Osmosis: Students should be able to plot, draw and interpret appropriate graphs.

The idea

Graph skills underpin osmosis investigations. When plotting, place the independent variable (salt or sugar concentration) on the x-axis and the dependent variable (change in mass) on the y-axis, using a sensible scale that fills the grid. Plot points accurately with small crosses, then draw a line or curve of best fit rather than joining dots. Interpreting means describing the trend: as concentration increases, mass change falls from positive to negative, crossing zero at the isotonic point where external solution concentration matches cell contents. Read values from the graph, including the intercept, and link steepness to the rate of water movement by osmosis.

What earns the marks

• Axes correctly labelled with quantity and unit, for example 'concentration of salt solution / mol dm⁻³' on the x-axis and 'change in mass / g' on the y-axis.
• Suitable linear scale chosen so plotted points occupy at least half the grid, with evenly spaced major divisions.
• Points plotted accurately as small crosses or dots, with a line or curve of best fit drawn smoothly rather than point-to-point.
• Trend described correctly: as concentration increases, change in mass decreases from positive through zero to negative.
• Zero change in mass identified as the point where the external solution is isotonic with the plant tissue, so no net water movement.
• Values read from the graph, such as the concentration at which mass change is zero, quoted with the correct unit.

Common mistakes and how to put them right

• Plotting concentration on the y-axis.

  Put it right: The independent variable always goes on the x-axis, so concentration belongs on the x-axis.

• Joining points dot-to-dot: draw a single line or curve of best fit that shows the overall trend.

• Forgetting units or using inconsistent units on an axis.

  Put it right: Label each axis with quantity and unit, for example 'change in mass / g'.

Exam tips

• Check the command word: 'plot' needs accurate points, 'draw' needs a best-fit line, and 'interpret' needs a described trend with data quoted.
• Quote at least one pair of values from the graph to support your description, for example the concentration where change in mass is zero.
• If asked to suggest why a point is anomalous, refer to the trend and state that the point lies away from the line of best fit.