Cambridge Natural Sciences Interview Explained

Kate P. · Physical Natural Sciences (Murray Edwards College, Cambridge)

My name is Katie, and I'm studying Natural Sciences at Murray Edwards College, Cambridge. I've just finished my second year and chosen to specialise in Chemistry. I want to explain how the Physical Natural Sciences interviews were structured when I applied three years ago, and then work through an example question, so you can see the level they expect and the best way to answer.

How the Natural Sciences interviews are set up

When you apply for Natural Sciences, you specify on the SAQ form whether you're applying for physical or biological Natural Sciences. This doesn't lock you in once you get to Cambridge, you can still change your mind later, but it decides the kind of interview structure they give you. I applied for physical Natural Sciences, so I had two interviews with my college, though this does vary between colleges.

Two rooms, two styles

My first interview was physics-based, with my physics supervisor and a materials sciences supervisor. It was mainly mathematical. I didn't sit an official maths test, but they brought along a list of maths questions on a sheet, laid out much like a test, and worked through them with me so they could see how I thought. We then discussed a few things from my personal statement and a couple more physics topics.

My second interview was with a chemistry supervisor and a geology supervisor. It covered mainly chemistry, a few geology questions, and a couple of biology ones tied to my personal statement. There was far less maths this time. It was much more about how I understood chemistry: there were diagrams to draw or interpret, and graphs and data to read. They wanted to see that I could recall a lot from A-level chemistry and then apply it to new situations, while genuinely understanding the reasoning behind it.

The example question they reuse

Here's a question from my chemistry interview, and I believe it's a fairly popular one between colleges, because I've heard others get something similar. Explain the trends in bond length and bond dissociation energy for the halogens, and explain why fluorine doesn't fit the trend. They don't expect you to remember the exact figures, so they gave me a table of the bond energies and bond lengths for each molecule. All they wanted to see was that I could understand and explain the trend. The first thing I did was look at the data and picture how it would appear on a graph.

Explaining the bond-length trend

The bond length for fluorine is about 143, which is quite small, and it increases fairly linearly all the way up to iodine at around 266. Explaining this part is simple: the atoms are bigger as you go down the group, so the molecule must be bigger too. Starting with the part you're confident about builds momentum and gives you something to reason from.

Explaining the bond dissociation energy trend

The more challenging half of the question is the bond dissociation energy, and it isn't a perfectly linear trend. As you go down from chlorine to iodine, the bond dissociation energy decreases. This makes sense once you connect it to the first half of the question: because the atoms are further apart, the bond is easier to break. Often, in a question with several parts, you can use the earlier parts to help with the later ones, and this is a perfect example.

Why fluorine breaks the trend

The genuinely hard bit is fluorine, because fluorine actually has a lower bond dissociation energy than chlorine, which doesn't fit the trend, it should be higher. The explanation comes down to the size of the atoms. Fluorine's atoms are very small and sit very close together, so the electrons around each atom interact and repel one another significantly. In iodine, the atoms are much further apart, so any repulsive forces between the electrons aren't strong enough to really affect the bond. If you picture the dot-and-cross diagram we draw at A-level, you can see the electrons on the two fluorine atoms are close enough to repel, and that repulsion makes the bond easier to break than you'd expect, lowering the bond dissociation energy. Looked at that way, it's actually quite a simple question, as long as you break it into parts and reason logically through each one.

What the interviewers are really testing

They aren't testing whether you memorised a data table, they handed you the table. They're testing whether you can:

• read data and picture it as a graph
• explain the confident part first, then use it to unlock the harder part
• reach for a physical reason when something breaks a trend, rather than just restating the anomaly
• break a multi-part problem into logical steps

Common mistakes to avoid

Watch out for trying to recall figures when the data is in front of you, treating the parts of the question as separate puzzles rather than letting one feed the next, and, when fluorine breaks the pattern, simply saying "fluorine is lower" without explaining the electron repulsion behind it.

Final thoughts

Expect to be given data rather than asked to recall it, visualise it as a graph, answer the confident part first, and reach for a physical cause when a trend breaks. That habit of breaking a complex question into logical parts is the single most useful one for a Natural Sciences interview. If you'd like help preparing, Oxbridge Solution can support you.