What you need to know
- Unit 3Identify apparatus, describe how the relevant variables are measured, and discuss how the data collected will be used.
Core Practical methods stay in their own guides. This lesson is the planning language the paper asks for, including in a context you have not carried out.
By the end, you should be able to…
- Name apparatus that matches the measurement, and give a reason.
- Describe the measuring technique, including the unit.
- State how the readings will be processed or displayed.
Build your understanding
Apparatus needs a reason
“Use a beaker” rarely answers the question. Identify the instrument that measures the dependent variable, or that sets the independent variable, and say why it is suitable. A thermometer is appropriate for a water-bath temperature because it reads that temperature. A colorimeter is appropriate for a coloured solution when you need a number rather than a personal judgement of colour.
Give the detail the method needs: the range or resolutionThe smallest distance between two points that can still be seen as separate points. when it matters, and the volume or size when the procedure depends on it. A 10 cm³ syringe and a 1 cm³ syringe are not interchangeable if the volume you must deliver is 0.5 cm³. Do not invent a catalogue of equipment the question did not require.
Describe the measurement itself
State what is read, in what unit, and at what moment. “Measure the rate” is the aim, not the technique. An illustrative technique is: “Start the timer when the enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. is added. Stop it when the iodine sample no longer turns blue-black. Record the time in seconds.”
If the dependent variable is a change, say the two readings and the interval. A bubble count is a proxy for gas production: say that you are counting bubbles, not that you have measured the volume of oxygen, unless a gas syringe or measuring cylinder was used. The same honesty applies to colour comparisons against standards.
Say how the data will be used
The specification asks you to discuss how the data will be used. Before the experiment, state the processing: a mean of the repeats, a rate as change divided by time, a graph of the dependent variable against the independent variable, or a comparison with colour standards.
That sentence stops a plan that collects numbers and then has no decision. It also shows you have not confused raw data with a conclusion. Units belong in the plan. A table heading or axis label without a unit is not yet a usable record.
Graph skills: SLAPUK and choosing a scalePlanning the graph you will plot: SLAPUK and the scale rules.
Make your reasoning visible
Reason, then instrument
“A colorimeter, because it gives a reading for colour intensity rather than a subjective match” is stronger than a bare list of glassware.
Move a familiar technique into a new setting
An unfamiliar extract or tissue can still be tested with a method you know, such as standards, a blank, or a timed end-point. Name the technique. Do not claim it is a core practical it is not.
Avoid this misconception
The claim: Writing the aim is the same as describing the method.
The correction: The aim names the relationship. The method says what is changed, what is measured, with which instrument, and how the numbers will be used.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative plan estimates the reducing-sugar concentration of a fruit juice using Benedict’s reagent. Why are known glucose standards part of the apparatus, not an optional extra?
Show suggested answer
The test is semi-quantitative. Standards treated in the same way as the juice give colours or precipitates to compare with. Without them the colour cannot be turned into a concentration estimate.
Practice question
Retrieval and explanation · suggested answer
A student writes “measure the bubbles”. Rewrite that as a measurement a reader could carry out.
Show suggested answer
Count the bubbles released from the cut end of the pondweed in a stated time, such as one minute, and record the number per minute. State that bubble counts are a proxy and that bubble volume is not measured unless a gas syringe is used.
Practice question
Retrieval and explanation · suggested answer
The plan will collect mass lost by leaves in five different wind speeds. How should those data be used?
Show suggested answer
Record the mass change for each leaf over the same stated time, calculate the rate of mass loss, and plot rate against wind speed. Keep the other relevant conditions the same so the graph is about wind speed.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
You are asked to plan a way to compare the vitamin C content of two juices. You may use DCPIP and a volumetric method you have met, but these juices are not a core-practical sample you have memorised. Outline the measurements and how the data will be used. State one assumption.
Show a hint
Transfer the titration idea. Do not invent a unique vitamin C value.
Explore the reasoning
Add each juice from a burette or syringe to a stated volume of DCPIP until the blue colour just disappears. Record the volume of each juice. The juice that needs the smaller volume has the higher vitamin C concentration, if the DCPIP volume and concentration are the same. A calibration against a known ascorbic acid solution would turn the volumes into concentrations. The assumption is that nothing else in the juice is decolourising the DCPIP.
Recall this lesson
Lesson recall questions
Question 1 of 5: A candidate plans to investigate how temperature affects the initial rate of an amylase-catalysed starch hydrolysis. State the independent variable and the dependent variable.
Unit 3 planning
Practice question
Retrieval and explanation · suggested answer
A candidate plans to investigate how temperature affects the initial rate of an amylase-catalysed starch hydrolysis. State the independent variable and the dependent variable.
Lesson: P2: Apparatus and the measurements you will take
Try this from memory first.
Beyond the classroomA number without a unit cannot be comparedMeasurement · units · Enrichment, not required recall
National measurement laboratories publish the SI so that a result can be traced to a stated unit. School apparatus is not a standards laboratory. The exam point is smaller and exact: name the unit of the quantity you measure, and do not change it halfway through a plan.
Back to the spec: Unit 3 planning: identify apparatus and describe the measurement, including the unit the data will be collected in.
NIST: the International System of UnitsQuestion
Two students both report “the length was 12”. Why can those results not yet be compared?
Explore a possible response
The unit is missing. Twelve millimetres and twelve centimetres are different lengths. A plan or a table has to state the unit before the numbers can be compared or plotted.