Practical Skills, Experimental Methods and Safety
This domain tests how you plan, carry out and evaluate experiments. You must choose apparatus, control variables, record reliable data and recognise hazards. Questions often ask you to improve a method rather than repeat it.
Measurement and apparatus
- Use a measuring cylinder for approximate volumes, a pipette for a fixed accurate volume and a burette for variable accurate volumes in titration.
- Read the bottom of the meniscus at eye level to avoid parallax error.
- Use a balance reading to two decimal places for masses and a thermometer with a suitable range and small scale divisions for temperature.
Fair tests and data
Identify the independent variable (what you change), the dependent variable (what you measure) and the control variables (what you keep the same). Repeat readings and calculate a mean, ignoring anomalies. Accuracy is how close a value is to the true value; precision is how close repeated readings are to each other.
Safety
Read hazard symbols and risk phrases. Wear goggles when heating or using acids. Keep flammable liquids away from flames. Heat test tubes at an angle and point them away from people. Never taste chemicals or return unused chemicals to stock bottles.
Worked example
A student measures the rate of reaction between marble chips and hydrochloric acid by collecting gas in a syringe. Describe the variables and one improvement. The independent variable is time; the dependent variable is the volume of gas collected. Control variables include the mass and size of marble chips, acid concentration and volume, and temperature. An improvement is to use a gas syringe with a tight seal rather than a delivery tube in a trough, because it reduces gas loss and gives more precise volume readings. Repeat the experiment and plot volume against time; the initial gradient gives the rate.
Traps that cost marks
- Treating accuracy and precision as the same thing.
- Changing more than one variable.
- Ignoring anomalous results instead of repeating them.
- Choosing a measuring cylinder when a burette is needed for accurate titration.