To compare a claim with its evidence, recalculate the claim from the data, say whether the numbers support it, and then state what the data cannot show. This is the evaluation skill behind many IGCSE Chemistry environmental questions. All data in the lesson is invented for teaching.
It draws on reading pollutant data and tracing carbon through a process. The full set of skills is at water, air and environmental chemistry.
What makes a good evaluation?
A full answer has four parts.
- Restate the claim exactly. Note any number, such as “50%”.
- Check the numbers by doing the calculation yourself.
- Judge: fully supported, partly supported or not supported, and why.
- State limits: sample size, repeats, conditions, missing measurements, and who produced the claim.
Judging a claim is not the same as judging a person. You comment on the data, not on whether someone is honest.
Worked example
Invented scenario: A company says its catalytic converter “cuts carbon monoxide emissions by 50%”. In one test on one car, the carbon monoxide emission was 4.0 g/km without the converter and 2.2 g/km with it.
Step 1, restate. The claim is a 50% cut.
Step 2, calculate. Decrease = 4.0 − 2.2 = 1.8 g/km. Percentage decrease = 1.8 ÷ 4.0 × 100 = 45%.
Step 3, judge. The data shows a large cut, but 45% is lower than the 50% claimed. The claim is not fully supported by this test, though the direction of the effect is supported.
Step 4, limits.
- Only one car and one test were used, so the result may not apply to other cars.
- There are no repeat readings to show how much the result varies.
- The test did not measure other pollutants or what happens as the converter ages.
Model conclusion: “The data shows a 45% decrease in CO, not 50%, so the claim is not fully supported. The data comes from one car in one test, so a stronger conclusion needs repeats on more vehicles.”
For the chemistry behind the claim: a converter helps change carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen, 2CO + 2NO → 2CO₂ + N₂. Check the balance: carbon 2 and 2, oxygen 2 + 2 = 4 and 4, nitrogen 2 and 2. Note that this still forms carbon dioxide.
The mistake to watch for
Mistaken answer: “The claim is true because the emission went down.”
A fall is not the same as the size of fall that was claimed. The answer also ignores the limits of a single test.
The correction is to compare the calculated figure with the claimed figure, using the same measure. A second common slip is judging the claim by how convincing it sounds instead of by the numbers. A third is saying the catalytic converter “removes pollution”, when it converts some pollutants and still produces carbon dioxide.
Check yourself
1. A fuel additive advertisement claims it cuts smoke particles by 30%. A test gives 5.0 mg/m³ without it and 3.0 mg/m³ with it (invented). Is the claim supported?
Show answer
Decrease = 2.0. Percentage = 2.0 ÷ 5.0 × 100 = 40%. The decrease is larger than the 30% claimed, so the claim is supported (the additive did at least as well), although one test is a limited basis.
2. Give two limits of evidence from a test on one sample taken on one day.
Show answer
Any two of: the result may not apply to other days, places or samples; there are no repeats to show variation; other factors were not measured, so a cause cannot be confirmed.
3. Balance: CO + NO → CO₂ + N₂
Show answer
2CO + 2NO → 2CO₂ + N₂. Check: carbon 2 and 2, oxygen 4 and 4, nitrogen 2 and 2.
Where this leads next
The module practice set mixes all five skills in one place, including a calculation and an evaluation. To keep track of errors you repeat, use the mistake log and retest queue. Come back to explaining separation steps in water treatment if particle reasoning needs another look.
Evaluation answers improve fastest when someone reads them back to you. Our teachers do this in online one-to-one Chemistry tuition, pointing to the exact phrase that needs more evidence.