Oxbridge Chemistry interview questions

Chemistry interviews at Oxford and Cambridge

Chemistry interview questions to practise

Say your first thoughts out loud or type them, then compare with how a strong answer goes. These questions are our own, written in the style tutors use.

1. Question 1

   Water boils at 100 °C, but hydrogen sulfide, H₂S, a heavier molecule, is a gas at room temperature. Why?

   What it shows a tutor: Whether you explain from structure and forces, and notice when the obvious rule fails.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. Heavier molecules usually have stronger London forces and higher boiling points, so on that rule alone H₂S should boil higher. Something extra is holding water together.
   2. Oxygen is much more electronegative than sulfur and much smaller, so the O–H bond is strongly polar and the hydrogen is left very exposed.
   3. Each water molecule has two hydrogens and two lone pairs, so it can form up to four hydrogen bonds. That builds a network that needs a lot of energy to break apart.
   4. Sulfur is larger and less electronegative. The S–H bond is barely polar and its lone pairs are spread out, so H₂S has only weak dipole and London forces between molecules.
   5. So boiling water means breaking hydrogen bonds, and boiling H₂S does not. That accounts for a gap of over 150 degrees.

   A tutor might then ask: Hydrogen fluoride forms stronger hydrogen bonds than water, yet water boils at a higher temperature. How can that be?

   The usual wrong turn: Saying water has hydrogen bonds and stopping. Say why oxygen allows them and sulfur does not, and why the number per molecule matters.

2. Question 2

   Dissolving ammonium nitrate in water makes the beaker cold. If it takes in heat, why does it dissolve at all?

   What it shows a tutor: Whether you know that energy is not the only thing that decides if a change happens.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. Getting cold means the process is endothermic: breaking the solid lattice costs more energy than the water releases by surrounding the ions.
   2. So energy alone says it should not happen. The missing factor is entropy: the ions go from a fixed lattice to moving freely through the solution, which is far more disordered.
   3. Whether a change happens on its own depends on the Gibbs energy, ΔG = ΔH − TΔS. ΔH is positive here, but TΔS is larger, so ΔG is negative.
   4. That predicts something testable: if the entropy term is doing the work, the salt should dissolve more readily at higher temperature, because T multiplies ΔS. Ammonium nitrate is indeed much more soluble in hot water.
   5. Cold packs use exactly this: dissolution happens anyway, and the heat it takes in comes from the surroundings.

   A tutor might then ask: Can you think of a change that releases heat yet does not happen at high temperatures?

   The usual wrong turn: Saying it dissolves because it is soluble. That restates the question.

3. Question 3

   Roughly how many water molecules are in a glass of water? If you poured that glass into the sea and let it mix completely, how many of its molecules would be in the next glass you scooped out?

   What it shows a tutor: Estimating with moles and scaling up to something enormous without losing track.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. A glass holds about 250 mL, which is about 250 g of water. Water's molar mass is 18 g/mol, so that is about 14 mol.
   2. 14 × 6 × 10²³ is about 8 × 10²⁴ molecules in the glass.
   3. The oceans hold about 1.3 billion cubic kilometres. One cubic kilometre is 10¹² litres, so that is about 1.3 × 10²¹ L.
   4. A new glass of 0.25 L is a fraction of about 2 × 10⁻²² of the ocean. If my original molecules are spread evenly, the new glass holds about 8 × 10²⁴ × 2 × 10⁻²², which is roughly 1,600 of them.
   5. So the surprising answer is: over a thousand. The assumption doing the work is complete mixing, which takes the oceans around a thousand years, so I would say the estimate needs a very long wait.

   A tutor might then ask: Is it likely that the air you are breathing contains a molecule from someone's breath 2,000 years ago? How would you estimate it?

   The usual wrong turn: Panicking about the ocean's volume. Say the number you would use and why it is about right; tutors care about the method.

4. Question 4

   Why does ice float? What would lakes be like if it did not?

   What it shows a tutor: Linking a molecular explanation to a large-scale consequence.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. Most substances are denser as solids, because the particles pack more closely. Water is unusual: ice is less dense than liquid water.
   2. In ice each molecule hydrogen bonds to four neighbours in a fixed, open arrangement with a lot of empty space. When ice melts, some of those bonds break and molecules can crowd closer, so the liquid is denser.
   3. Liquid water is densest at about 4 °C. As a lake cools in winter, the 4 °C water sinks and the colder water stays on top and freezes there.
   4. The ice layer then insulates the water underneath, so lakes rarely freeze solid and fish survive the winter at the bottom.
   5. If ice sank, each layer would sink as it froze and the lake would fill from the bottom with ice that summer might not fully melt. Fresh water ecosystems in cold places would look very different.

   A tutor might then ask: Silicon and gallium also expand when they freeze. What might their solids have in common with ice?

   The usual wrong turn: Saying ice floats because it contains air. Ice from pure water floats because of how its molecules are arranged.

Oxford interview dates, December 2026

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What tutors are listening for

• Thinking aloud

  Both universities say they want to hear how you work things out, not a memorised answer.

• Using a hint

  Tutors expect to help. What matters is what you do with a prompt or an objection.

• Checking

  Test your answer on a simple case, an example or a counterexample, and fix slips out loud.

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