IGCSE Biology (0610)

Calculating means, changes and rates

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

To find the change in temperature, subtract the final temperature from the starting temperature, not the other way around.
Common mistake: Students often reverse the order and get a negative value or subtract incorrectly.
To dilute a solution to a lower concentration, calculate the dilution factor by dividing the original concentration by the desired concentration, then divide the final volume by this factor to find the volume of stock solution needed.
Common mistake: Students often confuse which way to divide or subtract.
When calculating a rate from measured values, divide the total quantity by the total time and round the answer to the same number of significant figures as the least precise measurement.
Common mistake: Students often forget to match significant figures or use the wrong time unit.
To find the change in length, subtract the initial value from the final value (final - initial), so a negative result means a decrease.
Common mistake: Students often reverse the order and get the sign wrong, misinterpreting increase/decrease.
To find the new concentration after dilution, multiply the original concentration by the fraction of the original solution in the total volume.
Common mistake: Students often forget to divide by the total volume after adding water, leading to an incorrect answer.
To find the volume delivered from a syringe, subtract the volume left after use from the starting volume.
Common mistake: Students often mistakenly use the final volume or add the two values instead of subtracting.

Worked example

Complete Table 3.1 by calculating the change in the water temperature for water-bath Y.

Try it first, then show the working

change in temperature = starting temperature - final temperature
42 - 27 = 15
change in water temperature = 15 °C

Answer: 15 °C

Try one yourself

Calculate the volume of 8% glucose solution and the volume of distilled water needed to make 10 cm3 of 2% glucose solution.Answer it and get it marked → Calculate the rate of bubble production at 25 °C. Give your answer to an appropriate number of significant figures.Answer it and get it marked → Complete the row for beaker G2 by working out the volume, in cm3, of stock solution G that must be measured out. The distilled water W then makes the…Answer it and get it marked →

14 exam-style questions on Calculating means, changes and rates, each marked to the scheme.

Calculating means, changes and rates

Key ideas and common mistakes

Worked example

Try one yourself

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