Applies to: AQA, OCR Gateway, Edexcel (combined and separate), Edexcel International GCSE. Cambridge IGCSE doesn't name this as a fixed practical, but pH and enzyme-catalysed reactions sit squarely within their listed experimental contexts, so this is still directly useful revision.

Chew a plain cracker or a piece of bread for a full minute without swallowing. It starts tasting sweeter. That's not your imagination. Your saliva contains an enzyme called amylase, and it's breaking the starch in the bread down into sugar while it's still in your mouth. You can turn that everyday moment into a proper required practical.

What it tests: How pH affects the rate at which amylase breaks down starch.

What you need: A small amount of cornflour, water, a clean container, iodine solution (from a pharmacy), a teaspoon, and your own saliva.

How to do it: Mix a teaspoon of cornflour with warm water to make a thin, runny paste. This is your starch solution. Spit into a clean container to collect a small amount of saliva (yes, really) and stir it into the starch paste. Every thirty seconds, take a tiny sample out and add a drop of iodine to it. While starch is still present, the iodine turns blue black. Once the amylase has broken it all down, the iodine stays its usual orange brown colour. Time how long that takes. Now repeat the whole thing, but stir in a few drops of lemon juice or vinegar to the starch and saliva mixture first, to lower the pH. Time it again and compare.

Safety note: Use only your own saliva, don't share cups or spoons with anyone else, wash your hands before and after, and don't taste or drink any of the mixture once iodine has been added.

Why it matters for the exam: Enzyme questions almost always come back to optimum pH and what happens outside it, when an enzyme's shape changes and it stops working properly, which is called denaturing. Having actually timed the difference between neutral and acidic conditions makes that idea concrete rather than something you're trying to memorise cold.

On motivation: This is not a glamorous experiment. Spitting into a cup and timing a colour change is a bit ridiculous, and that's fine. Real science is unglamorous more often than textbooks let on. You don't need clean, tidy conditions to understand something properly. You need to actually watch it happen once.