Chemguide: Support for CIE A level Chemistry

This statement asks you to draw conclusions from given data derived from environmental monitoring.

Before you go on, you should find and read the statement in your copy of the syllabus.

You could waste an awful lot of time trying to prepare for this statement. In the first 13 exam sessions of the current syllabus, this statement has been tested only twice, once for 4 marks and once for 3 marks.

On both of those occasions, there was nothing in the question that needed any extra knowledge - just some common sense, and perhaps material from elsewhere in the syllabus.

One of the questions felt much more like a geography or general studies question than a chemistry one.

You were given bar charts of environmental testing of a river above and below a chemical factory over a succession of years. You were asked to identify what pollutants were produced by the factory, and which came from elsewhere. That just needed you to compare the data upstream of the factory with the downstream data for the same year.

You were also asked to compare the data for two years. In the second year, the pollution had decreased, and you were asked to suggest a reason why. All that they were looking for was something like better pollution control, or the factory had been shut down. This is trivial stuff.

A couple of points about the examples quoted in the syllabus:

PCBs

PCBs (polychlorinated biphenyls) are fat soluble. One question gave you some information about PCBs and then asked why berries and drinking water aren't contaminated by PCBs in the same way that oily fish, seal and whale meat are. If you know that PCBs are fat soluble and that berries don't contain much fat, this isn't a problem. If you don't know that, you could reasonably work it out.

You might also be expected to realise that molecules like PCBs can be detected using gas-liquid chromatography coupled to a mass spectrometer.

Isotopic ratios in ice cores

The other example talks about isotopic ratios in ice cores. They should give you enough information about this, but a small bit of background might be helpful.

Ice core dating relies on detecting the ratio of two isotopes of oxygen in the water that froze to make the ice - the common O-16, and the much less common O-18. The ratio can be detected using a mass spectrometer.

The ratio varies depending on the temperature of the atmosphere when the ice originally fell as snow. When it is cooler, the ice has less O-18 than when the atmosphere is warmer.

The variation is simply because the heavier water molecules containing O-18 evaporate into the atmosphere less readily than the lighter ones containing O-16. They also diffuse through the atmosphere less quickly as well. The colder it is, the more noticeable the effect.

When it is cold, there will be fewer water molecules in the atmosphere in arctic regions containing O-18, and so fewer of these molecules can fall as snow and turn to ice.

So there is a cyclical change every summer and winter, as well as long-term changes.

That means that you get a repeated pattern in ice cores rather like the pattern in tree rings. Counting the variations means that you can date the ice anywhere along core.

The ice contains small bubbles of air that was present when it originally formed. By analysing the air, it is possible to get a record of the amount of carbon dioxide (or anything else) in the atmosphere going back over hundreds of thousands of years.

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