A green chemistry question asks you to judge a process against stated criteria. Your job is to use those criteria and the data given, not to recite general opinions.
This lesson is part of SPM Chemistry and follows separating product claims from evidence. Check your textbook for the wording your school uses for green chemistry ideas.
How do you approach a criteria question?
Use four steps.
- List the criteria the question supplies.
- For each criterion, compare the two processes using the data given.
- State which process is better on each criterion, and by how much if numbers allow.
- Give an overall judgement and name any trade-off.
The criteria table is the plan of your answer.
Worked example: two ways to make a cleaning liquid
Here is an original case. Process X uses an organic solvent, runs in one step at 80 °C and produces 3 kg of waste for every 10 kg of product. Process Y uses water as the solvent, runs in two steps at 40 °C and produces 1 kg of waste for every 10 kg of product.
The given criteria are waste, hazard of the solvent and energy use.
| Criterion | Process X | Process Y | Better on this criterion |
|---|---|---|---|
| Waste per 10 kg product | 3 kg | 1 kg | Y |
| Solvent | Organic solvent | Water | Y, because water is a safer solvent than organic solvents |
| Temperature | 80 °C | 40 °C | Y, because the lower temperature needs less heat |
Waste falls from 3 kg to 1 kg, which is a reduction of 2 kg for each 10 kg of product.
The trade-off is that Y needs two steps. The data do not say whether two steps cost more time, so a good answer states this as a limit.
The mistake that loses marks
The one-sided answer says: “Y is greener because water is natural and X is dangerous.” It uses the idea of natural, and it makes an unsupported claim about danger.
The balanced answer says: “Process Y is greener on all three criteria. It gives 1 kg of waste against 3 kg, uses water instead of an organic solvent, and runs at 40 °C instead of 80 °C. It needs two steps, so its time and cost cannot be judged from the data.”
Check yourself
Process P gives 5 kg of waste per 10 kg of product at 60 °C. Process Q gives 2 kg of waste per 10 kg at 90 °C. The criteria are waste and energy use. Which process is greener?
Answer
It depends on the weight given to each criterion, so the answer must show the trade-off.
Q is better on waste (2 kg against 5 kg), and P is better on energy, since 60 °C needs less heating than 90 °C. If waste is considered more important, Q is greener. A good answer says this choice openly and does not claim one process wins on every criterion.
What to study next
Test the method in the consumer and industrial chemistry practice set. The fairness checking from the claims lesson also applies to comparisons like these.
For a teacher to mark a full discussion answer with you, see online one-to-one Chemistry tuition.