OCR A-Level Chemistry

## Planning an experiment A plan starts from what has to be measured and works back to the apparatus. If the aim is the enthalpy change of a reaction in solution, the quantity needed is a temperature change, so the plan needs a thermometer or probe, an insulated container and a way of measuring the mass or volume of reactant. If the aim is a rate, the plan needs a quantity that changes steadily with time, such as the volume of gas collected, the mass lost or the intensity of a colour, and a way of timing it. The variables come first. - The independent variable is the one you change on purpose. - The dependent variable is the one you measure. - Control variables are everything else that could affect the result, and each one is held constant: temperature, volumes, concentrations, the mass or surface area of a solid, stirring and the time allowed. A control experiment, which leaves out the thing being tested, shows that the effect comes from the independent variable and not from something else in the set-up. A good plan gives enough detail for someone else to repeat it. That means stating the apparatus with its sizes (a 25.0 cm³ pipette, a 50 cm³ burette, a 250 cm³ volumetric flask), the quantities used, the order of steps, how often each reading is taken and how results will be processed. The range should be wide enough to show a trend and the intervals even, so five concentrations such as 0.20, 0.40, 0.60, 0.80 and 1.00 mol dm⁻³ are more useful than two. Each measurement is…