Practical Skills in Chemistry

Why the method says what it says

Most people revise practicals as procedures: what goes into what, in which order, and what it looks like when it works. Then the written practical paper asks about an experiment they have never seen, and all that memorising turns out not to help. That paper is not testing whether you remember the method. It is testing whether you know why each step is in it. Every instruction in a prescribed method is there to remove ONE specific source of error: to stop something escaping, to hold one thing steady while another changes, to make a reading repeatable, or to make a change visible. Once you can say which, an unfamiliar experiment stops being unfamiliar. You are no longer trying to recall a method; you are reading one and asking what each line is protecting. That is a skill, and unlike a list of nine procedures it does not fall out of your head. So carry one question through this module: what would go wrong if I skipped this step, and which measurement would it spoil? The chemistry itself, the reactions and the calculations, belongs to the other modules on this course.

Five words used exactly

The last two are the pair that decides most of the evaluation marks, and they are the two most often used as though they meant the same thing.

Two that protect a control variable

Five instructions from a method. Tap the TWO that are there to stop something OTHER than the independent variable from changing.

• Place the lid on the container as soon as the solution has been added
• Use water from the same bath for every run, and check its temperature before each one
• Record the reading to the nearest division on the scale
• Use the same volume of solution in every run
• Repeat the whole experiment three times and take a mean

The reading that is always a little low

A student measures a gas given off. Every run comes out slightly lower than expected, by about the same amount each time, because a little escapes before the container is sealed. How should this be described in an evaluation?

• As a limitation of the method, because it happens the same way every time however carefully the work is done, and the fix is a change to the apparatus rather than more care
• As human error, because the student was not fast enough sealing the container
• As a random error, because measurements always vary a little
• As too small to matter, since the results still show the pattern

Two faults that need different answers

Almost every weak evaluation calls everything the left column and then suggests being more careful. Being more careful cannot touch the right column at all.

Five instructions, five different things they prevent

Planning an investigation

You are asked to plan an experiment to find out how one factor affects an outcome. Put the planning stages in a sensible order.

• State exactly what you are changing and exactly what you are measuring
• List everything else that could affect the outcome, and decide how each will be held steady
• Choose a range of values wide enough to show a pattern, with enough points to see its shape
• Do one trial run to check the range works and the measurement is readable
• Adjust the range or the quantities in the light of what the trial showed
• Plan repeats so you can tell whether results agree before you rely on them

What the written practical paper rewards

Three habits, and all three work on an experiment you have never seen before. Pair every weakness with a specific change. Not more care, not more repeats by reflex: a named change to the apparatus or the design, and a sentence saying which measurement it would improve. A weakness with no paired improvement is half an answer. Say what the pattern in the data is doing, before saying what it means. Does the outcome rise steadily, level off, or fall away? A conclusion that ignores the shape and only reports the direction has thrown away most of the information. ⭐ And treat an odd result as evidence, not as an embarrassment. One point far off the line is telling you something: either a mistake happened in that run, in which case say so and say why it should be repeated rather than averaged in, or the method behaves differently in that part of the range, which is more interesting still. Quietly averaging it away is the one thing that cannot earn anything.

Mistake, limitation, or neither

For each thing that happened during an experiment, decide what it is. Three lives.

The kitchen scale that reads in fives

Somebody is baking, and their scale only shows numbers going up in fives. They need forty-two grams of something. They cannot have it. They will get forty, or forty-five. Being more careful will not help. They can put the bowl down gently, read it in good light, lean over it with their eye level with the display, and do all of that perfectly. The scale still only shows fives. The problem is not in them; it is in the tool. Now suppose they also forgot they had already added some. That is a completely different kind of problem. It happened once, it would not happen again if they paid attention, and the fix is to start that bowl again rather than to buy anything. ⭐ Two faults, two different answers. One needs a different scale. The other needs doing again. Telling somebody to concentrate harder fixes exactly one of them, and people reach for it as the answer to both.

What earns evaluation marks

A student has written that their results were affected by human error and that they would be more careful next time. Select every change that would genuinely improve the evaluation.

• Naming the specific thing that went wrong and which measurement it affected
• Saying whether it was built into the method or happened on one occasion
• Proposing a specific change to the apparatus or the design, and saying what it would improve
• Adding that they would do more repeats
• Saying more firmly that they would take greater care

Finish the practical sentences

The one thing you deliberately change is the _____ variable, and anything you deliberately hold steady so it cannot explain your results is a _____ variable. Most of the instructions in a method exist to protect those. Getting closely similar values when you do the same thing again yourself means the work is _____, which is about the method being tight rather than about the answer being right. Something done wrong on one occasion, which doing it again would remove, is a _____. Something built into the equipment or the design, which no amount of care can remove, is a _____, and it is what an evaluation question is really asking about. The mark comes from pairing it with a specific _____ rather than with a promise to be careful.

Build the evaluation

Every result in an experiment came out slightly low, by a similar amount, because a little of what was being measured escaped before the container was sealed. Assemble the evaluation.

One investigation, four decisions

A student is investigating how one factor affects how quickly something happens. Each stage sets out where they are and hands you the decision.

• Their trial run finishes almost instantly, too fast to time reliably. Decide what to change.
• Four of five results sit neatly on a curve. The fifth is far off it. Decide.
• They realise the runs were done across a morning and an afternoon, and the room was noticeably warmer later. Decide how to handle it.
• The written paper asks about an experiment they have never seen, and gives the method. They freeze. Decide the best approach.

Evaluate an investigation

A student investigated how one factor affects how quickly something happens. Every result came out slightly low by a similar amount, one result was far off the pattern, and the runs were spread across a warm afternoon and a cool morning. Write the evaluation.

• Separate what is a mistake from what is a limitation, and say how you can tell
• Explain what the one odd result should be done about, and why averaging it in would be wrong
• Say which direction the uncontrolled temperature would have pushed the results
• Give ONE specific improvement to the apparatus or the design, not to the care taken
• Say what that improvement would actually change about the measurements