A fair comparison changes only one thing, measures one thing, and keeps everything else the same, so any difference in the result has one cause. A good plan states all three and says what the data will show.
This final lesson in measurement and chemical investigation combines reading apparatus, variables and observations versus inferences.
What is the five-part frame?
Use five short parts, in this order.
- Aim: one sentence starting “To compare…” or “To investigate…”.
- Variables: manipulated, responding and the fixed ones that matter.
- Method idea: three or four steps, with the key quantities.
- Data: what you record and how many repeats.
- Conclusion: what a higher or lower value would mean.
Worked example: comparing two fuels
Question. Plan an experiment to compare the heat released by burning ethanol and propanol. [6 marks]
Aim. To compare the heat energy released when 1 g of ethanol and 1 g of propanol burn.
Variables.
- Manipulated: the type of alcohol (ethanol or propanol).
- Responding: the rise in temperature of the water.
- Fixed: volume of water (100 mL), distance between flame and container, mass of alcohol burnt (1 g), same container.
Method idea. Put 100 mL of water in a metal can and record its temperature. Burn the alcohol in a spirit burner under the can until 1 g has been used, stirring the water.
Record the highest temperature. Repeat with the other alcohol and with fresh water.
Data. Record the initial and final temperatures, and work out the rise. Repeat each alcohol twice and take the average.
Conclusion. The alcohol that gives the larger average temperature rise releases more heat per gram.
Why a list of apparatus fails
A list of apparatus shows you know what is in the lab. It does not show that you know what the test is for.
Marks go to the variables, the control of the fixed ones, and the link between the result and the conclusion. If you list apparatus, do it in one line inside the method, not as the whole answer.
A quick self-audit
Before you hand in a plan, tick four things.
- Did you change only one variable?
- Did you name what you will measure?
- Are the fixed variables the ones that could change the result?
- Does your conclusion match the aim?
This audit takes thirty seconds and catches the marks students lose most easily. The thermochemistry topic applies this kind of plan to fuels and energy.
Check yourself
Plan, in five short lines, a fair test to compare whether powdered or lump calcium carbonate reacts faster with 1.0 mol dm⁻³ hydrochloric acid.
Answer
Aim: to compare the rate of reaction of powdered and lump calcium carbonate.
Variables: manipulated is the size of calcium carbonate, responding is the volume of gas in a set time (or the time for a set volume), fixed are the mass of calcium carbonate, the volume and concentration of acid, and the temperature.
Method: react 5 g of each form with 50 mL of acid and collect the gas in a syringe every 30 seconds.
Data: record the volume at each time, and repeat once.
Conclusion: the form giving more gas in the same time reacts faster.
What to study next
Practise the whole chapter with the measurement and chemical investigation practice set.
For a teacher to go through your plans with you, see online one-to-one Chemistry tuition.