Exploring and designing practice questions.

In an experiment testing how acid concentration affects reaction rate, which quantity should be kept constant to make it a fair test?

• The temperature of the reaction mixture
• The concentration of the acid used
• The time taken for the cross to disappear
• The volume of gas that is produced

Answer: The temperature of the reaction mixture

Concentration is the independent variable and the time (or gas volume) is the dependent variable, so neither is controlled. Temperature must be held constant so it does not confound the results.

A student plans to investigate how concentration affects reaction rate. Which choice of concentrations makes the best experimental design?

• A range of five evenly spaced concentrations
• Two concentrations that are very close together
• One concentration repeated five times over
• Five concentrations all above the solubility limit

Answer: A range of five evenly spaced concentrations

A spread of several evenly spaced concentrations reveals the trend and lets a graph be drawn. Two close values or a single value cannot show a relationship, and concentrations above the solubility limit cannot be prepared.

A student adds magnesium ribbon to hydrochloric acid of five different concentrations and records the volume of hydrogen collected in 30 s each time. Identify the dependent variable.

• The concentration of the hydrochloric acid
• The volume of hydrogen collected in 30 s
• The length of the magnesium ribbon used
• The fixed time of 30 s over which gas is collected

Answer: The volume of hydrogen collected in 30 s

The dependent variable is the one measured as an outcome, and here that is the gas volume; the concentration is set by the experimenter, so it is independent. The ribbon length and the collection time are held constant, which makes them control variables.

Identify the best research question for an internal assessment investigation.

• What happens to the reaction between an acid and a metal when the conditions in the flask are changed?
• How does the concentration of an acid affect the rate at which it reacts with a range of different metals under laboratory conditions?
• How does the concentration of ethanoic acid, from 0.20 to 1.00 mol dm⁻³, affect the initial rate of its reaction with magnesium?
• Is a more concentrated acid better than a dilute one when it is reacted with a piece of metal?

Answer: How does the concentration of ethanoic acid, from 0.20 to 1.00 mol dm⁻³, affect the initial rate of its reaction with magnesium?

A workable research question names the independent variable with its range, the dependent variable and the specific system, so the reader knows exactly what will be measured. The others leave the variables, the range or the meaning of a word like 'better' undefined, so no single experiment could answer them.

Identify the purpose of a control experiment in which the enzyme is left out of an otherwise identical reaction mixture.

• To use up the remaining reagents so that none of the prepared solutions is wasted
• To provide an extra repeat measurement, so that the mean of the results is more reliable
• To check that the whole apparatus has been assembled correctly before the real trials begin
• To show that any change observed is caused by the enzyme and not by the rest of the mixture

Answer: To show that any change observed is caused by the enzyme and not by the rest of the mixture

A control isolates the factor under investigation by removing it while holding everything else constant, so any change in the control must come from something other than that factor. It is not a repeat, because the conditions are deliberately different in exactly one way.

Identify the main purpose of a preliminary experiment carried out before the final data are collected.

• To find a range of the independent variable that gives measurable changes in a workable time
• To collect the first set of results, which are then averaged with the final data
• To confirm that the hypothesis is correct before the real investigation is started
• To let all of the apparatus reach a steady temperature before the final measurements begin properly

Answer: To find a range of the independent variable that gives measurable changes in a workable time

A preliminary run answers practical questions: whether the reaction is over in two seconds or two hours, and whether the chosen range produces a visible difference at all. Its results are not part of the final data set, because the conditions were still being adjusted.