Practical skills

8. Working Scientifically

Picking the right instrument for a measurement, and then reading it properly, is worth marks in every practical question. The two ideas that come up again and again are choosing an instrument whose scale suits the size of the thing you are measuring, and putting your eye in the right place when you read it.

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157 questions5 subtopicsFoundation tier6 required practicals

What practical skills covers

Every subtopic below has its own questions, mark schemes and notes.

WS 2.3Choosing apparatus and taking readings

WS 2.4Working safely and ethically

Every practical question can ask you to spot a hazard and suggest something sensible to do about it. The trick is to be specific: name the hazard, say what harm it could do, and give a precaution that actually addresses that harm.

WS 2.5Sampling

You almost never study every organism in a population, so you study a sample and use it to say something about the whole. That only works if the sample genuinely represents the population, which is why sampling has to be random.

WS 2.6Potometers and measuring cells

Two techniques where the marks are in the detail of the method: setting up a potometer so that no air gets in, and using a microscope with a ruler to work out how big a single cell really is.

WS 3.3Comparing results

When two sets of results did not start from the same value, comparing the raw change is misleading. Percentage change puts them on the same footing. Percentiles do a similar job for describing where one result sits within a whole data set.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Meniscus — The curved upper surface of a liquid in a narrow container.

Graduation — One of the marked divisions on a measuring scale.

Resolution — The smallest change in a quantity that an instrument can show.

Universal indicator — A mixture of indicators that changes colour gradually across a range of pH values, so it can estimate pH rather than just showing acid or alkali.

Displacement — Collecting a gas by letting it push water out of an upturned container, so the volume of water pushed out equals the volume of gas.

Hazard — Something with the potential to cause harm.

Risk — The chance that a hazard actually causes harm, and how serious that harm would be.

Control measure — Something you do to reduce the risk from a hazard.

Flammable — Catches fire easily.

Safety flame — The yellow, clearly visible flame produced when the air hole of a Bunsen burner is closed.

Population — All the organisms of one species living in a particular area at the same time.

Sample — A smaller part of a population that is actually studied.

Biased sample — A sample that does not accurately represent the population it was taken from.

Random sampling — Choosing sampling sites or individuals so that every one has an equal chance of being selected.

Quadrat — A square frame placed on the ground so that the organisms inside it can be counted.

Transect — A line along which samples are taken, used to study how a distribution changes across an area.

Potometer — Apparatus that measures the uptake of water by a plant shoot.

Transpiration — The loss of water vapour from a plant, mainly through the leaves.

Capillary tube — A very narrow tube, used in a potometer so that a small volume of water uptake moves the bubble a long, measurable distance.

Acclimatise — To be left long enough to settle into the new conditions before readings are taken.

Micrometre — One thousandth of a millimetre, written µm. The usual unit for the size of a cell.

Scale bar — A line on a drawing or image labelled with the real length it represents.

Percentage change — The change in a value expressed as a percentage of the value it started at.

Original value — The value measured at the start, before the change.

Percentile — One of a hundred equal chunks of a ranked data set; the value below which that percentage of the data falls.

Median — The middle value when the data is ranked in order, also known as the 50th percentile.

Outlier — A value that is much larger or much smaller than the other values in a data set.

Where marks get lost

The mistakes that come up again and again in this topic.

Reading from the top of the meniscus rather than the bottom is a classic lost mark - and it makes every reading too high.
If asked to choose apparatus, say why: name the instrument AND link it to the size of the measurement or the precision needed.
Litmus only tells you acid or alkali. If a question asks for a pH value, litmus is not enough - you need universal indicator to estimate one or a pH meter to measure one.
'Accurate' means close to the true value. Do not use it as a synonym for 'careful' or 'repeated'.
'Be careful' and 'wear a lab coat' score nothing on their own. Match the precaution to the specific hazard in that experiment.
The yellow flame is for safety because it is easy to see, not because it is cooler - do not write that it is used to heat things gently.
The tripod and gauze go over the Bunsen burner before it is lit, not after.
For ethics questions about people, the key words are consent and pressure - taking part must be voluntary.
'Random' does not mean casually wandering about and dropping a quadrat. It means every site had an equal chance of being chosen, which is why the grid and the random number generator both need mentioning.
A biased sample is not fixed by taking more samples from the same spot - the bias stays. It is fixed by sampling across the whole area.
When scaling a sample up to a whole population, say clearly which area or group you are scaling up to.
A potometer measures water UPTAKE. It only estimates transpiration, because not all the water taken up is transpired - some is used by the plant.
Cutting underwater and assembling underwater are two separate marking points. Both are about keeping air out.
Do not confuse the deliberate single bubble that you introduce to take readings with unwanted air in the xylem, which spoils the results.
1 mm = 1000 µm. Dividing by 100 or 10 000 instead is the most common way this calculation goes wrong.
More cells counted in the 1 mm means each cell is smaller, so the number you divide by goes on the bottom.
Divide by the ORIGINAL value, not the final one. Dividing by the wrong value is the single most common error in this calculation.
Do not drop the minus sign when a value has decreased - it is what shows the direction of the change.
A percentage change of -15% is a bigger change than one of +12%, even though -15 is a smaller number.
The 90th percentile means 90% are below you, not that you are in the top 90%.
The range and the 10th-to-90th percentile gap are not the same measurement, so do not use them interchangeably.

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