Christ's College Cambridge Natural Sciences Interview

Christ's College Cambridge has an outstanding tradition in the natural sciences, with alumni including Charles Darwin and a long history of producing significant scientists across biology, chemistry, and physics. Its Natural Sciences interviews reflect this tradition: supervisors are looking for students who engage with scientific problems the way scientists do — with curiosity, precision, and a willingness to push beyond what they already know. The interview is not a test of how much you have studied but of how well you reason, and the most successful Christ's candidates are those who can make genuine intellectual progress on problems they have never encountered before. There is no Natural Sciences pack; the nearest written practice is the Biological Sciences pack, £180.

Natural Sciences at Cambridge requires you to study three science subjects simultaneously in your first year, making scientific breadth essential. Christ's NST supervisors may probe any of the sciences in your application — physics, chemistry, or biology — and will specifically look for evidence that you can reason across disciplinary boundaries. Candidates who have prepared only in their strongest subject and are caught off-guard by questions in another area consistently underperform.

Christ's Natural Sciences Interview Format

NST applicants at Christ's typically have two interviews. Each lasts approximately 25 to 35 minutes and is problem-led. Supervisors present scientific questions — often involving data, diagrams, or novel scenarios — and observe as you reason through them aloud. Christ's NST interviews are known for following threads wherever they lead: a question that begins in chemistry may evolve into physics when the supervisor introduces a related constraint, or into biology when an application to living systems becomes relevant. This interdisciplinary probing is deliberate and designed to assess exactly the kind of scientific thinking that the NST curriculum demands.

The ESAT is used to shortlist NST candidates at Cambridge, and Christ's uses these scores as a primary shortlisting tool. Strong ESAT performance — particularly in the areas corresponding to your chosen science subjects — is important for securing an interview invitation.

Preparing for Christ's Natural Sciences Interviews

Prepare across all of your science subjects, not just your preferred one. Christ's NST supervisors will move between disciplines at will, and breadth of scientific reasoning is an explicit part of what they assess. Candidates with strong chemistry and weak physics, or strong biology and weak mathematics, regularly find Christ's interviews harder than expected.

Focus on understanding mechanisms rather than memorising facts. A-level biology and chemistry in particular can encourage superficial knowledge — knowing that something happens without understanding why it happens at the molecular or physical level. Christ's supervisors consistently distinguish between candidates who can describe phenomena and those who can explain them from first principles. The question is always “why does this happen?” not “does this happen?”

Develop comfort with uncertainty. Christ's supervisors will present questions you cannot fully answer. Your response to that situation — how you reason towards a partial answer, how you use what you do know, how you identify what you would need to know to go further — is a significant part of what they are assessing. Intellectual honesty and persistence under uncertainty are genuinely valued.

Example Christ's Natural Sciences Interview Questions

• Why does ice float on water? What would happen to life on Earth if it did not?
• Sketch the electron energy levels for hydrogen. What happens when an electron drops from a higher to a lower level?
• A reaction produces a gas at constant temperature. Sketch how the rate of reaction might change over time.
• What is a catalyst? How does it work at the molecular level?
• If you compress a gas very quickly, why does its temperature rise?
• What is the difference between mitosis and meiosis, and why does each process matter?

Why Does Ice Float? The Full Answer

The ice question on the list above is a fair example of what a Christ's supervisor does with a prompt that sounds like GCSE recall. It is not treated as one.

Question: why does ice float on water, and what would happen to life on Earth if it did not?

A natural first answer reaches for trapped air: ice looks bubbly, boats float despite being denser than water in bulk because of the air sealed inside them, so perhaps ice floats the same way. Say that out loud and the obvious test follows immediately: boil a tray of water first to drive the dissolved air out, then freeze it. The resulting ice comes out visibly clearer, and it still floats, at very close to the same density as ordinary, bubbly ice. If degassed ice floats just as well, trapped air cannot be doing the work. The explanation has to sit somewhere else, in what happens to the water molecules themselves when they freeze, not in what gets trapped between them.

Hints: think about what freezing actually changes. Not the molecules themselves, but how they are arranged. Water is bent, with a slightly negative oxygen end and two slightly positive hydrogen ends, and molecules pull on each other through hydrogen bonds. In the liquid those bonds keep breaking and re-forming on a timescale of picoseconds, so molecules tumble past each other constantly and rarely hold all their possible bonds at once, packing together about as tightly as bent, sticky molecules manage. Ask what changes as that tumbling slows near freezing, and what shape the bonds would settle into if the molecules stopped rearranging long enough to lock in place.

Worked answer: each water molecule can hydrogen-bond to up to four neighbours, two through its own hydrogens and two through lone pairs on its oxygen. In liquid water, constant thermal motion stops most molecules from holding all four bonds at once, so on average they sit closer together than the bonds alone would dictate. Below 0°C the molecules move too slowly to keep rearranging, and the four-bond geometry wins: each one locks into a fixed tetrahedral arrangement with its neighbours, building the open, hexagonal lattice of ordinary ice. That lattice holds molecules further apart on average than they sit in the liquid, so a given mass of ice takes up more space than the same mass of water, on the order of a tenth more. Ice is therefore less dense than the liquid it froze from, and it floats. Water's density actually peaks a few degrees above freezing, close to 4°C, for a related reason: below that point the liquid is already starting to build short-lived patches of the same open, hydrogen-bonded arrangement, which is why it stops getting denser as it cools further toward 0.

The second half of the question is where most candidates stop reasoning too early. If ice were denser than the water it froze from, the way almost every other solid is denser than its own liquid, it would sink as it formed. A lake losing heat over winter would then freeze from the bottom up: each new layer of ice would sink, exposing the water above it to the same cold air, and the whole body of water could freeze solid well before spring rather than staying liquid beneath a surface skin. There would be no floating lid slowing the loss of heat to the air above, and no liquid layer underneath for fish, invertebrates or aquatic plants to survive winter in. Oceans buffer climate, and land ecosystems depend heavily on the water cycle those oceans sustain, so an Earth where ice sank is plausibly one where surviving a full winter freeze had to be solved far earlier and far harder than it actually was, if complex aquatic life got established at all.

None of that is memorised chemistry. It is the same three moves each time: notice which everyday assumption the question is quietly testing, follow the physical reasoning until it produces a mechanism rather than a description, then ask what that mechanism would change if it ran in reverse. The Biological Sciences pack works through ten more questions this way: a question, Hints behind it, a full worked answer behind that.

A supervisor running this exchange is not marking you against a memorised fact sheet. They are watching whether a wrong first guess gets tested against a concrete case rather than defended out of stubbornness, whether the correction reaches for a mechanism instead of another guess pulled from the air, and whether the second half of the question gets the same care as the first. Candidates regularly nail the chemistry of the lattice and then rattle off “it would kill the fish” without saying why a frozen-solid pond is different from an ice-topped one, which is exactly the kind of shortcut a follow-up question is designed to catch.

For free samples in nine subjects and the full expert packs at £180, see our Cambridge Natural Sciences interview questions resource.

Frequently Asked Questions