Plan and interpret controlled evaporation measurements
Calculate net mass-loss rates from supplied data and distinguish a fair test from practical competence.
Section 2.2.3 · Melting, boiling and evaporation
Extended only (Supplement); Physics 0625, 2026–2028, section 2.2.3. Compare boiling with evaporation, explain temperature/surface-area/air-movement factors and cooling of a contacted object. Supplied data and written plans are self-checked practice
Before you begin
- Explain temperature, area and airflow effects.
- Calculate an amount change per elapsed time with consistent units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Liquid mass falls from 100 g to 94 g in 20 min. What is the average net loss rate?
What you will learn
- Plan a one-factor comparison with suitable controls and repeats.
- Calculate average net mass-loss rate and evaluate what a comparison establishes.
Measure a net change over a stated time
For a supplied trial, average net liquid-mass loss rate is (initial mass − final mass)/elapsed time. A liquid decreases from 100 g to 94 g in 20 min, giving 6/20 = 0.30 g/min. Use the same timing and comparable samples when comparing totals; a longer trial losing more mass does not automatically have a greater rate.
Weigh the same container/sample arrangement before and after so unchanged container mass cancels, while liquid spillage or another changing mass would invalidate a pure evaporation inference. Record conditions and distinguish the measured net mass loss from every microscopic escape event. The displayed table is original supplied teaching data, not newly recorded laboratory measurements.
Isolate a factor and evaluate evidence
To compare airflow, use the same liquid, exposed area, initial amount, starting temperature, trial duration and comparable humidity; change only the specified air movement. Monitor relevant conditions because evaporation can also cool the liquid during a trial. Repeat appropriate teacher-supervised trials, compare mean rates and consider spread before making a strong conclusion.
Suitable observations can support a difference for those conditions without proving a universal multiplier. If trials differ in exposed area or temperatures drift differently, attribution is weaker. A written method and correct division practise analysis and planning; they do not establish hands-on practical competence or complete Paper 5 or Paper 6 preparation by themselves.
Pause and explain
A loses 6 g in 20 min and B loses 3 g in 10 min. Which rate comparison is correct?
Worked example
Two supplied matched-airflow trials start at 100 g. After 20 min, still air leaves 94 g and moving air 90 g. Find average net rates.
Show the worked solution
- Still air loses 6 g: 6/20 = 0.30 g/min.
- Moving air loses 10 g: 10/20 = 0.50 g/min.
- The moving-air rate is greater for these supplied matched trials, not a universal factor.
Answer Still air 0.30 g/min; moving air 0.50 g/min.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate the average rates for the supplied 100-to-94 g and 100-to-90 g trials, each lasting 20 min.
Give me a hint
Find loss before dividing by the shared duration.
Compare my reasoning
- The losses are 6 g and 10 g.
- Rates are 6/20 = 0.30 and 10/20 = 0.50 g/min.
- The latter is greater for these stated trials.
0.30 g/min and 0.50 g/min.
Look for these in your work
- Use lost mass.
- Attach rate units and limit the conclusion.
A loses 8 g in 40 min; B loses 5 g in 20 min. Compare average net rates.
Give me a hint
Different durations require separate division.
Compare my reasoning
- A: 8/40 = 0.20 g/min.
- B: 5/20 = 0.25 g/min.
- B has the greater rate despite a smaller total loss.
A 0.20 g/min; B 0.25 g/min, so B is greater.
Look for these in your work
- Normalise by each duration.
- Do not rank by total loss alone.
An airflow trial also changes dish area and has no repeats. Propose improvements and state what written planning cannot demonstrate.
Give me a hint
Isolate airflow and obtain evidence of variability.
Compare my reasoning
- Keep liquid, area, initial amount, temperature, duration and humidity suitably comparable.
- Change airflow alone, monitor conditions and repeat suitable trials to examine spread.
- Written planning does not demonstrate supervised apparatus competence or complete the practical route.
Control other factors, repeat and monitor; practical competence still needs supervised evidence.
Look for these in your work
- Identify meaningful controls.
- Retain evidence and competence limits.
Common mistakes
- Compare rates, not losses over unequal times.
- Spillage is not evaporation evidence.
- One supplied comparison is not a universal law or practical certification.
What can you explain now?
A trial loses 3 g in 10 min. What is its average net rate?
Compare with the explanation
0.30 g/min.
Divide the net loss by elapsed time, retaining a mass-per-time unit.
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Make it stick
Calculate net loss per time and explain controls, repeats, sample boundaries and the limits of supplied data.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.