The effect of fluoride on teeth can be modelled in a practical using eggshells and toothpaste. Apply the toothpaste to an egg, place it in a small plastic bag or cover with plastic film and leave for several days, then rinse off and place into vinegar. Keep a control egg in another plastic bag but without the toothpaste. After a few hours in the vinegar, the untreated egg shell will have dissolved but the toothpaste-treated shell will not. Students could be asked to design their own experiment to investigate whether different types of toothpaste work equally well, or whether a sodium fluoride solution gives the same effect.
*Original article online at https://eic.rsc.org/feature/why-you-need-a-level-chemistry-to-study-dentistry/3010835.article