Choosing apparatus and measuring correctly

Pick the instrument with suitable resolution

Resolution is the smallest change an instrument can show.

• Volumes: measuring cylinder (about 1 cm³ resolution for a 100 cm³ cylinder), graduated pipette or syringe (0.1 cm³), burette (0.1 cm³ graduations, read to 0.05 cm³).
• Mass: top-pan balance reading to 0.01 g.
• Length: ruler (1 mm), eyepiece graticule calibrated against a stage micrometer (micrometres).
• Time: stopwatch (0.01 s display, but human reaction time is much larger, about 0.2 to 0.3 s).
• Colour intensity: colorimeter (absorbance or transmission) instead of judging by eye.

Correct techniques

• Read a liquid volume at the bottom of the meniscus with your eye level with the mark (avoids parallax). The June 2024 mark scheme credited exactly this for 'how to measure an accurate volume with a pipette'.
• Zero (tare) a balance before weighing.
• Use a colorimeter with the same filter and zero it with a blank (water or reagent only) before each set of readings.
• Use a thermostatically controlled water bath and let tubes equilibrate for a few minutes before mixing.
• Start timing at the moment solutions are mixed; use a colour standard or a colorimeter to judge the end point consistently.

Making biological material comparable

Living material varies, so standardise it:

• cut tissue with the same cork borer and a ruler and scalpel to the same length (same surface area to volume ratio);
• rinse beetroot pieces to remove pigment from cut cells (CP3);
• take plants or seeds from the same source, of the same age;
• use the same region of a root tip (the tip 5 mm) in CP6.

The June 2024 squid question credited 'equal sized pieces give equal surface area for the enzyme to act on'.

Worked example

Suggest the most suitable apparatus to add 2.5 cm³ of enzyme solution to a test tube, and explain why a 10 cm³ measuring cylinder is less suitable.

1. A 5 cm³ graduated pipette or syringe reads to 0.1 cm³.
2. A 10 cm³ measuring cylinder is typically graduated in 0.2 cm³ divisions, so the volume is less precise and some liquid stays on the walls.

Answer: A graduated pipette or syringe, because it has a finer resolution than the measuring cylinder, so the volume is more precise.

Check yourself

1. What is meant by the resolution of a measuring instrument?
   • How close its reading is to the true value
   • The smallest change in the quantity that it can show
   • The largest value it can measure
   • How many times it has been used
   Show answer

   The smallest change in the quantity that it can show. Resolution is the smallest division or digit change.

2. How should the volume in a pipette be read to avoid parallax error?
   • At the top of the meniscus, looking down on it
   • At the bottom of the meniscus from below
   • By tilting the pipette towards you
   • At the bottom of the meniscus with the eye level with the mark
   Show answer

   At the bottom of the meniscus with the eye level with the mark. Eye level with the bottom of the meniscus.

3. Why is a colorimeter better than the eye for judging the colour of Benedict's tests?
   • It is faster to use
   • It gives a quantitative reading that does not depend on the observer
   • It needs no calibration
   • It makes the colour darker
   Show answer

   It gives a quantitative reading that does not depend on the observer. Absorbance is a number, removing subjective judgement.

4. Why are enzyme and substrate tubes left in the water bath for several minutes before mixing?
   • So the enzyme can denature
   • To let the substrate evaporate
   • So the two solutions reach the required temperature before the reaction starts
   • To start the timer
   Show answer

   So the two solutions reach the required temperature before the reaction starts. Equilibration means the reaction happens at the stated temperature.

5. Why is a cork borer used to cut beetroot pieces in CP3?
   • To give pieces of the same diameter and so the same surface area
   • To kill the cells
   • To remove the pigment
   • To keep the pieces cold
   Show answer

   To give pieces of the same diameter and so the same surface area. Same diameter (and cut length) standardises surface area.

6. A stopwatch reads to 0.01 s. Why is the uncertainty in a hand-timed reading larger than 0.01 s?
   • Stopwatches are not accurate
   • Time cannot be measured in biology
   • Human reaction time is about 0.2 to 0.3 s
   • The display is too small
   Show answer

   Human reaction time is about 0.2 to 0.3 s. Starting and stopping by hand adds far more uncertainty than the display.

7. What should be done to a top-pan balance before weighing a sample in a weighing boat?
   • Place the boat on the balance and tare (zero) it
   • Weigh the sample twice and add the readings
   • Heat the balance
   • Remove the boat and add the sample directly
   Show answer

   Place the boat on the balance and tare (zero) it. Taring means only the sample's mass is recorded.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson Examiners' Report WBI13/01, June 2024
• Pearson mark scheme WBI13/01, June 2024 (via Physics & Maths Tutor)

Last updated 8 November 2026. Spotted a mistake? Tell us.