cannot be turned back into batter.
- APhysical change
- BChemical change
Answer: Physical change
Each piece is still paper. Only the size and shape have changed.
- APhysical change
- BChemical change
Answer: Chemical change
Bacteria change substances in the milk into new substances (including an acid), which changes its smell and taste.
- APhysical change
- BChemical change
Answer: Physical change
The bubbles are water vapour, which is still water. When it cools it condenses back into liquid water.
- APhysical change
- BChemical change
Answer: Chemical change
New substances form, and large amounts of light, heat and sound are released.
- APhysical change
- BChemical change
Answer: Physical change
The metal changes shape but it is still the same metal.
Part B: Evidence of a chemical change
Observations such as fizzing or a colour change are evidence of a chemical change, but they are not proof. Scientists look for several pieces of evidence and for a new substance.
Answer: Any four of: a gas is produced (bubbles or fizzing, not caused by boiling); a colour change; a temperature change (getting hotter or colder); light (or sound) given off; a precipitate (a solid forming when two solutions are mixed); a new smell.
Answer: It is a precipitate. Its formation is evidence that a chemical change has happened and a new, insoluble substance has formed.
Answer: Gas is produced (fizzing; the gas is carbon dioxide) and there is a temperature change (it gets colder).
Answer: False
Bubbles in boiling water are water vapour, which is a physical change. Bubbles escaping when a fizzy drink is opened are dissolved carbon dioxide coming out of solution, also a physical change.
Answer: False
Mixing blue and yellow paint gives green, but no new substance forms; the paints are just mixed. The best test is whether a new substance with different properties has been made.
- AA new substance with different properties has formed
- BThe substance has changed shape
- CThe substance has changed from a solid to a liquid
- DThe substance has dissolved
Answer: A new substance with different properties has formed
The defining feature of a chemical change is the formation of a new substance. The other changes are all physical.
Part C: Particles and atoms
Answer: The water particles slow down and become fixed in a regular arrangement, vibrating in place. The particles themselves are still water particles, so no new substance forms.
Answer: magnesium + oxygen → magnesium oxide
Answer: The magnesium atoms and oxygen atoms join together (bond) to form a new substance, magnesium oxide, which has different properties from both. The atoms are rearranged; none are created or destroyed.
Part D: Conservation of mass
The law of conservation of mass: in a closed system, the total mass before a change equals the total mass after it, because atoms are not created or destroyed.
Answer: 50 g
- Melting is a physical change and nothing enters or leaves the sealed container.
- Mass is conserved, so the water has a mass of 50 g.
Answer: 20 g
- Mass is conserved in a closed system.
- 12 g + 8 g = 20 g
Answer: 1.6 g
- mass of oxygen = mass of product − mass of magnesium
- = 4.0 g − 2.4 g = 1.6 g
Answer: No. Oxygen from the air (which was not weighed at the start) combined with the magnesium. The total mass of magnesium plus oxygen equals the mass of magnesium oxide formed.
Answer: Most of the mass went into the air as gases (mainly carbon dioxide and water vapour) and smoke. If the gases were collected and weighed, the total mass of the products would equal the mass of the wood plus the oxygen it used.
Part E: Explain it
Model answer: It is a chemical change. Evidence: there is a colour change (shiny grey metal to reddish-brown), and the coating has different properties from iron (it is flaky and crumbly, not strong and shiny). A new substance, rust (a hydrated iron oxide), has formed because the iron reacted with oxygen and water. The change cannot easily be reversed: scraping off the rust does not turn it back into iron.
Success criteria:
- States it is a chemical change
- Gives two valid pieces of evidence
- Names rust (iron oxide) as the new substance
- Mentions oxygen and water reacting with the iron
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
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