Coaching Centre
Chemical change review
A summary of the whole Chemical Change chapter, key words, and a mixed test covering physical and chemical change, signs of reactions, equations and conservation of mass, energy, cooking, nature, polymers and medicines.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 5A Physical and chemical change | Physical: same substance, particles unchanged, often reversible. Chemical (reaction): new substances, bonds break and re-form, usually hard to reverse, bigger energy changes. Mass conserved in both. |
| 5B Signs of chemical change | Gas, permanent colour change, precipitate, temperature change, light, substance disappearing. Not proof on their own. Gas tests: H₂ pops, O₂ relights a glowing splint, CO₂ turns limewater milky. Rusting needs oxygen and water; salt speeds it up. |
| 5C Equations and mass | Reactants → products. Word and symbol equations; big numbers = how many particles. Combination, decomposition, combustion, corrosion, displacement, neutralisation. Conservation of mass (Lavoisier); open vs closed systems. |
| 5D Energy in reactions | Breaking bonds takes in energy; making bonds releases it. Exothermic (burning, respiration, hand warmers) vs endothermic (photosynthesis, cooking). Activation energy; catalysts and enzymes lower it. Thermite, dynamite and Nobel. |
| 5E Kitchen chemistry | Browning, caramelising, setting proteins. Baking soda + acid or heat → CO₂; baking powder. Fermentation: glucose → ethanol + carbon dioxide (yeast); lactic acid (bacteria). First Nations leaching and fermenting of cycad seeds. |
| 5F Chemistry in nature | Photosynthesis (chloroplasts, endothermic) and respiration (mitochondria, exothermic); starch and limewater tests. Chemical weathering: oxidation, carbonic acid, limestone caves. pH; acid rain; acid sulfate soils. |
| 5G Fuels and polymers | Fossil fuels, hydrocarbons, fractional distillation of crude oil, petrochemicals. Monomers → polymers; natural and synthetic; thermoplastics and thermosets. Fabrics. Plastic waste, recycling and bioplastics. |
| 5H Medicines and materials | Pharmaceuticals; aspirin from willow; First Nations medicines (tea tree). Penicillin: Fleming, Florey, Chain. Resistance, screening, clinical trials. Aluminium, oxygen from liquid air, dyes, Shrilk: collaboration across fields. |
Putting it together
One idea links this whole chapter: in a chemical reaction, atoms are rearranged into new substances, but never created or destroyed. That's why mass is conserved, why breaking and making bonds involves energy, and why we can write equations that balance. The same rearrangements raise bread, rust bridges, build sugar in leaves, carve limestone caves, turn oil into plastic and turn mould into medicine. Chemists use this understanding to make the substances society needs, and to deal with the problems some of them cause.
Key words
acid · acid rain · activation energy · anaerobic · antibiotic · catalyst · chemical change · chemical reaction · chemical weathering · chlorophyll · chloroplast · clinical trial · conservation of mass · crude oil · decomposition · endothermic · enzyme · exothermic · fermentation · fossil fuel · fractional distillation · hydrocarbon · mitochondria · monomer · oxidation · petrochemical · pharmaceutical · photosynthesis · physical change · pH · polymer · precipitate · product · reactant · resistance · respiration · symbol equation · thermoplastic · thermoset · word equation
Test yourself
Which is a physical change?
It separates a mixture by boiling point; no new substances.
A glowing splint relights in a gas. The gas is
Oxygen supports burning.
24 g of magnesium burns completely to make 40 g of magnesium oxide. What mass of oxygen reacted?
Mass is conserved: 40 − 24 = 16 g.
Respiration is
It releases energy in every living cell.
Enzymes are
They lower activation energy in living things.
Which rock is most damaged by acid rain?
Calcium carbonate reacts with acids.
Polyethylene is made by joining ethene molecules. Ethene is the
Monomers join to make polymers.
Yeast makes bread rise by producing
glucose → ethanol + carbon dioxide
Label the monomers and the polymer.
Classify each change.
The substances made in a reaction are the . A solid formed when two solutions react is a . The energy needed to start a reaction is the energy. In a reaction, atoms are rearranged, so is conserved. Bacteria that survive antibiotics can become .
A student burns a piece of paper in a metal dish on a balance and finds the dish and contents get lighter. Another student burns steel wool and finds it gets heavier. Explain both results, show how they are consistent with the law of conservation of mass, and explain how both experiments could be changed to show that mass is conserved.
When paper burns, it reacts with oxygen to make carbon dioxide and water vapour (plus some ash). These products are gases, and in an open dish they escape into the air. The balance only weighs what is left behind, so the reading goes down.
When steel wool burns, the iron combines with oxygen from the air to make iron oxide, a solid that stays in the dish. The solid now contains the mass of the oxygen atoms it has gained, so the reading goes up.
Both results agree with the law of conservation of mass. In each reaction atoms are only rearranged, never created or destroyed: in the first, mass seems to be lost because some atoms leave as gas; in the second, mass seems to be gained because atoms from the air join in. To show mass is conserved, each reaction should be done in a sealed, closed container (for example, a sealed flask with enough air inside, lit with an electric spark or by focusing light). Then no gas can get in or out, and the total mass before and after would be the same.