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. Compounds Y and Z are fictional, and their data are invented for practice. 1 L = 1000 cm³.
Question 1 (multiple choice · foundation)
Which description fits the particles in an amorphous solid?
- (A) They are far apart and move freely, but the sample keeps its shape.
- (B) They touch and vibrate about fixed positions, with no repeating pattern.
- (C) They touch and sit in a regular repeating pattern, but slide past one another.
- (D) They touch, slide past one another and have no repeating pattern.
Answer and explanation
Answer: (B). An amorphous solid is still a solid: its particles stay in fixed positions and only vibrate. What makes it amorphous is the lack of a regular, repeating arrangement.
- (A) describes widely spaced particles, which is a gas. A gas cannot keep its own shape.
- (C) mixes up a crystalline arrangement with liquid-like motion. Particles in a solid do not slide past one another.
- (D) describes a liquid.
Question 2 (multiple choice · core)
A sealed syringe contains nitrogen gas. The plunger is pushed in slowly until the gas has half its original volume. The temperature stays constant. Which statement describes what happens to the N₂ molecules?
- (A) The molecules become smaller, so more of them fit into the smaller space.
- (B) The average spacing decreases, and the molecules collide with each other and with the walls more often.
- (C) The average spacing does not change, because gas molecules are always far apart.
- (D) The molecules move more slowly because they have less room.
Answer and explanation
Answer: (B). The same number of molecules now share half the volume, so on average they are closer together. Closer molecules meet each other and the walls more often.
- (A) Molecules do not change size when a gas is compressed; only the empty space between them changes.
- (C) The molecules are still far apart compared with their size, but the average spacing has decreased.
- (D) The speed of the molecules depends on temperature, which is constant here.
Question 3 (multiple choice · core)
Compound Z has a molar mass of 120.0 g mol⁻¹. Its densities are: solid 1.60 g cm⁻³; liquid 1.45 g cm⁻³; gas 3.66 g L⁻¹ at 400 K and 1 atm. The molar volume of gaseous Z at 400 K and 1 atm is closest to how many times the molar volume of liquid Z?
- (A) 0.40
- (B) 1.1
- (C) 2.5
- (D) 400
Answer and explanation
Answer: (D). Molar volume = M ÷ density. Liquid: 120.0 ÷ 1.45 = 82.8 cm³ mol⁻¹. Gas: 120.0 ÷ 3.66 = 32.8 L mol⁻¹ = 32 800 cm³ mol⁻¹. Ratio: 32 800 ÷ 82.8 ≈ 396, which is closest to 400.
- (A) compares 32.8 (in L) with 82.8 (in cm³) without converting units.
- (B) is the ratio of the liquid to the solid molar volume (82.8 ÷ 75.0), not gas to liquid.
- (C) divides the two density numbers (3.66 ÷ 1.45) and ignores that one is per litre and the other per cm³.
Question 4 (multiple choice · core)
Which statement best explains why a liquid takes the shape of its container but keeps a fixed volume?
- (A) Its particles are far apart, so they spread out to fill any space.
- (B) Its particles stay in close contact, but they can move past one another.
- (C) Its particles are held in a regular pattern that can bend.
- (D) Its particles are smaller than those of the solid, so they can flow.
Answer and explanation
Answer: (B). Because the particles touch, the liquid cannot be squeezed into much less space or spread into much more, so its volume is fixed. Because they can move past one another, the liquid flows into the shape of its container.
- (A) describes a gas, which does not have a fixed volume.
- (C) A regular pattern belongs to a crystalline solid; a liquid has no long-range pattern.
- (D) Particles have the same size in every phase.
Question 5 (constructed response · core)
A student draws particulate models of ammonia, NH₃, in two boxes of the same size.
- Liquid box: twelve NH₃ molecules, each separated from its neighbours by a gap about three times the width of a molecule.
- Gas box: twelve N atoms and thirty-six H atoms, drawn separately and heaped at the bottom of the box.
(a) Identify two errors in the student’s drawings and explain why each is wrong. (b) Describe a correct model for gaseous ammonia. (c) The student says: “When ammonia boils, the N–H bonds break.” Evaluate this claim.
Worked solution and suggested Marlbridge rubric
(a) Any two of:
- In the liquid, the molecules should be in contact, with only small gaps. Large gaps suggest a gas.
- In the gas, the particles should be whole NH₃ molecules, not separate N and H atoms.
- In the gas, the molecules should be spread through the whole box, not heaped at the bottom, because gas molecules move constantly and fill the container.
(b) A small number of NH₃ molecules, each drawn as one N atom bonded to three H atoms and identical to those in the liquid box, spread far apart across the whole box. Arrows may show straight-line motion if a key explains them.
(c) The claim is incorrect. Boiling overcomes the attractions between NH₃ molecules (hydrogen bonds and other intermolecular forces). The covalent N–H bonds inside each molecule do not break, so the gas still consists of NH₃ molecules.
| Point | What earns it |
|---|---|
| 1 | First error identified with a correct reason |
| 1 | Second, different error identified with a correct reason |
| 1 | Gas model: whole NH₃ molecules, far apart, filling the box |
| 1 | Claim rejected and linked to intermolecular forces being overcome, not covalent bonds |
Do not award the point in (c) for “the bonds do not break” without saying which forces are overcome.
Question 6 (calculation · core)
Compound Y has a molar mass of 60.0 g mol⁻¹. Its densities are: solid 0.88 g cm⁻³; liquid 0.80 g cm⁻³; gas 2.09 g L⁻¹ at 350 K and 1 atm.
(a) Calculate the molar volume of Y in each phase, in cm³ mol⁻¹. (b) Use your answers to explain, at the particle level, why melting changes the molar volume of Y only a little but boiling changes it a lot. (c) Estimate how many times farther apart, on average, the molecules are in gaseous Y than in liquid Y.
Worked solution and suggested Marlbridge rubric
(a) Solid: 60.0 ÷ 0.88 = 68.2 cm³ mol⁻¹. Liquid: 60.0 ÷ 0.80 = 75.0 cm³ mol⁻¹. Gas: 60.0 ÷ 2.09 = 28.7 L mol⁻¹ = 28 700 cm³ mol⁻¹.
(b) Liquid ÷ solid = 75.0 ÷ 68.2 = 1.10, so melting increases the volume by only about 10 per cent: the molecules are in contact in both phases and just lose their fixed positions. Gas ÷ liquid = 28 700 ÷ 75.0 ≈ 383: in the gas the molecules have separated completely and are mostly surrounded by empty space.
(c) Distance scales with the cube root of volume: ∛383 ≈ 7.3, so the molecules are roughly 7 times farther apart in the gas.
| Point | What earns it |
|---|---|
| 1 | Correct molar volumes of solid and liquid |
| 1 | Correct molar volume of gas, converted to cm³ mol⁻¹ |
| 1 | Melting: particles in contact in both phases, so a small change (ratio about 1.1) |
| 1 | Boiling: particles separate completely, so a very large change (ratio about 380) |
| 1 | Cube root of the volume ratio, giving about 7 |
Accept answers from (a) carried forward. A method that finds the volume per molecule in each phase (divide by 6.022 × 10²³) and compares cube roots is equally valid: it gives about 0.50 nm in the liquid and 3.6 nm in the gas.
Question 7 (explanation · stretch)
Liquid water and liquid hexane, C₆H₁₄, are both at 20 °C.
(a) Describe how the arrangement of neighbouring molecules differs in the two liquids, and explain the difference in terms of interparticle forces. (b) Predict how the arrangement in liquid water changes when it is heated from 20 °C to 80 °C, and explain your prediction.
Model answer and suggested Marlbridge rubric
(a) In both liquids the molecules touch and keep moving. In water, the molecules form hydrogen bonds with their neighbours, so at any moment most molecules are oriented with an H atom of one molecule pointing towards the O atom of another. These bonds keep breaking and re-forming. Hexane molecules are nonpolar and attract each other only through dispersion forces, which have no strong direction, so neighbouring molecules show no strong preferred orientation.
(b) At 80 °C the molecules move faster. Hydrogen bonds break more often and last for less time, so the short-range order in the liquid decreases, and the molecules spread slightly further apart on average.
| Point | What earns it |
|---|---|
| 1 | Water: hydrogen bonding gives a preferred orientation (H towards O of a neighbour) |
| 1 | Hexane: only dispersion forces, so no strong preferred orientation |
| 1 | Higher temperature: faster motion disrupts hydrogen bonds, so less ordered arrangement |
How did you do?
- Q1 or Q4 wrong: re-read the solids and liquids sections of the study guide.
- Q2 wrong: look again at the gas table in the study guide.
- Q3 or Q6 wrong: check your unit conversion and follow Worked example 2.
- Q5 or Q7 incomplete: use the drawing rules under Figure 1, and name the forces involved.
Then tick off the topic checklist.