Variables in an experiment

A controlled variable is a condition kept the same in every trial so that only the intended factor differs. When comparing different soaps, the type of soap is the independent variable, and cleaning ability is the dependent outcome. Other conditions, including the stain, water volume, washing time, and temperature, should be kept comparable.

The stain size must be kept the same because a larger stain could require more cleaning regardless of the soap used. If stain sizes differed, one soap might appear better simply because it was tested on a smaller stain. Thus stain size is an appropriate controlled variable, so option C is correct. Cleaning ability and amount removed are outcomes, not controls.

The independent variable is the factor deliberately changed, and the dependent variable is the result measured in response. A controlled variable is kept unchanged so that it does not confuse the comparison. This distinction is useful whenever an experiment asks whether one quantity affects another: the changed quantity is the possible cause and the measured quantity is the possible effect.

The student changes the amount of sugar added to water. That amount is therefore the independent variable. The density of the resulting solution is measured to see whether it changes with sugar quantity, so solution density is the dependent variable. Water volume, temperature, and other relevant conditions should be controlled. Hence option D is correct.

Direct answer: Option A is correct. The independent variable is the factor deliberately changed to investigate its effect. Here the plant is placed at different distances from the light source, so distance is the independent variable. The amount by which the plant bends is measured and is therefore the dependent variable. Option A works because distance is the experimental input that changes from one trial to another. Option B is wrong as it is the measured response, not the deliberately changed factor. Option C is wrong because plant species should normally be kept the same to make a fair comparison, so it is a controlled variable. Option D is also normally controlled by giving plants equal water; it is not the factor being tested. The logical chain is: change distance → light conditions change → measure bending → compare results. Memory cue: ask “What did the investigator change?” The answer is the independent variable.

Keeping controlled variables constant ensures that mainly the independent variable affects the result. This is the basis of a fair test.

The duration of light is deliberately changed so it is the independent variable. In exams identify the changed factor.

Increase in height is the measured outcome so it is the dependent variable. In exams choose a measurable result.

Water amount can also affect growth so it should be kept the same. In exams identify other influential factors.

Changing sugar amount prevents isolation of the temperature effect. In exams change one main factor at a time.

Drop height is changed and bounce height is measured. In exams identify the cause first and the outcome second.

Changing the ball type may affect bounce so it should be kept the same. In exams identify the controlled variable.

The direct answer is C: boiling point of water. The independent variable is the amount of salt, because that is what the student deliberately changes. The dependent variable is the result that responds and is measured. Here, the student observes the temperature at which the water boils, so the boiling point is dependent on the salt amount. Option A, amount of salt, is independent, not dependent. Option B, initial amount of water, should be kept constant for a fair comparison and is a control condition. Option C is correct because it is the measured outcome. Option D, container size, should also be controlled unless it is being studied; it is not the stated result. A clear method is to label the experiment as “effect of salt amount on boiling point”: the first part is independent and the second part is dependent.

Different water amounts may affect the result so they should be controlled. In exams focus on fair comparison.

Cloth thickness is changed by the experimenter so it is the independent variable. In exams identify the deliberately changed property.

The time taken to reach a fixed temperature is the measured outcome. In exams choose a clearly measurable result.

Lamp distance is changed while light intensity is measured. In exams distinguish the changed and measured variables.

Different lamps may produce different power so the same lamp should be used. In exams check apparatus consistency.

Surface type is deliberately changed so it is the independent variable. In exams identify the factor being changed.

Different pushes may affect the distance so the push should be kept the same. In exams control initial conditions.

The number of leaves is measured as the effect of fertilizer. In exams identify the counted outcome as the dependent variable.

Different initial heights may affect the final result. In exams keep or record starting conditions.

The independent variable is the factor that the investigator deliberately changes. The dependent variable is the response that is measured to see the effect of that change. In this experiment, the student uses different temperatures, so temperature is the factor being varied. The student then records the percentage of seeds that germinate, so germination percentage is the measured response.

Option A is correct because the order is independent variable followed by dependent variable: temperature and germination percentage. The question is asking for cause-like input and observed outcome, not for two controlled conditions. Other factors, such as seed type, amount of water and light, should normally be kept constant if the investigation is to compare temperature fairly. Thus, changing temperature and measuring germination clearly produce the pairing in A.

Keeping the same number of seeds in every group makes the comparison of germination percentages fair. If one group has more seeds, comparing only the total number germinated can give a misleading result. The number of seeds is a controlled variable, so the effect of the independent variable can be tested more reliably. Exam tip: In a fair test, keep sample size and other conditions the same.

The direct answer is C, change in potato mass. The experiment changes salt concentration and measures how potato mass changes. Thus salt concentration is the independent variable, while mass change is the dependent variable because it responds to concentration. Step by step: prepare solutions with different concentrations; use potato pieces that are comparable in size and starting mass; keep solution volume, time and temperature similar; measure final mass; calculate change as final mass minus initial mass, or compare the loss or gain; the calculated mass change is the measured outcome. A is wrong because concentration is deliberately changed, so it is independent. B is wrong because initial size should normally be controlled, not the result. C is correct because mass change is what is measured. D is wrong because solution volume should usually be kept constant as a control. The terms “dependent variable” and “outcome variable” are closely related, but only C is offered as the measured change. Memory cue: the variable that answers “what happened to the potato?” is dependent.

If potato pieces have different sizes, their surface area and volume will differ. This can change the amount of water entering or leaving them by osmosis. Therefore, size is kept constant as a controlled variable, so that differences in the results are mainly due to the salt-solution concentration. Option A is incorrect because solution concentration may be the independent variable; it is not a reason to vary potato size. Exam tip: In a fair test, keep all factors that may affect the result constant.

Voltage is deliberately changed so it is the independent variable. In exams identify the changed electrical quantity.