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Cambridge Natural Sciences Questions & Answers: Chemistry

Oxbridge Application Specialists

If you’re applying to study Natural Sciences at Cambridge, you should be ready for a range of questions covering Chemistry, as well as others covering the range of Sciences. You should have a framework ready to answer questions, especially those that are more difficult, or that stray further from the typical syllabus.

Initially, you should work to put the question into context. What area does it fall in? Through considering it further and working to place it within a framework that you understand, you’ll be better able to answer it.

Next, apply your A Level knowledge. That means deploying as much relevant knowledge as you can, and using it as the basis for your answer. Of course, this knowledge might not make much of a dent in the question – but it should lay the foundations.

After this, you ought to bring in your extra knowledge. That means outside reading, experiments you’ve done, your EPQ, other projects, etc. Try to use this to set yourself apart from other students.

Lastly, try to build out an answer that develops on the above knowledge, and uses prompts and information supplied by the tutor that you’re talking to as well. The key here is to develop a rapport with the tutor, and show that you’re the kind of student that they can teach. Show a willingness to take on new ideas and develop them.

Question 1: How do amino acids bond to form a peptide?

A peptide is a molecule made up two, or more, amino acids. The amino acids are held together by a covalent bond – which is a peptide bond. A condensation reaction occurs between the amino group of one amino acid and the acid group of another amino acid.

Question 2: Why are explosions a risk in flour mills, whilst bags of flour in a kitchen don’t explode?

Flour is an organic material that is dry – it’s devoid of water. As such, it can be highly explosive. However, when compacted down it won’t burn, as the flour dust is compacted too much for oxygen to penetrate throughout it. A bag of flour has very little oxygen within it, and cannot fuel an explosion. On the other hand, a flour mill has a cloud of loosely dispersed flour within it. Within the cloud, there will be a significant amount of oxygen. As such, a spark, loose flame, or other ignition source can cause particles nearby to ignite, and as there is a mixture of both oxygen and dry organic material, the fire will spread rapidly, causing an explosion.

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Question 3: What issues might there be if you wanted to create a metallic oxide that has good conductive properties but is also transparent?

Transparent conductors are typically very fragile – this is due to cross strain constraints. They must be very thin, which adds to the issues with fragility. They may also degrade over time when subjected to mechanical stresses.

Question 4: How does a glow-stick work?

There are two approaches to this question. The first relies on you knowing the answer, the second on you reasoning a sensible approach. The first answer would be that glow sticks emit light when the two solutions inside them are mixed, and these two chemicals are diphenyl oxalate and hydrogen peroxide. They have an ampule within them that contains one solution – when the glow stick is bent, the ampoule shatters, bringing the solution within the ampoule into contact with the chemical in the rest of the stick. The reaction is typically catalysed by sodium salicylate. There will also be a suitable dye used. The chemical reaction yields two moles of phenol and one mole peroxyacid ester, and this decomposes spontaneously to carbon dioxide, which in turn excites the dye, releasing photons. Thus the reaction releases energy as light rather than heat.

If you didn’t know this, you should be able to provide a reasonable explanation nonetheless. You should deduce that the bending motion shatters something within the glow stick, and this results in two chemicals mixing. The question, of course, is how light is emitted – so you therefore need to make the jump to realising that a dye is contained within the stick as well, and that it is excited.

Question 5: Why doesn’t a fish freeze?

Fish ought to freeze if the water temperature drops below 1 degree Celsius, as they are ectothermic. However, they do not, as they have developed antifreeze proteins to combat this problem. These proteins lower the freezing point of the fish’s blood by preventing ice crystals developing beyond a certain size. This results in the fish’s blood having a freezing point of -2.5C rather than -1.9C, which is the freezing point of the salt water in which they swim.

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Question 6: What is the concentration of water?

1L of water is 1000mL, so this is 1000g. The molecular weight of water is 18gmol-1. Therefore, 1000/18 = 55.5 (recurring) mol. 1000mL of water is 1dm3, which means that the concentration for water must therefore also be 55.5moldm-3.

Question 7: Why does iron rust? How can we prevent iron rusting?

Iron rusts due to an oxidation reaction – iron reacts with water and oxygen to form hydrated iron oxide.

Iron + water + oxygen → hydrated iron (III) oxide

This explains why iron will rust in the presence of water, and why it rusts when left in a humid environment, and why it will be better preserved in an environment that is maintained at a lower humidity. In terms of how to prevent rust – we should consider maintenance of the environment, or protection of the object itself. Protection of the object could involve oiling, or painting, but can also be performed through galvanising. This process involves coating the object in a thin layer of a different metal – normally zinc or magnesium. This is called the sacrificial metal, and it is more reactive than iron and therefore will lose electrons in preference to it.

Question 8: How does blood maintain its pH?

The pH of blood is maintained within a tight band of 7.35 to 7.45. An alteration in pH is called either an acidosis or an alkalosis, and then further categorised as being metabolic or respiratory. So one might have a respiratory acidosis, for example, which is an acidosis stemming from a respiratory problem, with the lungs unable to remove sufficient amounts of CO2.

Considering short term maintenance of pH, an altered pH can be amended by the respiratory system – an increased pH would result in the respiratory rate decreasing, and vice versa. This is due to the reaction between H2O and CO2 which can form H2CO3, which in turn can form H+ and CO3-. Considering an example of longer term maintenance, the kidneys maintain the acid-base balance by reabsorbing bicarbonate from the urine and excreting hydrogen ions into the urine. If acidosis is detected, the tubular cells reabsorb more bicarbonate, and the conducting cells secrete more hydrogen whilst generating more bicarbonate.

Question 9: Discuss the bonding in benzene

This is a straightforward question that can be answered from A Level knowledge. Benzene’s chemical formula is C6H6, and each carbon atom bonded to other carbon atoms and a single hydrogen. The 4th bond pair of electrons from each carbon is delocalised, which creates a delocalised cloud of elections. Benzene is a hexagonal ring.

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