Variables in an experiment

The initial volume of water should be the same in all containers. This allows a proper comparison of surface area.

The direct answer is D: an additional influential variable. In a surface-area experiment, evaporation may be the measured result, while surface area is usually the factor being tested. Air movement from a fan can also change evaporation: faster moving air may remove water vapour more quickly. If only one container is near the fan, air speed is not part of the intended test and is not kept equal. Therefore it is an additional factor that influences the result, often called a confounding variable. A is not the air speed itself; the dependent variable is what is measured. B would be the deliberately changed factor. C is the recorded result, not a cause. Memory cue: an accidental influence is an extra variable.

The independent variable is the factor deliberately changed by the experimenter. In this investigation, the stirring speed is changed, and its effect on the time taken by salt to dissolve or on the rate of dissolving is observed. Therefore, stirring speed is the independent variable. Factors such as the amount of salt and water temperature should be kept constant because they can also affect the result. Exam tip: the factor that is changed is the independent variable.

The dissolving time is measured to see how it changes when the stirring speed is changed. Therefore, it is the dependent variable because its value depends on the independent variable, stirring speed. Controlled variables are conditions kept the same, such as the amount or temperature of water. Exam tip: The result that is measured or observed is usually the dependent variable.

Stirring speed is the independent variable because it is deliberately changed. The time taken to dissolve is the dependent variable because it is measured. Water temperature also affects the rate of dissolving, so it must be kept constant in every trial for a fair test. Exam tip: Change only the factor being investigated and control other factors that can affect the result.

The amount of salt can affect outcomes such as dissolving time or the concentration of a solution. If it changes in every trial, it becomes impossible to tell whether a change in the result was caused by the variable being tested or by the salt amount. Therefore, the amount of salt should be kept constant as a control variable. Exam tip: In a fair test, keep all factors that can affect the result constant except the variable being investigated.

Environmental temperature is the factor deliberately changed in this experiment, so it is the independent variable. The time taken for the ice to melt, or the amount of ice melted, may be measured as the dependent variable. Factors such as the size and initial mass of the ice piece should be kept constant. Exam tip: the variable intentionally changed by the experimenter is the independent variable.

The time taken for ice to melt is the outcome measured in the experiment. It may change with temperature, ice-cube size, or other changed conditions, so it is the dependent variable. An independent variable is deliberately changed by the experimenter, such as the surrounding temperature. Exam tip: a variable that is measured or observed as the result is usually the dependent variable.

The direct answer is C: the initial size of the ice pieces. A fair comparison changes only the factor being tested, such as environmental temperature, while starting with equivalent ice pieces. If one piece is larger, it contains more ice and may naturally take longer to melt, so size could explain the result. Thus initial size is a controlled variable. A may be the independent variable if temperature is being tested, so it should change rather than remain the same. B, melting time, is normally the dependent variable being measured. D is a conclusion, not an experimental variable. Memory cue: keep starting materials alike before comparing results.

Ice size can affect melting time. This prevents the effect of temperature from being isolated.

The independent variable is the factor deliberately changed by the experimenter. In this investigation, sound intensity is varied, while plant growth, such as increase in height, is measured. Therefore, sound intensity is the independent variable. Factors such as the amount of water and plant species should be kept the same as controlled variables. Exam tip: the factor that is changed is the independent variable.

Average plant height gain is the outcome measured in the experiment. Its value may change in response to the independent variable, so it is the dependent variable. The independent variable is the factor deliberately changed, such as the amount of water or light. Exam tip: the result that is measured or observed is usually the dependent variable.

The direct answer is C: duration of light. When testing how sound affects plant growth, sound intensity is the intended independent variable, and height gain is a possible dependent result. Light also affects photosynthesis and growth, so every group should receive the same amount and duration of light. Otherwise a growth difference might be due to different lighting rather than sound. A is wrong because sound intensity is the factor being tested. B is the measured growth outcome, not a control. D is a calculated result, not a condition controlled during the experiment. The question says duration, and equal light duration helps make the comparison fair. Exam cue: control other growth factors.

The amount of fertiliser can affect plant growth. If plants exposed to louder sound also receive more fertiliser, any change in growth could be due to sound or to fertiliser. Therefore, fertiliser amount is a confounding variable, not a controlled variable; a controlled variable must be kept the same in all groups. Exam tip: Identify a confounding variable as an extra factor that affects the result and changes along with the independent variable.

The independent variable is changed the dependent variable is measured and the controlled variable is kept the same. Identify them by role.

The governing concept is the distinction among independent, dependent and controlled variables. The independent variable is deliberately changed by the investigator; the dependent variable is measured as the response; controlled variables are kept constant. In option B, the amount of fertiliser is intentionally changed between plant groups, so it is the independent variable. Measured time or height would generally be outcomes and therefore dependent variables, not independent or controlled ones. Same soil could be a controlled condition if kept constant, but it is not a dependent variable. Thus B is the only correctly classified statement.

The answer is C because it gives an incorrect classification. A changed temperature is an independent variable: it is the factor deliberately varied. B measured time is a dependent variable: it is the observed result. C says that the same amount of water is dependent, but a factor kept the same is a controlled variable, not a measured outcome; this is the error. D correctly classifies the same container as a controlled variable, because its condition is held constant to make comparisons fair. A, B, and D therefore use the roles correctly. A helpful rule is that ‘same’ usually signals controlled, ‘measured’ signals dependent, and ‘changed’ signals independent. Do not confuse what is held fixed with what is recorded as the result.

The result may change because of natural variation or an uncontrolled factor. Check the procedure and all controlled variables.

Height gain and the seven day period provide a clear measurement method. Replace vague terms with measurable outcomes.

The direct answer is B. Different levels of an independent variable are used to discover how changing that factor affects the dependent variable. For example, if light intensity is the independent variable, plants may receive low, medium, and high light, while water and soil remain the same. Their growth can then be measured and compared. Option A is wrong because controlled variables should not be removed; they should be kept constant. Option B is correct because comparison across levels reveals a possible relationship between the changed factor and the measured outcome. Option C is wrong because an experiment does not aim to force identical results; differences may be the evidence. Option D is wrong because measurement is essential for observing the effect. Memory cue: change one factor, measure the response, keep other important factors constant.

The governing concept is that an independent variable can be tested at more than one level. In this investigation, the student deliberately changes the amount or intensity of light, so light is the independent variable. The three light levels are three different values or conditions of that same variable, such as low, medium and high intensity. Plant growth is the response measured, making it the dependent variable. Therefore option C is correct. The levels are not automatically three control groups, because a control group normally provides a comparison condition, and they are certainly not conclusions, which are formed only after analysing results.

A dependent variable is the result that is observed or measured in an experiment. For the result to be compared fairly, it should be described using a clear and repeatable measurement. Words such as “very good” and “poor” do not have a fixed meaning. Different people may use them for different levels of performance, so the recorded result may vary even when the actual observation is similar.

The main problem is therefore that the measurement becomes subjective and vague, which is option D. A better record might use a count, time, height, distance, score or another stated scale, depending on the experiment. The independent variable does not disappear, and controlled variables do not increase merely because the description is unclear. Clear numerical or defined categories make results easier to compare.

In the first experiment the duration of light exposure is changed. Therefore time is used as the independent variable.

The governing concept is determined by the role of a quantity in the investigation, not merely by whether it is recorded. Temperature is deliberately changed by the investigator, so it is the independent variable. Time is then measured as the response or outcome, so it is the dependent variable. Option C correctly identifies these different roles. Recording a value does not automatically make it dependent; the experimental purpose and procedure decide its classification. Both cannot be independent if only temperature is manipulated, and temperature cannot be called controlled when it is intentionally varied. Therefore C is the unambiguous answer.

A change in a controlled variable makes the trial unfair. Record it and repeat the trial under correct conditions.