New experimental evidence may lead to a scientific model being changed or…

The idea

A scientific model is the best current explanation of the evidence, not a permanent truth. When an experiment produces a result the existing model cannot account for, the model has to be adapted or thrown out and rebuilt. Alpha particle scattering is the classic case: the plum pudding model could not explain why some alpha particles bounced back, so the nuclear model replaced it. A replacement model must still explain everything the old one explained, plus the new results. Before it is accepted, other scientists repeat the work and peer review it to check the observations are right. Models are not abandoned because they are old or because they are simple.

What earns the marks

• Stating that new evidence which the existing model cannot explain leads to it being changed or replaced.
• Stating that the new model must explain the earlier observations as well as the new ones.
• Giving an example, such as alpha scattering replacing the plum pudding model.
• Stating that results are peer reviewed or repeated by other scientists before being accepted.

Common mistakes and how to put them right

• Saying models are replaced because they are old fashioned or too simple.

• Claiming scientists replace models on a fixed timetable.

• Treating a single unexpected result as immediate proof that the old model is wrong.

• Saying models change because the public does not understand them.

Exam tips

• Use the mark scheme wording, new evidence that the existing model cannot explain, in multiple-choice and short answers alike.
• Have one worked example ready, the alpha scattering experiment, to support a general statement.
• Mention peer review when a question asks why results are checked before a model changes.