(i) selection and labelling of axes with appropriate scales, quantities…

The idea

When you plot data from a practical, the graph must let a reader read values without guessing. Decide which variable is independent (you chose or controlled it) and place it on the x-axis; place the dependent variable on the y-axis. Label each axis with the quantity and its unit, for example 'Time / s' or 'Rate of oxygen uptake / cm³ min⁻¹', using a solidus or brackets so the unit is unambiguous. Choose a scale that uses most of the grid: pick a simple step such as 1, 2, 5 or 10 units per major square so intermediate values are easy to read, and keep the step constant along the whole axis. Start each axis at zero unless the data range makes that wasteful, in which case a clear break may be shown. Number the major gridlines rather than every small square, and give the graph a title stating what is plotted.

What earns the marks

• Identifies the independent variable and places it on the x-axis, with the dependent variable on the y-axis.
• States that each axis label must give the quantity and its unit, e.g. 'Concentration / mol dm⁻³'.
• Explains that a constant, simple scale step (such as 1, 2, 5 or 10 per major square) makes values easy to read.
• Recognises that the scale should use most of the available grid so plotted points are spread out and readable.
• Notes that axes normally start at zero, with any break clearly indicated, and that major gridlines are numbered.
• Describes checking the finished graph so that any point can be read to the nearest small division.

Common mistakes and how to put them right

• Putting the dependent variable on the x-axis.

  Put it right: The variable the experimenter sets or controls goes on the x-axis and the measured outcome on the y-axis.

• Labelling an axis with a bare number or a bare word such as 'time' without a unit.

  Put it right: Give both quantity and unit, for example 'Time / s'.

• Choosing an irregular scale so that equal distances represent unequal steps.

  Put it right: Use one constant step along the whole axis.

Exam tips

• Read the axis labels in the question stem before answering; the quantity and unit must both be present.
• Check the scale by counting the value of one small square and confirming it stays the same across the axis.
• If asked to improve a graph, name the specific fault (axis swapped, missing unit, uneven scale) and state the correction.