Experimental and investigative skills
Experimental and investigative skills tests planning, variables, measurement, data processing and evaluation.
Planning and variables
The independent variable is the one changed; the dependent variable is measured; control variables are kept constant to make the test fair. A hypothesis is a testable prediction. Repeat measurements and use a control experiment where needed. Choose apparatus that matches the required precision: a burette for titration, a measuring cylinder for approximate volumes, a thermometer with a suitable range, and a balance reading to 0.01 g or better.
Recording and processing data
Record raw data in a table with units and consistent decimal places. Plot the dependent variable on the y-axis and the independent variable on the x-axis. Draw a line of best fit and identify anomalous points; do not simply join every dot. Calculate a mean from concordant titres, ignoring rough or anomalous values. Use the gradient of a tangent to find a rate from a curve.
Errors and evaluation
Random errors scatter results; systematic errors shift all results in one direction. Accuracy is closeness to the true value; precision is closeness of repeated values to each other. Evaluate by discussing the main source of error, its effect on the result, and a realistic improvement.
Worked example
Three titres are 24.10, 24.20 and 24.15 cm3. The mean is (24.10 + 24.20 + 24.15) ÷ 3 = 24.15 cm3. If a fourth titre were 25.50 cm3, it would be anomalous and excluded. For a 25.0 cm3 pipette of alkali titrated by 0.100 mol dm−3 acid, n(acid) = 0.100 × 0.02415 = 2.415 × 10−3 mol; the equation ratio then gives the alkali concentration.
Traps
Do not include an anomalous titre in a mean without comment. Read the bottom of the meniscus at eye level to avoid parallax. Do not quote more significant figures than the apparatus justifies. A conclusion must refer to the hypothesis and the evidence, not just repeat the results.