Alternative to Practical (Paper 4): where the marks go and how to get them

In Cambridge O Level Physics 5054, Chemistry 5070 and Biology 5090, the practical 20% is examined in one of two ways. Paper 3 is a practical test in a laboratory. Paper 4, the Alternative to Practical, is a written paper about experiments. Your school decides which one you sit, so ask your teacher.

Paper 4 gives you the experiment on the page: a diagram of the apparatus, a scale to read, some results to process. Then it asks what someone doing the experiment would do next. The same few skills come up again and again, and skills can be practised.

Many Paper 4 questions start with a reading from a diagram: a thermometer, a measuring cylinder, a burette, a stopwatch. Read to the precision the scale allows, usually its smallest division, and read a liquid level at the bottom of the meniscus.

Then comes the table. Three habits earn the marks:

• Quantity and unit in every heading, for example "length / cm" or "time / s". The unit goes in the heading, not after every number.
• The same number of decimal places down a column of readings, matching the precision of the instrument. A burette read to the nearest 0.05 cm³ gives 23.45, not 23.4 or 23.450.
• Processed values worked out correctly, such as a mean or a period, given to a sensible number of significant figures.

A graph question usually carries several marks, each for one specific thing. Work through them in order:

1. Axes the right way round. The quantity you changed goes on the x-axis, and the one you measured on the y-axis, unless the question says otherwise.
2. Label both axes with the quantity and its unit, as in the table.
3. Choose an easy scale. Steps of 1, 2 or 5 per large square are easy to plot and read. Steps of 3 are not.
4. Use the space. The points should spread across at least half the grid in each direction.
5. Plot precisely, with small, neat crosses.
6. Draw one thin line of best fit, straight or a smooth curve, with the points scattered evenly either side. Do not join the dots.

To find a gradient, draw a large triangle on your line, using most of its length. Read the two corners from the line itself, not from your table, and write their coordinates down so the working is visible. The gradient is the change in y divided by the change in x.

Give the gradient a unit when it has one: the y unit divided by the x unit. In the pendulum experiment, for example, plotting T² in s² against length in m gives a gradient in s²/m.

A conclusion describes the pattern in the results in front of you. "As the length increases, the period increases" is a conclusion. So is "the volume of gas is directly proportional to the time", but only if the graph is a straight line through the origin. If it does not pass through the origin, it is not proportional, even if it is straight.

When a question says "use the data", quote figures from the table or graph, with units. When it asks for an explanation, add the science that accounts for the pattern.

Improvement questions reward answers that fit this particular experiment. "Be more careful" and "human error" earn nothing on their own. Name the problem, then give the fix for it:

• Reaction time when starting and stopping a stopwatch, so time 20 oscillations and divide by 20, instead of timing one.
• Readings that vary, so repeat and calculate a mean.
• Parallax when reading a scale, so read it at eye level.
• Heat lost to the surroundings, so insulate the beaker or add a lid.
• A colour change that is hard to see, so put the flask on a white tile.

In Chemistry, an observation is what you see, hear or feel. A conclusion is what it means. "Bubbles form" or "effervescence" is an observation. "Hydrogen is produced" is a conclusion. If the question asks what you would observe, do not name the gas.

The gas tests come up often:

For ion tests, learn the results from the list in your own syllabus, not from what you remember of a lesson. In a titration, use only the accurate titres that agree closely when you calculate the mean.

Reading about graphs does not make you quicker at drawing them. Drawing them does. Take a Paper 4 question, do every part on real graph paper, then mark it against the mark scheme line by line. Our guide on how to practise past papers sets out a method.

On Eudika Learn, the virtual labs have you take readings, fill in a results table and, in most labs, plot the graph and name an improvement. The labs check that your headings carry units, that your scale steps are 1, 2 or 5, and that your points fill at least half the grid. The pendulum, titration and osmosis labs are free to try. The past papers for Physics and Biology include real Paper 4 questions to answer and mark. None of this replaces a school laboratory.