Appreciate that results of research by different groups in the scientific…

The idea

This statement is about how science becomes policy, so the credit is for the process, not the chemistry. A laboratory prediction in the 1970s that chlorine radicals could catalyse ozone loss was only a hypothesis. It gained weight when independent teams measured the effect in the real atmosphere: ground-based measurements over Antarctica showed a seasonal collapse in ozone, satellite data confirmed the same hole, and laboratory work reproduced the radical chemistry. Because separate groups using different methods agreed, and the work was published and peer reviewed, governments treated the conclusion as reliable and signed an international agreement restricting chlorofluorocarbons as refrigerants and solvents. Replication by independent groups, not the authority of any one scientist, is what turned evidence into legislation.

What earns the marks

• Saying the evidence came from more than one independent research group.
• Naming a check such as peer review, publication or reproducing the result.
• Naming the kind of evidence, such as measurements of falling ozone concentration over Antarctica.
• Linking the accepted evidence to legislation banning chlorofluorocarbons as solvents and refrigerants.

Common mistakes and how to put them right

• Answering with the ozone depletion mechanism when the question asks how the evidence came to be accepted.

• Claiming one scientist's result was enough to change the law.

• Saying chlorofluorocarbons were banned because they are toxic or flammable, when they are neither.

• Describing the ban as based on theory alone, ignoring the atmospheric measurements.

Exam tips

• Use the words independent, reproduced and peer reviewed; these are the terms the mark scheme looks for.
• Give one concrete example of evidence rather than a general claim that scientists did research.
• Finish by naming the outcome, the international restriction on chlorofluorocarbon use.