Study Companion
Topic 04 | Chemical sciences

Physical vs chemical changes

Year 8 (Levels 7-8 band): telling physical changes from chemical reactions, the evidence of chemical change, and the standard laboratory tests for oxygen, carbon dioxide and hydrogen.

45-60 min Printable practice Answer key Challenge included
VC2S8U08
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What you will learn

Worked example 0 Real-world example: frying an egg

You crack an egg into a hot pan. The clear part turns white and solid. Is this a physical or chemical change? How do you know?

  1. The new white solid cannot be turned back into the clear liquid by cooling or any simple physical method.
  2. The colour, texture and structure of the egg have all changed at once.
  3. This is a chemical change — heat has caused the protein molecules in the egg to rearrange permanently (the proteins “denature” and link together).

Key idea: if you cannot easily reverse a change, it is almost always chemical.

1. Physical vs chemical change

FeaturePhysical changeChemical change (reaction)
New substance formed?NoYes
Atoms rearranged?No — atoms/molecules stay the same, positions changeYes — atoms regroup to form new molecules
Easily reversed?Usually yesUsually no
Mass change?NoneNone (mass is conserved)
ExamplesMelting, boiling, dissolving, cutting, crushingBurning, rusting, cooking, photosynthesis, digestion
Worked example 1 Sorting changes

Classify each as physical (P) or chemical (C): (a) sugar dissolving in tea; (b) a log burning; (c) ice melting; (d) iron rusting; (e) a nail being cut in half.

  1. (a) Sugar dissolving — the sugar molecules are still there, you can evaporate the water and recover them. P.
  2. (b) Burning — wood combines with oxygen to form CO2_2, water, and ash. You cannot get the log back. C.
  3. (c) Ice melting — still H2_2O, just rearranged from solid to liquid. P.
  4. (d) Rusting — iron combines with oxygen to form iron oxide. C.
  5. (e) Cutting a nail — the pieces are still iron. P.

2. Evidence of a chemical change

Several signs tell a chemist that a chemical reaction has happened:

  1. Colour change (and not just mixing of colours — a permanent new colour). Example: copper turning green (patina).
  2. Gas produced — bubbles or fizzing. Example: vinegar and baking soda.
  3. Precipitate — a solid that forms when two liquids are mixed. Example: silver nitrate + sodium chloride produces a white precipitate.
  4. Temperature change — some reactions release heat (exothermic), others absorb it (endothermic).
  5. Light or sound emitted — as in a burning match or fireworks.
Worked example 2 Identifying evidence

A student mixes two clear solutions in a test tube. A white solid appears and the tube feels colder. Which two pieces of evidence show a chemical reaction has occurred?

  1. A precipitate (the white solid) has formed from two clear liquids — a new substance.
  2. The temperature has dropped — energy was absorbed from the surroundings.

Key idea: two independent lines of evidence make the chemical change certain.

3. Laboratory gas tests

When you make a gas in a reaction, there are quick tests to identify it.

GasTestPositive result
Oxygen, O2\text{O}_2Place a glowing wooden splint in the gasSplint re-lights (bursts into flame)
Carbon dioxide, CO2\text{CO}_2Bubble the gas through limewaterLimewater turns cloudy/milky
Hydrogen, H2\text{H}_2Place a lit splint at the mouth of the tube”Squeaky pop” sound
Oxygen, carbon dioxide and hydrogen testsO₂: re-lightsCO₂: milkypop!H₂: squeaky pop
Three standard gas tests. Glowing splint for O2 (re-lights), limewater for CO2 (goes cloudy), lit splint for H2 (squeaky pop).
Worked example 3 Identifying an unknown gas

A student collects a gas from a reaction between magnesium metal and dilute hydrochloric acid. A lit splint at the mouth of the test tube gives a squeaky pop. Which gas is it?

  1. The “squeaky pop” is the standard test result for hydrogen, H2\text{H}_2.
  2. The reaction fits: magnesium + hydrochloric acid →\to magnesium chloride + hydrogen gas.

Key idea: each test uses the distinctive behaviour of the gas. Oxygen feeds combustion, CO2_2 reacts with calcium hydroxide in limewater, hydrogen is explosive in small amounts.

4. Word equations and conservation of mass

A word equation describes a reaction in plain language:

reactants⟶products\text{reactants} \longrightarrow \text{products}

For example:

Conservation of mass: in a chemical reaction, total mass does not change. The atoms before the reaction still exist after — just rearranged into new substances.

Worked example 4 Where did the mass go?

A 5.0 g candle burns in open air and only 0.2 g of ash is left behind. Has mass been lost?

  1. No — the wax combined with oxygen from the air to form carbon dioxide and water vapour.
  2. Those gases drifted away unseen, so the remaining solid is much lighter.
  3. If the reaction were done in a sealed jar and everything weighed, the total mass would be exactly the same before and after.

Key idea: mass is always conserved. “Disappearing” mass has usually become a gas you cannot see.

5. Everyday reactions

Practice: Year 8

Fluency

Physical or chemical?

    1. State whether each is a physical (P) or chemical (C) change: (a) water boiling, (b) a banana rotting, (c) iron rusting, (d) shredding paper, (e) a candle burning, (f) sugar dissolving in tea.
    2. List five pieces of evidence that a chemical change has happened.
    3. Give an example of (a) a physical change used at home, (b) a chemical change used in cooking.
    4. True/false: after a chemical reaction in a sealed container, the total mass changes.
    5. Explain why “mixing paint colours” is a physical and not a chemical change.
Fluency

Gas tests

    1. Which gas re-lights a glowing splint?
    2. Which gas turns limewater cloudy?
    3. What is the test for hydrogen?
    4. A gas does not re-light a splint, does not pop, and does not affect limewater. Could it be hydrogen? Could it be nitrogen?
    5. What does limewater actually react with in the CO2_2 test?
Reasoning

Explain

    1. Explain, using the atomic theory, why a chemical change produces a truly new substance while a physical change does not.
    2. A student claims the rust on a car means mass has been lost. Argue against this claim using conservation of mass.
    3. A reaction that feels cold to the touch is still a chemical reaction. Explain how this is possible.
    4. A gas is produced when vinegar meets baking soda. Design a test to confirm it is carbon dioxide.
Problem solving

Applied contexts

    1. A Year 8 student heats a small piece of magnesium in air. A bright white light is given off, and a white powder is left. State two pieces of evidence for a chemical reaction, and write a word equation.
    2. A sealed 200 g bottle of fizzy drink is shaken and the lid opened. Gas escapes. Predict whether the bottle now weighs exactly 200 g, more, or less. Explain.
    3. A student mixes solutions of lead nitrate and potassium iodide. A bright yellow solid appears. Classify the change and explain.
    4. Explain why scientists always test the gas produced in a reaction rather than just assuming what it is from what was added.

Challenge

Reasoning

Harder reasoning

    1. A 10 g strip of magnesium is burned in a crucible. The crucible + magnesium started at 50 g. After burning, the crucible + white powder weighs 56.6 g. Use conservation of mass to explain where the extra mass came from.
    2. A scientist bubbles a gas through limewater and nothing happens. Next they test with a glowing splint and it re-lights. Identify the gas and suggest a chemical reaction that could produce it in the lab.
    3. A chemical reaction is described as both exothermic and combustion. Explain how these two terms fit together and give an everyday example.
    4. Carbonic acid (H2CO3\text{H}_2\text{CO}_3) breaks down into water and carbon dioxide. Write this as a word equation and explain why it looks like “something has been lost” to an observer.
Answers

Answer key

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Year 8 answers

Fluency

Physical or chemical?

    1. (a) P, (b) C, (c) C, (d) P, (e) C, (f) P.
    2. Colour change, gas produced, precipitate formed, temperature change, light/sound emitted.
    3. (a) Any reasonable physical change, e.g. melting butter, dissolving sugar, cutting vegetables. (b) Any reasonable chemical change, e.g. baking a cake, frying an egg, browning toast.
    4. False — mass is conserved in a sealed container.
    5. Each pigment is still the same substance after mixing; you could in principle separate them again. No new substance has formed, so the change is physical.
Fluency

Gas tests

    1. Oxygen (O2\text{O}_2).
    2. Carbon dioxide (CO2\text{CO}_2).
    3. A lit splint at the mouth of the tube gives a squeaky pop.
    4. Not hydrogen (no pop). Could be nitrogen, which gives no response to any of those tests.
    5. Calcium hydroxide, Ca(OH)2\text{Ca(OH)}_2 — it reacts with CO2_2 to form calcium carbonate, a white solid that clouds the water.
Reasoning

Explain

    1. In a chemical change, atoms are rearranged into new groupings (new molecules), which have new properties. In a physical change the atoms stay in the same molecules — only their arrangement or state changes.
    2. Rusting adds oxygen atoms from the air to the iron atoms, forming iron oxide. Total mass of iron + oxygen before equals mass of rust after. Mass was not lost — the rust is heavier than the iron was.
    3. Endothermic reactions absorb energy from the surroundings, so the reaction mixture feels cold. Whether heat is taken in or given out, a new substance has formed, so it is still a chemical reaction.
    4. Bubble the gas through limewater. If it turns cloudy/milky, the gas is CO2_2.
Problem solving

Applied contexts

    1. Evidence: bright white light given off; new white powder formed (was silvery metal). Word equation: magnesium + oxygen →\to magnesium oxide.
    2. Less than 200 g. Carbon dioxide escaped as gas, taking some mass with it.
    3. Chemical change. A precipitate (the yellow solid, lead iodide) has formed — a new substance.
    4. Different gases can sometimes come from similar-looking reactions, and some reactions produce a mixture. Testing confirms which gas you actually have and avoids wrong conclusions.
Reasoning

Challenge

    1. The extra 6.66.6 g came from oxygen atoms in the air that combined with the magnesium to form magnesium oxide. Total mass (Mg + O + crucible) is still conserved; it just was not all on the scales at the start.
    2. The gas does not react with limewater (rules out CO2_2) but re-lights a glowing splint — so it is oxygen. Example lab preparation: decomposition of hydrogen peroxide using a catalyst (manganese dioxide): hydrogen peroxide →\to water + oxygen.
    3. Combustion is a fuel reacting rapidly with oxygen to give out heat and light. It is exothermic because it releases energy. Everyday example: a gas stove burning natural gas.
    4. carbonic acid →\to water + carbon dioxide. The CO2_2 leaves as a gas, so the liquid volume shrinks — it looks like something has “been lost,” but in a sealed container the total mass is unchanged.

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