Practical skills questions: variables, repeats and errors.
Experimental skills are tested in the written papers, often as 1 to 3 mark questions inside a longer one. These answers follow a small set of patterns. Learn them here, then plan a full investigation with CORMMSS.
Three kinds of variable
- Independent variable: the one you change on purpose.
- Dependent variable: the one you measure, to see the effect.
- Control variables: everything else that could affect the result, kept the same so the test is fair.
- Name a control variable precisely, as a quantity: “the volume of starch solution”, not “the amount of stuff”.
Model line: The independent variable is the temperature; the dependent variable is the time taken for the starch to disappear; control variables include the pH and the volume and concentration of the amylase.
Quick check A student counts bubbles from pondweed with the lamp at 10, 20, 30 and 40 cm. Name the independent variable, the dependent variable and one control variable.
Answer: Independent: the distance of the lamp (light intensity). Dependent: the number of bubbles per minute. Control variable: for example the temperature of the water, the same piece of pondweed, or the same carbon dioxide concentration.
A control experiment is not a control variable
- A control experiment is a version of the experiment without the factor you are testing, such as boiled enzyme, or no light. It shows that the factor, and nothing else, caused the result.
- A control variable is a quantity kept the same in every test.
- Always say which kind of control you mean. “It is a control” earns nothing on its own.
Model line: A control experiment with boiled (denatured) amylase shows that the starch is broken down by the enzyme, not by something else in the tube.
Quick check Why might a student set up a tube of germinating seeds and a tube of boiled (dead) seeds when testing whether seeds release heat?
Answer: The boiled seeds are a control experiment: they do not respire, so any temperature rise in the living seeds can be put down to their respiration, not to the surroundings.
Repeats, means and anomalies
- Repeat each test (often three times) and calculate a mean. This reduces the effect of random variation in single readings.
- Results are repeatable if you get similar results when you repeat the test yourself, and reproducible if other people, or other methods, get similar results.
- An anomaly is a result that does not fit the pattern. Leave it out of the mean, say so, and suggest a cause if asked.
Model line: The reading of 41 s at 30 °C is anomalous, so I left it out and calculated the mean of the other two repeats.
Quick check Three repeats give 12, 13 and 25 seconds. What is the mean, and what should the student do first?
Answer: Spot that 25 s is an anomaly and leave it out. The mean of 12 and 13 is 12.5 seconds.
Errors and improvements
- Name a specific source of error from this method: judging a colour change by eye, a lamp that also heats the water, timing by hand.
- Match each problem with a specific improvement: a colorimeter, a heat shield or a beaker of water between the lamp and the plant, a data logger.
- “Human error” and “be more careful” are not credited. Say what went wrong and what would fix it.
Model line: Judging when the iodine stops turning blue-black is subjective, so compare each drop with a colour chart, or use a colorimeter.
Quick check The lamp in a pondweed experiment also warms the water. Why is this a problem, and how could it be fixed?
Answer: Temperature also affects the rate of photosynthesis, so the results would not show the effect of light alone. Put a heat shield (such as a beaker of water) between the lamp and the plant, or use an LED lamp, and check the water temperature with a thermometer.
Risks and precautions
- Name the hazard, the harm it could do, and one precaution that matches it.
- Heating Benedict’s reagent: use a water bath rather than a direct flame, and wear eye protection.
- Microorganisms: use aseptic technique and incubate cultures at no more than 25 °C in school, so harmful microorganisms that grow at body temperature are less likely to grow.
Model line: Hot water could scald, so use tongs or a test-tube holder to move the tubes in and out of the water bath.
Quick check Give one hazard and one precaution when cutting potato cylinders with a scalpel.
Answer: Hazard: the sharp blade could cut skin. Precaution: cut on a tile, cutting away from your fingers.
Conclusions
- A conclusion says what the results show, using the data: “As X increased from … to …, Y …”.
- Link it to the biology only if the question asks you to explain.
- Do not claim more than the results show: a result from one plant, or a few readings, can only weakly support a general claim.
Model line: As the sugar concentration increased from 0 to 1.0 mol/dm³, the percentage change in mass of the potato decreased from +8% to −12% (illustrative).
Quick check Why can a student not conclude that “all enzymes work best at 40 °C” from an experiment on amylase?
Answer: Only one enzyme was tested. Different enzymes have different optimum temperatures, so the result cannot be extended to all enzymes.