PassNova has 30 free KS3 Science practice questions on Chemistry, each with a clear explanation. Practise them in the browser with instant feedback — 100% free, no sign-up, on any device. Updated for 2026.
Chemistry: example questions & answers
30 worked examples with answers and explanations below. Practise them in the browser with instant feedback on every answer.
What are the three states of matter?
- AHot, warm, cold
- BSolid, liquid, gas✓
- CBig, medium, small
- DHard, soft, runny
Answer: Matter usually exists as a solid, a liquid or a gas.
In which state are the particles packed closely together and vibrating in fixed positions?
- ALiquid
- BGas
- CPlasma
- DSolid✓
Answer: In a solid, particles are tightly packed in a fixed arrangement and only vibrate.
What is the smallest part of an element?
- AAn atom✓
- BA cell
- CA mixture
- DA compound
Answer: An atom is the smallest particle of an element that still has its properties.
Water is made of hydrogen and oxygen joined together. Water is therefore a:
- AMetal
- BSingle element
- CMixture
- DCompound✓
Answer: A compound is formed when two or more elements chemically bond — H₂O is a compound.
What colour does universal indicator turn in a strong acid?
- ABlue
- BRed✓
- CPurple
- DGreen
Answer: Universal indicator turns red in a strong acid (low pH).
On the pH scale, a neutral substance like pure water has a pH of:
- A14
- B0
- C1
- D7✓
Answer: pH 7 is neutral; below 7 is acidic and above 7 is alkaline.
Which process turns a liquid into a gas?
- AFreezing
- BMelting
- CEvaporation✓
- DCondensing
Answer: Evaporation (or boiling) changes a liquid into a gas.
What is formed when a metal reacts with oxygen?
- AA salt
- BA metal oxide✓
- CA metal hydroxide (alkali)
- DHydrogen gas
Answer: Metals react with oxygen to form metal oxides (e.g. magnesium + oxygen → magnesium oxide).
The rows in the Periodic Table are called:
- AFamilies
- BGroups
- CColumns
- DPeriods✓
Answer: Horizontal rows are 'periods'; vertical columns are 'groups'.
Which of these is a chemical change (not a physical one)?
- AMelting ice
- BBoiling water
- CBurning wood✓
- DCutting paper
Answer: Burning wood makes new substances and is irreversible — a chemical change.
A mixture of sand and water can be separated by:
- AFiltering✓
- BBurning
- CMagnetism
- DFreezing
Answer: Filtering traps the insoluble sand and lets the water pass through.
Which gas makes up most of the air we breathe?
- ACarbon dioxide
- BNitrogen✓
- COxygen
- DHelium
Answer: About 78% of air is nitrogen; oxygen is around 21%.
What do we call the change of a gas into a liquid?
- ACondensation✓
- BCrystallisation
- CMelting
- DChromatography
Answer: Condensation is when a gas cools and turns into a liquid (e.g. steam on a cold window).
Salt is stirred into water until it can no longer be seen. What is the salt called in this mixture?
- AThe solvent
- BThe solute✓
- CThe residue
- DThe precipitate
Answer: The substance that dissolves is the solute and the liquid it dissolves in is the solvent, so here the salt is the solute and the water is the solvent. A residue is what stays behind in the filter paper, and a precipitate is a solid formed by a reaction in a solution.
Which layer of the Earth lies directly beneath the crust?
- AThe inner core
- BThe outer core
- CThe atmosphere
- DThe mantle✓
Answer: Working outwards from the centre, the Earth has an inner core, an outer core, the mantle and finally the thin rocky crust, so the mantle is the layer immediately below the crust.
Which particles are found in the nucleus of an atom?
- AProtons and neutrons✓
- BProtons and electrons
- CNeutrons and electrons
- DElectrons only
Answer: The nucleus holds the protons and the neutrons, which together account for almost all of an atom's mass. The electrons move around the nucleus in shells.
An atom of an element has 11 protons. What else must be true of that atom?
- AIt contains exactly 11 neutrons
- BIt carries an overall charge of +11
- CIt contains exactly 11 electrons✓
- DIt must be a noble gas
Answer: An atom has no overall charge, so the number of negative electrons must exactly balance the number of positive protons, giving 11 electrons. The number of neutrons is not fixed by the number of protons.
Why are the elements in Group 0 of the periodic table so unreactive?
- ATheir outer electron shell is already full✓
- BThey are heavier than all the other elements
- CThey are all liquids at room temperature
- DThey hold more neutrons than protons
Answer: The Group 0 elements, the noble gases, already have a complete outer shell of electrons, so they have no tendency to gain, lose or share electrons and take part in almost no reactions.
Lithium, sodium and potassium are all Group 1 metals. Which of them reacts most vigorously with water?
- ALithium
- BThey all react at the same rate
- CSodium
- DPotassium✓
Answer: Reactivity increases going down Group 1, and potassium sits below the other two, so it reacts fastest, often igniting the hydrogen it releases with a lilac flame.
Which technique would you use to obtain pure water from salty water?
- AFiltration
- BDistillation✓
- CChromatography
- DSieving
Answer: Distillation boils the water off and leaves the dissolved salt behind, then cools the steam so it condenses back into pure liquid water. Filtering cannot remove a solid that has dissolved.
Which of these substances is a mixture?
- AWater
- BCopper
- CAir✓
- DCarbon dioxide
Answer: Air is a mixture: it is mostly nitrogen and oxygen, which are not chemically joined and can be separated by physical means. Water and carbon dioxide are compounds whose atoms are chemically bonded, and copper is an element made of just one kind of atom.
Magnesium ribbon burns with a bright white flame in oxygen. Which word equation describes this reaction?
- Amagnesium + oxygen → magnesium hydroxide
- Bmagnesium oxide → magnesium + oxygen
- Cmagnesium + water → magnesium oxide
- Dmagnesium + oxygen → magnesium oxide✓
Answer: The two reactants, magnesium and oxygen, combine to make the single product magnesium oxide. Any metal burning in oxygen forms a metal oxide, and the reactants always go on the left of the arrow.
In the balanced equation 2Mg + O₂ → 2MgO, how many oxygen atoms are there on each side?
- A2✓
- B1
- C3
- D4
Answer: On the left, O₂ is one molecule holding 2 oxygen atoms. On the right, 2MgO means two units each holding 1 oxygen atom, which is 2 again. A balanced equation always has the same number of each kind of atom on both sides.
A solution is tested and found to have a pH of 2. What does this tell you?
- AIt is a weak alkali
- BIt is a strong acid✓
- CIt is neutral
- DIt is a strong alkali
Answer: The pH scale runs from 0 to 14 with 7 as neutral. A value well below 7 means a strongly acidic solution, so pH 2 is a strong acid.
Hydrochloric acid is neutralised by sodium hydroxide solution. Which two products are formed?
- AHydrogen and oxygen
- BSodium and chlorine
- CSodium chloride and water✓
- DSodium hydroxide and hydrogen
Answer: Neutralising an acid with an alkali always produces a salt plus water. Hydrochloric acid produces chloride salts, so with sodium hydroxide the products are sodium chloride and water.
Which of these metals will displace copper from copper sulfate solution?
- AMagnesium✓
- BSilver
- CGold
- DCopper
Answer: A metal can only push another metal out of its compound if it sits higher in the reactivity series. Magnesium is well above copper, whereas silver and gold are both below it.
When 24 g of magnesium burns completely it combines with 16 g of oxygen. What mass of magnesium oxide is produced?
- A8 g
- B24 g
- C384 g
- D40 g✓
Answer: Mass is conserved in a chemical reaction, so the mass of the product equals the total mass of the reactants: 24 + 16 = 40 g. The apparent gain in mass is simply the oxygen that has joined the metal.
Why can a gas be squashed into a much smaller volume while a liquid can hardly be squashed at all?
- AGas particles are far smaller than liquid particles
- BGas particles weigh less than liquid particles
- CGas particles have large spaces between them✓
- DGas particles stop moving when they are squashed
Answer: In a gas the particles are widely spaced with big gaps between them, so they can be forced closer together. In a liquid the particles are already touching, so there is almost no space left to remove.
Which type of rock forms when layers of sediment are buried and compressed over long periods of time?
- AIgneous rock
- BMetamorphic rock
- CSedimentary rock✓
- DMagma
Answer: Sediment settles in layers, and the weight of the layers above squeezes the water out and cements the grains together, forming sedimentary rock. Igneous rock forms when molten rock cools, and metamorphic rock forms when heat and pressure alter an existing rock.
A bottle of perfume is opened at one end of a still room and the smell is soon noticed at the other end. Which process explains this?
- ADiffusion✓
- BCondensation
- CFiltration
- DFreezing
Answer: The perfume particles move randomly and spread out from where they are most concentrated until they are mixed evenly among the air particles. That spreading out is called diffusion.