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Chemguide: Support for CIE A level Chemistry Paper 5 Safety considerations Introduction Most papers will have a question worth probably only a single mark asking you to suggest necessary safety precautions during an experiment. All the information needed is given to you, and it would be silly to waste this mark. In each case, you will normally be given a list of hazards relating to substances in an experiment. Assorted hazards Corrosive or irritant Corrosive is more dangerous than irritant, but in each case you need to avoid skin contact. The obvious answer here is to wear gloves. | |
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Note: In truth, gloves are rarely necessary, and you simply warn students to wash their hands immediately if they get such compounds on their skin. There are, however, some things (such as concentrated nitric acid or bromine or phenol) which are so corrosive that you would use gloves. However . . . in an exam, say that you would use gloves even for something which is simply an irritant. CIE will always accept that. | |
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Flammable Flammable means that a substance will catch fire if exposed to a naked flame. Perhaps confusingly, inflammable means exactly the same thing! Obviously, here you have to keep the substance away from any naked flames. If you had a flammable liquid, for example, you wouldn't want to heat it in a test tube using a bunsen burner. Instead, you could heat it in a beaker of hot water on an electric hotplate. Toxic or harmful Toxic means that the substance is poisonous; harmful is less poisonous, but you will still need to take care with it. What you need to do about this depends on how the substance poisons you. The substance is a gas or a volatile liquid This could be a gas like nitrogen(IV) oxide or a liquid like ethanal. You shouldn't breath in the gas or vapour, and the obvious precaution is to use a fume cupboard, or a fume hood. The substance is a liquid or solid, poisonous by skin contact or if swallowed In both cases you would need to use gloves, washing the gloves carefully before you took them off to avoid any skin contact. The need to take care when answering questions Most of the time, these questions are obvious and straightforward, but CIE do ask questions where you need to look very carefully at the information given. This example comes from October/November 2016 Paper 51 Q1(d). This came at the end of a question spreading over 4 pages, and referred to three substances which were used during an experiment - barium chloride, potassium chromate(VI) and silver nitrate. You were given information about the hazards involving all of these - and in each case, the information varied with the concentration of solutions of the compounds. So, for example, barium chloride was classified as toxic if it was solid, and then either moderately hazardous or non-hazardous depending on the concentration of its solution. For each of the compounds, you therefore had to search back through the question to see what concentration was actually used so that you could identify what sort of hazard (if any) was present. Of the three compounds mentioned, only potassium chromate(VI) turned out to be hazardous at the concentrations used. The question asked you to identify one hazard and describe a precaution (other than eye protection) to keep risks to a minimum. You were told that potassium chromate(VI) solution at the concentration used was a health hazard, which may cause skin, eye and respiratory irritation. The Examiner's Report complained that students weren't being precise enough in their answers. Your answer should look something like this:
You can't use the fact that it causes eye irritation, because the question says "other than eye protection", and it is difficult to see how you could get respiratory irritation from a solution. There was only one mark for all this, so you had to work for it!
© Jim Clark 2017 |
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