Sunday, April 15, 2012

Exam Technique 1

I've never written a blog post about exam technique before, but after reading the following question on an exam paper, I felt the time had come to broach the subject.

Imagine being a High School Chemistry student who is confronted by the following scenario on an exam paper:
  • Written information that tells you that the diagram refers to an experiment to model a process and observe the colour change of crystals.
  • A diagram of a dessicator (labelled as a sealed container) containing a watch glass of copper(II) sulphate pentahydrate (no indication of colour) and a beaker labelled Liquid X.
  • And the instruction that the student must identify Liquid X and explain the colour change of the crystals.

So, where do you start, given that you have so very little information to go on?

You might start with the dessicator (assuming you have recognized it as a dessicator and that you know what it is used for). This would suggest that, whatever this 'process' is, it is effected by water.

That copper(II) sulphate pentahydrate could also be a clue. It is copper(II) sulfate that is hydrated, CuSO4.5H2O, so if you are taking the trouble to keep moisture out of the dessicator, then maybe Liquid X in the beaker is a dessicant, something that will remove water from the air, and hence from the copper(II) sulphate pentahydrate.

Now, you would need to know, or infer from the question, that there is a colour change involved. Copper(II) sulphate pentahydrate is actually blue, while anhydrous copper(II) sulphate is off-white in colour.

But you still haven't answered the first part of the question, you must identify Liquid X. And this is where it becomes very, very, difficult. If you are sensible at this point, you might just call Liquid X a dessicant, and hope the exam markers will accept that. Otherwise you are going to come up against the nasty problem of naming a dessicant that is a liquid (not a solution, not a gel, but a liquid). So, is there such a dessicant?

Silica gel is a commonly used dessicant, but it is not a liquid, it is a very solid-like gel.

Lots of solids are commonly used as dessicants, including activated carbon, calcium sulphate, calcium chloride, and swelling-clay minerals like bentonite. You can even use sodium hydroxide as a dessicant.

Strangely enough, the term "liquid dessicant" is used in the air-conditioning industry, but the dessicant is not a liquid, it is usually solid lithium chloride.

Well, I haven't had any luck coming up with the name of a liquid dessicant.

Clearly, we have started off in the wrong direction. The resemblance of the vessel to a dessicator is probably a red-herring, something put into the question in order to get you to think along the wrong lines. Is the use of hydrated copper(II) sulphate also a red-herring?

Well, I'll keep working on the problem, but, if I was a student trying to complete the exam within the time-limit, I would have realized that this question was only worth 3 marks, and I couldn't afford to spend more than about 5 minutes on it....

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