Practical investigations
Read our general notes on Risk Assessment
Planning your investigation
In any practical assessment you do need to understand the criteria for assessment. You must plan an investigation that gives you a chance of meeting all the criteria.
For Nuffield candidates full details of what has to be included in your report for high marks are given on pp 56 to 60 of the Chemistry (Nuffield) specification from Edexcel You can download the specification from the Edexcel web site. Consult your teacher if you do not understand what is required.
If you are studying the Salters course you might find this article in Education in Chemistry helpful.
RESEARCHING THE LITERATURE
Marking criterion: Uses sources of information including computer data bases if necessary
The Nuffield Student’s book and textbooks are an obvious starting point. For some investigations you will find useful guidance on this web site.
Always make a note of your sources as you use them – this is so very much easier than having to go back for the information later. In your report you must show that you have consulted a range of sources of information by giving a list of references. This should be easy but very many students fail to do this. List all the sources you use including this web site.
DECIDING ON YOUR RESEARCH QUESTION
Marking criterion: Identifies the problem and states it clearly in a way that can be tested.
Choosing the right question to investigation to investigate in very important because you need to pick a problem to tackle that you can realistically solve with the equipment available and in the timescale allowed.
Various investigations are suggested in the Nuffield book – but you should not assume that they are all equally practical. Some of them would be very hard to do well. For example on page 261, investigation 11.5b suggests you might investigate the effect of agitation on the activation energy or the rate of reaction when magnesium reacts with an acid. This is possible but it is hard to quantify the degree of agitation and the questions to this web site suggest that it is not a particularly fruitful line of inquiry.
PLANNING A PRACTICAL APPROACH
Consulting references
Marking criterion: Proposes an experimental procedure of good quality
In most investigations you are likely to use apparatus and procedures that you have come across during the rest of the course. As well as the Nuffield book, you will find that there are other textbooks with information about practical activities suitable for advanced courses.
If you are investigating a rate of reaction refer to the commonly used methods are summarised on pages 247-8 of the Nuffield Advanced Chemistry Students Book. You should go through these in turn and discuss in your plan whether each of them is suitable for your particular reaction. You should then state which method you think is the one you are actually going to use and why.
In general you are unlikely to gain high marks if you only determine one value by one method. You can do better if you determined one value by two different methods and then compared them or if you used two different methods to determine two different values.
Preliminary investigations
Marking criterion: Uses laboratory trials and modifies plans as necessary
It is well worth spending some time trying things out on a test tube scale. This will give you a ‘feel’ for the way the chemicals behave and how quickly they react. If investigating the decomposition of copper carbonate you could try heating a small amount in a test tube, seeing how hot the solid has to get before it decomposes and getting some sense of the type and volumes of any gases evolved.
This type of preliminary work will help you to decide on the key variables and make a better plan.
Often you will have to decide on suitable concentrations and quantities of solution. Here you would need to try various different concentrations until you hit on the best ones. In a rate investigation you want to be sure that the reaction is not too fast (difficult to make measurements) or too slow (can't be done in the time available).
Then decide on what measurements you are going to make and how many you will make.
A trial run is always a good idea to see if any modifications need to be made to your plan.
Calculations – quantities
Marking criterion: Uses theory to justify procedures and scale of working
A good starting point is to use the equation for the reaction and to do some preliminary calculations. Suppose you want to collect a gas and can measure up to 100 cm3 – then you can calculate roughly how much of the reagents you need to produce about the right amount of gas.
Measuring instruments
Marking criterion: Uses theory to justify precision of measurement
You need to decide which measurements are critical to your results and must be made accurately. There is no point in using a burette to measure out a reagent that you are going to add in excess but it might be essential to do so for a limiting reagent.
Assessing the hazards and risks
Marking criterion: Considers safety and includes a risk assessment in which safety procedures are clearly related to the hazard.
You will find guidance on pages 517-520 of the Nuffield Students book. Consider asking your laboratory technician or teacher if you can refer to the Nuffield Advanced Chemistry Teachers’ Guide. One of the appendices, lists “hazardous chemicals”. Your technician should also have the relevant “hazcards”, which you may be able to use.
A NOTE ON HELP
Before you do any practical work, show your plan to your teacher to ensure that what you are about to do is safe and practicable and that it will give you some results you can process. Your teacher is allowed to advise you at this point (or, for that matter, at any other point); what teachers are not allowed to do is to plan it all for you.
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updated: 12 January 2007
