Variables in an experiment

Option A changes the light condition and compares the rate of germination, so it tests the relationship between light and germination. Options B, C, and D test the effects of water, seed size, and temperature respectively, not light. Exam tip: In a fair test, change only one factor at a time.

For a fair comparison, both groups must be measured using the same method. For example, temperature in both groups should be measured with the same type of thermometer and procedure. The independent variable is usually the factor changed for comparison, so it need not be the same. Exam tip: In comparison questions, check that the measurement method and units are consistent.

Plant growth is affected by several factors, such as light, soil, temperature, and type of seed. If these are kept the same, any difference in growth can be linked to the amount of water only. Therefore, option A is correct. If other factors also change, it will not be clear whether water or another factor caused the result. Exam tip: In a fair test, change only one independent variable.

Option A directly uses the number of leaves on a plant as the outcome to be counted, so it is related to leaf number. Option B measures plant height, which is a different observation. Exam tip: Identify the measurement or observation named in the question; it is usually the dependent variable.

The measured result depends on the independent variable so it is the dependent variable. In exams choose the measured effect.

The amount of water is deliberately changed so it is the independent variable. In exams look for the factor changed by the experimenter.

Plant height is the measured result so it is the dependent variable. In exams identify the outcome variable.

Keeping the seed type the same allows a fair test of temperature. In exams keep other factors constant.

The direct answer is C, the dependent variable. A variable is any feature that can change. The independent variable is the factor the investigator deliberately changes. The dependent variable is the response that changes because of that factor and is usually measured. A controlled variable is kept the same, and a constant is another name for something held unchanged. Step by step: identify the deliberate cause; identify the measured effect; the measured effect depends on the cause; therefore it is the dependent variable. A is wrong because a controlled variable is not supposed to change. B is wrong because the independent variable is the cause or input, not the response. C is correct because it changes as a consequence of another variable. D is wrong because a constant remains unchanged. Example: if fertiliser amount is changed and plant height is measured, fertiliser amount is independent and height is dependent. Exam cue: “what is measured?” usually points to the dependent variable.

The surface type is deliberately changed so it is the independent variable. In exams identify the changed condition.

The direct answer is A, the dependent variable. The surfaces are deliberately different, so surface type is the independent variable. The distance travelled is observed and measured after the ball rolls, so it is the response to the surface. Step by step: change the surface; roll the ball under the same starting conditions; measure how far it travels; distance may change because the surface changes; therefore distance is dependent. A is correct. B is wrong because the independent variable is what is deliberately changed, namely the surface, not the measured distance. C is wrong because a controlled variable must be kept the same, such as ball, starting point, push, and slope. D is wrong because distance is not a constant if it varies between surfaces; “constant” means unchanged. A fair test also repeats trials and uses the same method. Memory cue: changed factor is independent; measured result is dependent.

Changing only one independent variable at a time helps identify its effect. In exams remember the idea of a fair test.

Temperature is changed in the experiment so it is the independent variable. In exams identify the quantity being changed.

The dissolving time is measured and depends on temperature. In exams treat the measured time as the dependent variable.

The experimenter changes the fertilizer amount so it is the independent variable. In exams choose the deliberately changed factor.

The number of leaves is the measured result so it is the dependent variable. In exams identify the measure of growth.

Keeping pot size the same allows a fair comparison of fertilizer effects. In exams identify it as a controlled variable.

When two factors change it becomes difficult to know which caused the result. In exams change only one independent variable.

The direct answer is A: the independent variable is usually shown on the horizontal axis, also called the x-axis. In an experiment, the independent variable is the factor deliberately changed by the investigator, while the dependent variable is the result measured in response. Graphing convention places the changed factor on the horizontal axis and the measured response on the vertical axis. Option A is correct because this arrangement makes the relationship easier to read. Option B is usually wrong because the vertical axis is normally used for the dependent variable, though a graph must always be checked and labelled. Option C is not the usual answer because one independent variable is normally placed on the horizontal axis, not both axes. Option D is less correct because axes should not be chosen randomly; the standard convention improves clarity. Always write the variable name and unit on each axis. Memory cue: independent is the input on x; dependent is the response on y.

The dependent variable is usually placed on the vertical axis. In exams place the measured quantity on this axis.

A controlled variable keeps experimental conditions the same. In exams identify constant factors by this name.

The drop height is deliberately changed so it is the independent variable. In exams identify the changed quantity.

The bounce height is the measured result so it is the dependent variable. In exams choose the measured effect.

Keeping the ball type the same allows a fair test of drop height. In exams treat it as a controlled variable.

Fan speed is changed so it is the independent variable. In exams identify the change controlled by the experimenter.