Is this right? – When a practical ‘doesn’t work’ by Sarah Longshaw

Is this right? – When a practical ‘doesn’t work’ by Sarah Longshaw

Is this right?

Along with the phrase ‘My practical hasn’t worked’ – these are probably the two statements I find most frustrating as a Science teacher; when what the student means is ‘My results are not what I expected’. As a scientist before I became a teacher (I spent my summer holidays between the ages of 18 and 22 working for Shell Agrochemicals and I did a PhD after my degree); I dislike recipe style practicals where students can fill in the results table before they have carried out the practical and I find it hard not to smile when the results are unexpected and have to be explained. Perhaps this is why I also like to try out new practicals with my classes. Now, don’t get me wrong, I am not suggesting we throw caution to the wind and mix any combinations of chemicals together, what I mean is finding new ways of doing things as suggested by reputable sources such as CLEAPPS; Learn Chemistry; SAPs; Nuffield etc. and I certainly wouldn’t do anything without risk assessing it first.

When students first come to secondary school, they are often excited by the idea of doing more practical work; but sometimes this enthusiasm can be lost if the practical work becomes perfunctory and predictable. The open ended investigation of the primary science curriculum is more difficult to recreate in the more confined secondary time slots and this can turn practical into a more affirmatory and less investigative experience, where students are able to predict outcomes. This in itself is not all bad, it is useful for students to have an idea of the underlying scientific principles; what we as teachers need to do, is to be sure of the purpose of the practical. For example – students carrying out a displacement reaction and placing an iron nail in copper sulphate solution often describe the pink colour that they see forming on the nail as rust, which enables us to deal with their misconception.

Can we retain their focus by making the practical less predictable?

Students enjoy practical sometimes because they see it as a chance to ‘switch off’; can we retain their focus by making the practical less predictable? For example by taking the classic thiosulphate rate practical and asking students to change variables (temperature and concentration) to achieve the end result at a given time.

At A-level, I like to carry out practical work to introduce a topic; for example I introduce structure and bonding by using a variety of solids and solvents which we mix and then as I teach the topic we refer back to the results and explain why water and ethanol mix (hydrogen bonding) but water and hexane don’t.

Returning to the idea that a practical doesn’t work and sometime they don’t – my response is always to ask students to analyse why. In scientific research results are not always what we expect and this may be for a number of reasons including the effect of human error on the ability to recreate the same experimental conditions; or there may be problems with the equipment or reagents (I remember one class wanting to speed up a fermentation reaction – so they stood the reaction mixture containing yeast in boiled water, then had to explain that they had denatured their enzyme!)

A recent A-level required practical asked students to research a redox reaction which needed to be carried out under different conditions – acid and neutral and this involved students having to find out suitable quantities of the reagents.

In another A-level practical we discovered the limitation of the equipment we were using for an enthalpy change. We started off using a thermochromic (forehead) thermometer and trying to carry out a microscale reaction. It didn’t work because the temperature change wasn’t great enough – even though we could extrapolate that the reaction started at 21oC by finding room temperature. What we were able to do though was to adapt the practical to carry it out in a test tube and measure the temperature change and use this information to predict what range we would have needed for our thermochromic strip.

So – if we are training students to think scientifically, we need to ensure that we equip them to evaluate why the results they gain may not be what they expect and how to test what they are suggesting has happened.

About the author

Sarah Longshaw is an SLE (Specialist Leader of Education), a Chartered Science Teacher and the Head of Science at Eaton Bank Academy. Eaton Bank hosts the Science Learning Partnership for Cheshire and Stockport.

Authored by: Lee Acton

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