Chemistry PAG Review

What the question is really asking

There are eight required practicals on this course, and a practical question can draw on any of them. That sounds like a lot to revise, and it would be if the questions were about remembering what you did. They are not. They ask what a particular decision was PROTECTING. ⭐ Here is the idea that covers all eight. In any experiment, ONLY ONE THING IS ALLOWED TO CHANGE, and every other decision in the method is there to protect that. So every step is doing one of three jobs. Holding something still so it cannot affect the result. Reading something more finely so the number can be trusted. Or checking that the first two actually worked. ⚠️ Learn those three and you can answer about a practical you have never seen, which is precisely the situation the exam puts you in.

Three jobs a method step can do

Take any instruction in any method and ask which of these it is. Almost every one is exactly one of the three, and saying which is usually the whole answer. The first column is about fairness, the second about the quality of a number, the third about trusting what you got.

Order the planning

Put the stages of planning an investigation in the order they should happen.

• Decide exactly what is being investigated
• Choose the one variable that will be changed
• List everything else that could affect the result and decide how to hold each one still
• Choose apparatus fine enough for the measurement being taken
• Decide how many times to repeat, and how results will be compared

Precise and wrong

A student takes five readings and they all agree closely with each other, but every one of them is well above the true value. How should that set of results be described?

• Precise but not accurate, since precision is about how close the readings are to each other and accuracy is about closeness to the true value
• Accurate but not precise
• Both accurate and precise, because they agree so closely
• All five are anomalies

The words this topic is made of

Five terms that are constantly confused with one another. Learn each one against the one it is usually mixed up with.

Find the decision that protects fairness

A student writes up a method. Select the ONE sentence describing a step whose job is to hold something still rather than to read more finely or to check.

• I used a burette rather than a measuring cylinder so I could read the volume more finely.
• I repeated each measurement three times and compared the results.
• I used the same mass of solid, broken into the same size pieces, for every run.
• I ignored one result that was far away from the other two.
• I read the scale at eye level each time.

Which steps hold something still

Three of these five exist to stop a second variable changing. Choose the three.

• Keeping every run at the same temperature
• Using the same volume of solution in each run
• Using pieces of solid of the same size
• Choosing apparatus with finer divisions
• Repeating each run three times

One question for any practical

You may be asked about an experiment you have never done. That is deliberate, and you are not stuck. ⭐ Ask of each step: is this holding something still, reading more finely, or checking? Then say what would go wrong without it. That second half is where the marks are, because it shows you know what the step was protecting. ⚠️ And keep the words straight. Precise means the readings agree with each other. Accurate means they are close to the truth. Repeatable means you could get them again.

Match the decision to what it protects

Which runner is faster

Leave the laboratory aside. Two friends want to settle who is quicker over four hundred metres, so they each run it and compare the times. One runs on Tuesday, on a track, in still weather, in proper shoes. The other runs on Saturday, on grass, into a headwind, after football. ⭐ The stopwatch was the same, and it was a good stopwatch, reading to a hundredth of a second. That is not the problem. The problem is that four things differed besides the runner. ⚠️ And notice that a better stopwatch could not help even slightly. You could time both to a thousandth and still not know which of them is quicker, because the question you measured was never the question you asked. So they do it again: same track, same day, same shoes, one after the other. Now the only thing that differs is the runner. ⭐⭐ And they run it three times each, because on one attempt somebody might stumble, and an answer that rests on a single try is an answer nobody can check. Fairness first, then the quality of the measurement, then check it. In that order.

Complete the practical skills facts

The one thing deliberately changed in an investigation is the _____ variable, and everything else that could affect the result is a _____ variable and must be held still. How close a result is to the true value is its _____, while how closely repeated readings agree with each other is their _____. A result that does not fit the pattern of the others is an _____ and should be left out of a mean rather than quietly deleted. Getting the same result again with the same equipment shows that the measurement is _____.

Build the improvement

A student compares reaction times but changes the temperature between runs without meaning to. Complete the advice.

Practical skills quick-fire

Five quick decisions. Three lives.

Evaluate a method you have never seen

You are shown a method for an experiment you have not carried out, and asked to improve it. Work through your approach.

• What do you look for first?
• The controls look sound. What is the next thing worth examining?
• You have one improvement left to suggest. What earns most?

Explain what protects a result

Explain how a practical method protects its result, using the ideas of control variables, apparatus choice and repeats. You may refer to any experiment you have carried out.

• Explain what the independent variable is and why only one thing may change
• Give one control variable and say what would go wrong if it were allowed to change
• Explain what choosing finer apparatus improves, and what it cannot fix
• Explain the difference between accuracy and precision, using both words correctly
• Explain what should be done with an anomalous result, and why it should not simply be deleted