(b) use of appropriate instrumentation to record quantitative…

The idea

Choosing appropriate instrumentation means matching the instrument to the quantity being measured and the precision required. A colorimeter records the absorbance or transmission of light through a coloured solution, so it quantifies colour change objectively rather than by eye; it is calibrated with a blank or distilled water before use and a suitable filter is selected. A potometer records the rate of water uptake by a shoot as a proxy for transpiration rate; a bubble of air moves along a capillary scale as the shoot takes up water, and the distance moved in a set time is recorded. Both instruments give quantitative data, reducing subjective judgement. The choice depends on what is being measured: colour intensity for a colorimeter, water uptake for a potometer.

What earns the marks

• Matches the instrument to the quantity measured: a colorimeter measures absorbance or transmission of light through a coloured solution.
• Explains that a colorimeter is calibrated with a blank (e.g. distilled water) and an appropriate filter is selected before readings are taken.
• Describes a potometer as recording the rate of water uptake by a shoot, with an air bubble moving along a capillary scale as a proxy for transpiration.
• Recognises that both instruments produce quantitative data, reducing subjective judgement compared with visual estimation.
• Selects the appropriate instrument for a stated investigation, e.g. a colorimeter for colour change and a potometer for water uptake.

Common mistakes and how to put them right

• Claiming a colorimeter measures the colour directly by eye.

  Put it right by stating it measures absorbance or transmission of light quantitatively.

• Using a potometer to measure transpiration directly.

  Put it right by explaining it measures water uptake, which is used as a proxy for transpiration rate.

• Omitting calibration of the colorimeter with a blank.

  Put it right by including calibration with distilled water or a blank solution before readings.

• Assuming any filter will do for a colorimeter.

  Put it right by selecting a filter of appropriate wavelength for the colour being measured.

Exam tips

• Identify the variable being measured in the stem, then match it to the instrument designed for that variable.
• Reject options that describe qualitative visual judgement when the question asks for quantitative recording.
• Check that the described procedure includes calibration or a control step, since appropriate use of instrumentation depends on it.