9.19B Calculate rate changes in the decay of biological material

The idea

Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. The rate of decay is how quickly material is broken down, and it changes with conditions: warmth, moisture and oxygen speed up microbial activity, while cold, dry or airless conditions slow it. To calculate a rate, divide the change in a measured quantity by the time taken, for example rate = mass lost ÷ time, giving units such as g per day. If 20 g of material decays to 8 g over 6 days, the mass lost is 20 − 8 = 12 g, so the rate is 12 ÷ 6 = 2 g per day. Comparing rates before and after a change in conditions shows how temperature, water or oxygen affect decay. Always subtract to find the change, keep units consistent, and state the rate with its unit.

What earns the marks

• Identify the change in the measured quantity, for example final mass subtracted from initial mass.
• Divide the change by the time taken to give a rate, such as g per day.
• Use consistent units and state the unit with the answer.
• Compare rates under different conditions to show how temperature, moisture or oxygen affect decay.

Common mistakes and how to put them right

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

  Put it right: Subtract first, then divide.

• Mixing units, for example days and hours.

  Put it right: Convert so time units match before dividing.

• Giving a rate without a unit.

  Put it right: Attach the correct unit such as g per day.

Exam tips

• Write the formula rate = change ÷ time before substituting numbers.
• Show the subtraction step so the change in mass is clear.
• Check the unit of the answer matches the quantity and time used.