Yr12 Back, Marble Chips and the School Science Technician Role

Yr 12 are back and with a vengeance – starting the practical-heavy part of their A2 course before the students or teachers are whisked away to participate in all the end-of-year expeditions and events like the World Challenge Expedition, Do E, Yr7 nature reserve trip, Yr12 Essex Uni Chemistry and DNA all-day workshops, etc…

In chemistry it is the nitration of methyl benzoate which is an exercise in preparing a chemical product, purifying it and assessing its purity with melting point apparatus. P and I have a pact between us to empty the ice maker every chance we get in order to keep the freezer full as the nitration process makes heavy use of ice baths and crushed ice for crystallisation. We still haven’t found a satisfactory affordable machine for the latter, the device we use only takes 7 ice cubes at a time – slow!

The first two days of the Stepping Stones course have gone very well, everyone seems to enjoy themselves and this year we actually got through all the prepared practical activities. We achieved this by preparing beforehand all the constituents and doing some of the activities like the acid/base titration step-by-step and led from the front, Me on one side and P on the other, Little and Large!

‘Ah, but that’s not what science technicians do’, I can almost hear my imaginary readers say, ‘we just get the stuff out and wash up’. Well that is certainly true when things are busy (which is rather a lot of the time) and during a new science technician’s first year when they are learning the ropes. After that, we try things out.

An example. In the set up above, which is for a classic GCSE rate of reaction investigation, the purpose is to collect and measure the time taken for a fixed volume of gas to be given off when marble chips are reacted with bench dilute hydrochloric acid. Variables can be size of marble chips down to calcium carbonate powder, temperature at which the reaction takes place or more likely the concentration of the acid.

Sometimes the pupils have to find out for themselves appropriate ranges of concentration, what size measuring cylinder to use, etc. More usually they have less than hour for the practical and lesson, and we want to provide them with the ingredients that will actually give them satisfactory results and enthuse them. So, beforehand, the science technician would experiment with the plastic measuring cylinder sizes, bench HCl and other equipment available. So, say, surface area is being investigated - what amount of chips gives a reasonable result using a 100 cm3 plastic measuring cylinders, (of which we have loads)? We find the a volume of 80 cm3 is easy to read off on the inverted measuring cylinder. We also find that constantly swirling the conical flask once the stopper is in each time is the only way to get consistent and reasonable results. We also find that too large a quantity - the small chips react too fast to be controllable, use too small a quantity - and it takes over three minutes to get the 80cm3 of gas. The best results are with 2 grams of each size of chips which results in about 30secs with the small chips and 90secs with the large chips to produce 80cm3 of gas. That aspect of rate of reaction, surface area, can now be investigated successfully by the class in half an hour leaving time for the write-up theory and of course clearing up. DON’T forget the sieves and reminder to avoid the curse of the science technician’s work – BLOCKED SINKS!