These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Substances Z, R and G in Questions 4 and 5 are fictional, and their data are invented for practice. Limewater turns cloudy when carbon dioxide is bubbled through it.
Question 1 (multiple choice · foundation)
Which process involves breaking covalent bonds inside molecules?
- (A) Boiling ethanol
- (B) Ice melting on a warm day
- (C) Passing an electric current through water to make hydrogen and oxygen
- (D) Mixing ethanol with water
Answer and explanation
Answer: (C). Electrolysis turns H₂O into H₂ and O₂. The O–H covalent bonds break, and new H–H and O=O bonds form. New substances appear.
- (A) Boiling separates whole ethanol molecules. Only hydrogen bonds and other attractions between molecules are overcome.
- (B) Melting breaks some hydrogen bonds between H₂O molecules. The O–H bonds inside each molecule stay intact.
- (D) Ethanol and water molecules mix intact. Hydrogen bonds form between ethanol and water molecules, but no covalent bonds break.
Question 2 (multiple choice · foundation)
A few drops of liquid bromine, Br₂, are left in a closed gas jar at room temperature. A brown vapour slowly fills the jar. Which best describes the particles in the vapour?
- (A) Separate bromine atoms, Br
- (B) Bromine molecules, Br₂
- (C) Bromide ions, Br⁻, and bromine cations, Br⁺
- (D) Molecules of a new compound formed between bromine and the air in the jar
Answer and explanation
Answer: (B). Evaporation is a phase change. Only the London dispersion forces between Br₂ molecules are overcome, so whole Br₂ molecules escape into the gas phase. The brown colour of the vapour is the colour of Br₂ itself.
- (A) would need the Br–Br covalent bond to break, which needs far more energy than evaporation at room temperature.
- (C) Splitting a molecule into ions needs even more energy, and a gas of separate ions does not form at room temperature.
- (D) Seeing a colour in the gas does not mean a new substance formed. The jar contains the same Br₂, now spread out.
Question 3 (multiple choice · core)
Which observation is the strongest evidence that a chemical change has taken place?
- (A) A gas forms when a liquid is heated.
- (B) A solid disappears when it is stirred into water.
- (C) After heating, a solid has a different colour and a different melting point, and cooling it does not bring back the original solid.
- (D) A solution becomes colder as a solid dissolves in it.
Answer and explanation
Answer: (C). A new set of properties that does not return when the temperature change is reversed shows that a new substance has formed. That means chemical bonds broke and new ones formed.
- (A) Boiling makes a gas from a liquid without any reaction.
- (B) Dissolving a molecular solid such as sugar is a physical change.
- (D) Some salts take in energy as they dissolve; the change in attractions makes the solution colder without a new substance forming.
Question 4 (constructed response · core)
Substance Z is a molecular liquid. Two energy values are measured for 1 mol of Z:
- Energy needed to turn liquid Z into gaseous Z: 35 kJ
- Energy needed to break every covalent bond in gaseous Z molecules into separate atoms: 1650 kJ
(a) State which value belongs to a physical process. Justify your answer. (b) Explain, in terms of the attractions involved, why the two values are so different. (c) A sample of Z is heated gently until it boils. Predict whether the vapour contains Z molecules or the separate atoms, and explain.
Worked solution and suggested Marlbridge rubric
(a) The 35 kJ value. Turning liquid Z into gas is a phase change: the Z molecules stay intact and only the attractions between them are overcome.
(b) Vaporizing overcomes intermolecular attractions, which are weak. Breaking the molecules apart needs every covalent bond to break, and covalent bonds are much stronger. The bond-breaking value is about 47 times the vaporization value.
(c) The vapour contains Z molecules. Gentle heating supplies enough energy to overcome intermolecular attractions (35 kJ mol⁻¹) long before it could break covalent bonds (1650 kJ mol⁻¹).
| Point | What earns it |
|---|---|
| 1 | Chooses 35 kJ, with the reason that molecules stay intact (phase change) |
| 1 | Links the small value to weak intermolecular attractions and the large value to strong covalent bonds |
| 1 | Predicts intact Z molecules in the vapour, using the energy comparison |
Question 5 (constructed response · core)
A student tests two substances.
| Measurement | Trial 1: blue liquid R | Trial 2: green solid G |
|---|---|---|
| Mass before | 1.84 g | 3.00 g |
| Treatment | Heated in a sealed apparatus; vapour cooled and collected | Heated strongly in an open test tube |
| Observations | Boils at 64 °C; a blue liquid collects in the cooled flask | Turns black; a gas forms that turns limewater cloudy |
| Mass afterwards | 1.84 g of blue liquid | 1.93 g of black solid |
| Further test | Collected liquid boils at 64 °C | Black solid stays black on cooling |
(a) Make a claim about the type of change in each trial. (b) Support each claim with two pieces of evidence from the table. (c) Explain each claim in terms of what happens to bonds or attractions.
Worked solution and suggested Marlbridge rubric
(a) Trial 1 is a physical change (boiling, then condensing). Trial 2 is a chemical change.
(b) Trial 1: the mass is unchanged (1.84 g); the collected liquid has the same colour and the same boiling point (64 °C). Trial 2: a gas that turns limewater cloudy (carbon dioxide) forms; the solid changes colour from green to black and does not change back on cooling; the mass falls by 1.07 g.
(c) Trial 1: boiling and condensing only overcome and then re-form the attractions between molecules of R. The molecules themselves are unchanged. Trial 2: carbon dioxide is a new substance. Bonds in G must break and new bonds form, giving CO₂ and a different black solid.
| Point | What earns it |
|---|---|
| 1 | Both claims correct |
| 1 | Two valid pieces of evidence for Trial 1 |
| 1 | Two valid pieces of evidence for Trial 2 |
| 1 | Trial 1 reasoning: only intermolecular attractions change; molecules stay intact |
| 1 | Trial 2 reasoning: bonds broken and formed, giving new substances |
Do not award a reasoning point for a label alone (“it is physical because it is a phase change”) unless the answer also says what happens to the particles.
Question 6 (constructed response · core)
Solid potassium bromide, KBr, dissolves in water.
(a) Describe what happens to the particles when KBr dissolves. Name the attractions that are broken and the attractions that form. (b) Give one argument for classifying the process as physical and one argument for classifying it as chemical. (c) Explain why dissolving glucose, C₆H₁₂O₆, is more clearly a physical process than dissolving KBr.
Worked solution and suggested Marlbridge rubric
(a) The ionic bonds between K⁺ and Br⁻ ions in the lattice are broken. The ions separate and each becomes surrounded by water molecules. Ion–dipole attractions form: the partly negative oxygen ends of water molecules face K⁺, and the partly positive hydrogen ends face Br⁻. Some hydrogen bonds between water molecules are also broken to make room.
(b) Physical: the same K⁺ and Br⁻ ions are present before and after, and evaporating the water gives back solid KBr. Chemical: ionic bonds are broken and new interactions (ion–dipole) form, and the solution has new properties such as electrical conductivity.
(c) Glucose is molecular. Its molecules stay whole; no covalent or ionic bonds break. Only attractions between molecules change (glucose–glucose hydrogen bonds are replaced by glucose–water hydrogen bonds).
| Point | What earns it |
|---|---|
| 1 | Ionic bonds in the lattice broken |
| 1 | Ion–dipole attractions form, with the correct orientation of water for at least one ion |
| 1 | A valid argument for physical and a valid argument for chemical |
| 1 | Glucose molecules stay intact; only intermolecular attractions change |
Question 7 (explanation · stretch)
Three solids are melted: ice (H₂O), sodium chloride (NaCl) and quartz (silicon dioxide, SiO₂, a covalent network solid).
(a) For each solid, name the attractions that must be overcome or broken for it to melt. (b) Melting sodium chloride and melting quartz both involve breaking chemical bonds, yet melting is usually classified as a physical change. Explain why. (c) Describe one test a student could do to show that melting sodium chloride produced no new substance.
Model answer and suggested Marlbridge rubric
(a) Ice: hydrogen bonds (and other intermolecular attractions) between H₂O molecules; the O–H bonds stay intact. Sodium chloride: ionic bonds in the rigid lattice, so the ions can move past one another (they still attract each other in the liquid). Quartz: Si–O covalent bonds in the network must break for the solid to flow.
(b) Melting does not change the composition. Molten NaCl still contains Na⁺ and Cl⁻ in a 1 : 1 ratio, and molten quartz is still SiO₂. No new combination of elements forms, so the substance keeps its identity; only its state changes. This is one of the cases where a physical process involves breaking chemical bonds, so the “bonds broken means chemical” rule needs judgement.
(c) Let the liquid cool and solidify, then show that the solid has the same properties as before: for example the same melting point, the same appearance, and the same mass. (Showing that the molten salt conducts is not enough on its own, because it shows ions are present, not that the substance is unchanged.)
| Point | What earns it |
|---|---|
| 1 | Correct attractions for all three: intermolecular (hydrogen bonds) for ice, ionic for NaCl, covalent for SiO₂ |
| 1 | Explains that composition is unchanged (same ions or same formula), so no new substance forms |
| 1 | A valid test comparing a property of the re-solidified sample with the original |
How did you do?
- Q1 or Q2 wrong: re-read “Two kinds of attraction, two kinds of change” and Figure 1 in the study guide.
- Q3 wrong: re-read “Linking what you see to what the particles do”.
- Q4 wrong: look again at “Why the energy scale is so different”.
- Q5 incomplete: compare your answer with the claim–evidence–reasoning steps in Worked example 2.
- Q6 incomplete: re-read “When the line is blurred: dissolving a salt”.
- Q7 incomplete: re-read “A decision routine” and “When the line is blurred”, and look at what stays the same when a solid melts.
Then tick off the topic checklist.