How to tabulate data and plot graphs (SPM Paper 3)

Presenting results is a science process skill in its own right, and SPM Chemistry Paper 3, the practical paper (Paper 3 is a practical test assessing science process skills) of SPM Chemistry, gives marks for a tidy table and a well-drawn graph even before you interpret them. Many students lose these marks not because they cannot do chemistry but because their table has no units or their graph uses a poor scale. This guide sets out exactly what a table and a graph must contain, works through the steps, and lists the errors that cost marks.

Tabulating data

A data table communicates your readings clearly and in order. The marking scheme looks for several specific features, and each is worth being deliberate about.

Column headings with quantity and unit. Every column must be headed with the quantity it holds and its unit, written in an accepted form such as “Time / s” or “Temperature / °C”. The slash-and-unit style is the one examiners expect. A number in the table then needs no unit beside it, because the heading already carries it.

A column for the manipulated variable and one for the responding variable. Arrange the table so the quantity you changed and the quantity you measured each have their own column, with the manipulated variable usually on the left.

Readings in order and to consistent precision. List the values of the manipulated variable in increasing order, and record every reading of a quantity to the same number of decimal places, matching the precision of the instrument. If you take repeat readings, include a column for each repeat and one for the average.

Include every reading you are given or take. Do not leave gaps; a complete table is part of the mark.

Choosing what to plot

Usually you plot the responding variable on the vertical (y) axis against the manipulated variable on the horizontal (x) axis. Sometimes the question asks you to plot a derived quantity, such as 1/time as a measure of rate; read the instruction carefully and plot exactly what is asked.

Drawing the graph

Label both axes with quantity and unit. Use the same “quantity / unit” style as the table, for example “Volume of gas / cm³”.

Choose a scale that uses more than half the grid. Pick a simple scale (1, 2, 5 or 10 units per square) so that the plotted points fill more than half of the graph paper in both directions. A cramped graph that fills only a corner loses a mark, and so does an awkward scale like 3 units per square that makes plotting hard.

Plot the points accurately. Mark each point with a small cross or an encircled dot, placed precisely at the coordinates. Careless plotting is a common way to lose marks.

Draw a smooth line or line of best fit. If the points lie on a straight line, draw a single straight line of best fit with roughly equal numbers of points on each side. If they follow a curve, draw a single smooth curve through the trend. Do not simply join the dots point to point, and do not force the line through the origin unless the data support it.

Reading values from the graph

You are often asked to read a value off the graph, for example, the responding variable at a manipulated value you did not measure (interpolation), or the gradient of a straight line. Show your working on the graph with dashed construction lines, and for a gradient use a large triangle and state the change in y over the change in x with units. Reading between your points is interpolation; extending beyond them is extrapolation, which is less reliable and should be done only if the question asks.

Common errors

• No units in the table or on the axes. Every heading and axis needs its unit; this is one of the easiest marks to lose.
• Inconsistent precision. Recording some readings to one decimal place and others to two suggests carelessness; keep the precision consistent.
• A poor scale. A scale that fills only part of the grid, or an odd scale like 3 per square, loses the scale mark; use a simple scale that fills more than half.
• Joining points dot to dot. Draw a single best-fit line or smooth curve, not a zig-zag.
• Plotting points inaccurately. Take care to place each point exactly; a misplaced point can spoil the line and the reading taken from it.
• Forgetting to show working for a gradient or reading. Draw the construction lines or triangle so the examiner can see how you obtained the value.

Tips for full marks

Rule your table before you start and write the headings with units first, so no reading goes down without a home. When you reach the graph, decide the scale by looking at the largest and smallest values and choosing a simple step that spreads them across more than half the grid. Plot lightly, check each point, then draw the line firmly. Finally, when a value is read from the graph, always leave the dashed lines or the gradient triangle visible as evidence of your method.

How our teachers build this skill

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we mark students’ tables and graphs the way an examiner would, checking headings, units, scale, points and the best-fit line one by one, because these are dependable marks in the Paper 3 practical of SPM Chemistry that are lost only through untidiness. Once a student rules a proper table and chooses a sensible scale automatically, these marks stop slipping away, and the clean data presentation also makes the inference and conclusion that follow far easier to write.