Students should be able to: • calculate rate changes in the decay of…

The idea

Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. The rate of decay changes with temperature, moisture, oxygen availability and the type of material. To calculate a rate change, measure how a quantity such as mass, temperature or gas volume alters over a timed interval, then divide the change by the time taken. For example, if a piece of fruit loses 12 g in 4 days, the average rate is 12 g ÷ 4 days = 3 g per day. You must also move between numbers and graphs: read values from a plotted curve, use a table to plot points, and choose axes with equal, sensible scales so the pattern is clear. A well-drawn graph lets you compare rates by comparing gradients.

What earns the marks

• Rate of decay is calculated by dividing the change in a measured quantity by the time taken, for example mass lost ÷ time or gas produced ÷ time.
• A steeper gradient on a mass-loss or gas-volume graph indicates a faster rate of decay, while a flatter gradient indicates a slower rate.
• Numerical data from a table can be translated into a graph by plotting time on the x-axis and the measured quantity on the y-axis.
• When plotting graphs, choose a scale that uses most of the available axis and gives evenly spaced major intervals, then label each axis with quantity and unit.
• Changes in temperature, moisture and oxygen availability alter the rate of decay because they affect the activity of decomposers.

Common mistakes and how to put them right

• Dividing time by the change in mass instead of dividing the change in mass by time.

  Put it right by keeping the rate as change in quantity per unit time.

• Plotting points without checking the scale, so the line is cramped or runs off the grid.

  Put it right by choosing a scale that fits the largest value and uses most of the axis.

• Joining points with straight lines when the pattern is a smooth curve, or drawing a line of best fit through scattered points without judging the trend.

  Put it right by using a smooth curve or a straight line of best fit as appropriate.

Exam tips

• Always include units in rate calculations, such as g per day or cm³ per minute, and show the division clearly.
• When reading a graph, use a ruler to line up the value on the axis with the curve, then read across to the other axis.
• Check that the scale on each axis increases by equal steps and that the axes are labelled with both the quantity and its unit.