Chemical changes: GCSE questions with answers

Answer: C Cl₂

An element contains only one kind of atom, so its formula has only one symbol. Hydrogen, oxygen, nitrogen and the halogens exist as molecules of two atoms, so they are written H₂, O₂, N₂ and Cl₂.

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Answer: B CO₂

A formula shows the atoms in one molecule. Water is H₂O, carbon dioxide is CO₂, ammonia is NH₃ and methane is CH₄. The small number after a symbol counts the atoms of the element just before it.

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Answer: C Copper sulfate

A name ending in -ide, such as chloride, oxide or bromide, usually means just two elements. A name ending in -ate, such as sulfate, nitrate or carbonate, means oxygen is there too: copper sulfate is CuSO₄.

Checked against Pearson Edexcel GCSE Chemistry (1CH0) specification, statement 1.25

Answer: C The simplest whole number ratio of atoms

The molecular formula shows the actual atoms in one molecule; the empirical formula shows the simplest whole number ratio. Ethane, C₂H₆, has 1 carbon for every 3 hydrogen atoms, so its empirical formula is CH₃.

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Answer: C Na₂O

Count each kind of atom or ion in the model and write the ratio. For a giant ionic lattice the formula is the simplest ratio of the ions, so two Na⁺ for every O²⁻ gives Na₂O. For a small molecule, count every atom in it.

Checked against AQA GCSE Chemistry (8462) specification, section 4.2.1.3 and 4.2.1.4

Answer: B Atoms are not made or destroyed in a reaction

In a reaction, atoms are rearranged into new substances, but none are made or destroyed. So the total mass stays the same, and an equation must show the same number of each kind of atom on both sides.

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Answer: B The numbers in front of the formulae

Changing a small number inside a formula would make a different substance, so you only change the big numbers in front. In 2H₂O the 2 in front means two molecules, each with two hydrogen atoms, so four hydrogen atoms in all.

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Answer: B 2H₂ + O₂ → 2H₂O

Count each kind of atom on both sides. In 2H₂ + O₂ → 2H₂O there are four hydrogen and two oxygen atoms on each side. The formulae must stay correct: magnesium oxide is always MgO, so you balance with the numbers in front.

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Answer: C MgCl₂

The positive and negative charges in an ionic compound must cancel out. One Mg²⁺ needs two Cl⁻, so magnesium chloride is MgCl₂. Brackets go round a group ion such as OH⁻ when you need more than one of it, as in Ca(OH)₂.

Checked against Pearson Edexcel GCSE Chemistry (1CH0) specification, statement 1.26

Answer: C Dissolved in water

State symbols show the state of each substance: (s) solid, (l) liquid, (g) gas and (aq) aqueous, which means dissolved in water. Pure water is (l), but an acid or a salt dissolved in water is (aq).

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Answer: D A glowing splint relights

Things burn much better in oxygen than in air, so a glowing splint pushed into oxygen bursts back into flame. A lit splint is the test for hydrogen, not for oxygen.

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Answer: A A lit splint gives a squeaky pop

Hold a lit (burning) splint at the mouth of the test tube. Hydrogen burns very quickly with a squeaky pop, which is a small explosion as hydrogen reacts with oxygen in the air to make water.

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Answer: B Limewater turns milky

Bubble the gas through limewater, which is calcium hydroxide solution. Carbon dioxide reacts with it to make tiny solid particles of calcium carbonate, which turn the limewater milky (cloudy).

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Answer: C Damp litmus paper is bleached

Hold damp litmus paper in the gas: chlorine bleaches it white. Blue litmus may turn red first, because chlorine dissolves in the water on the paper to make an acidic solution, and then it is bleached.

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Answer: A Hydrogen

Metals above hydrogen in the reactivity series react with dilute acids: metal + acid → salt + hydrogen. The salt is named from the metal and the acid, so zinc and sulfuric acid make zinc sulfate.

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Answer: A Calcium chloride, water and carbon dioxide

Acid + metal carbonate → salt + water + carbon dioxide. The fizzing is carbon dioxide escaping, which you can show by bubbling it through limewater. Hydrogen comes from acids reacting with metals, not with carbonates.

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Answer: B Nitric acid

The first part of a salt name comes from the metal and the second part from the acid. Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.

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Answer: D Hydrogen ions, H⁺

An acid releases hydrogen ions, H⁺, when it dissolves in water: hydrogen chloride gives H⁺ and Cl⁻. Solutions of alkalis contain hydroxide ions, OH⁻. These ions give acids and alkalis their typical reactions.

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Answer: A Neutral

The pH scale runs from 0 to 14. A solution at pH 7 is neutral, acids are below 7 and alkalis are above 7. The further the pH is from 7, the more acidic or alkaline the solution.

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Answer: D Green

Universal indicator is a mix of dyes that changes colour across the whole pH range: red when very acidic, green when neutral, and blue to purple when alkaline. A pH probe or meter gives a more exact number.

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Answer: A A salt and water

In neutralisation, acid + alkali → salt + water. For example, hydrochloric acid and sodium hydroxide make sodium chloride and water. No gas is given off when an acid reacts with an alkali.

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Answer: D A base that dissolves in water

A base is a substance that neutralises an acid to make a salt and water, such as a metal oxide or hydroxide. An alkali is a base that dissolves in water, like sodium hydroxide. Copper oxide does not dissolve, so it is a base but not an alkali.

Checked against Pearson Edexcel GCSE Chemistry (1CH0) specification, statement 3.9 and 3.10

Answer: A To make sure all the acid reacts

Adding the insoluble base until no more reacts makes sure all the acid is used up. Filtering removes the leftover solid, leaving only the salt solution. Evaporating some water and leaving it to crystallise gives crystals of the salt.

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Answer: D H⁺(aq) + OH⁻(aq) → H₂O(l)

Whatever acid and alkali you use, the real change is the same: H⁺ ions from the acid join OH⁻ ions from the alkali to make water. The other ions stay in solution and make up the salt.

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Answer: B Sodium atoms form positive ions more easily

When metals react, their atoms lose electrons to form positive ions. The more easily a metal does this, the more reactive it is. Potassium and sodium form positive ions very easily; copper does not.

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Answer: B Potassium

The most reactive metals, such as potassium and sodium, fizz with cold water to make a metal hydroxide and hydrogen. Calcium reacts steadily, magnesium only very slowly, and copper not at all.

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Answer: B Copper

Metals above hydrogen in the reactivity series react with dilute acids to give hydrogen. Magnesium fizzes quickly, zinc and iron more slowly, and copper, which is below hydrogen, does not react at all.

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The right order:

1. Sodium
2. Calcium
3. Magnesium
4. Zinc
5. Iron
6. Copper

This order comes from how vigorously each metal reacts with water and dilute acids, which shows how easily it forms positive ions. Sodium fizzes fast on cold water, while copper reacts with neither water nor dilute acids.

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Answer: C Zinc and copper sulfate solution

A more reactive metal displaces a less reactive metal from a solution of its compound. Iron is more reactive than copper, so iron + copper sulfate → iron sulfate + copper. A less reactive metal cannot displace a more reactive one.

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Answer: D X, Z, Y

To rank metals, compare how vigorously each reacts with water or dilute acid, and which displaces which. A metal that displaces another is more reactive than it. A metal that reacts with nothing is the least reactive.

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Answer: C A molten or dissolved ionic compound

An electrolyte is an ionic compound that has been melted or dissolved in water, so its ions are free to move. In a solid the ions are held in a lattice and cannot move, so no current flows and no electrolysis happens.

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Answer: A The cathode, the negative electrode

Opposite charges attract. Positive ions (cations) move to the negative electrode, the cathode. Negative ions (anions) move to the positive electrode, the anode. At the electrodes the ions are discharged and form elements.

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Answer: C At the cathode

Metal ions and hydrogen ions are positive, so metals or hydrogen form at the negative cathode. Non-metal ions such as halide ions are negative, so non-metals such as chlorine, bromine or oxygen form at the positive anode. Inert electrodes, such as graphite, do not react themselves.

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Answer: C Lead

A molten binary ionic compound contains only two kinds of ion, so there is no competition. The metal forms at the cathode and the non-metal at the anode: molten lead bromide gives lead and bromine, and molten sodium chloride gives sodium and chlorine.

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Answer: B When the metal is more reactive than hydrogen

A salt solution also contains H⁺ ions from water. At the cathode, if the metal is more reactive than hydrogen, as sodium is, hydrogen is given off instead. If the metal is less reactive, as copper is, the metal is deposited.

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Answer: C Oxygen

At the anode, oxygen is given off unless the solution contains halide ions (chloride, bromide or iodide), when the halogen forms instead. Sulfate ions are not discharged, so sodium sulfate solution gives hydrogen and oxygen.

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Answer: B From water molecules

A salt solution contains the ions of the salt plus some H⁺ and OH⁻ ions from water. These compete at each electrode, and which one is discharged depends on reactivity and on whether halide ions are present.

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Answer: D Copper and chlorine

Copper is less reactive than hydrogen, so copper forms at the cathode, and chloride is a halide, so chlorine forms at the anode. Potassium is more reactive than hydrogen, so potassium bromide solution gives hydrogen and bromine.

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Answer: D Gain of oxygen

Oxidation is gain of oxygen and reduction is loss of oxygen. When magnesium burns it gains oxygen, so it is oxidised to magnesium oxide. When a metal oxide loses its oxygen to form the metal, the oxide is reduced.

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Answer: A Iron oxide

Look at which substance loses oxygen and which gains it. Iron oxide loses its oxygen, so it is reduced; carbon gains oxygen, so it is oxidised. It is the metal oxide that is reduced, not the metal that forms.

Checked against AQA GCSE Chemistry (8462) specification, section 4.4.1.3