What is the effect of acid on teeth, and how does toothpaste help prevent cavities?
Put a few drops of food colouring into a little water and ask a pupil to swill it around their mouth and then spit it into the sink. The colouring will highlight areas of plaque on their teeth. Ask the class to note where the plaque is. Then ask the pupil to clean their teeth using a new toothbrush and toothpaste, and repeat the experiment. You should find that there is less plaque on the teeth.
Prepare the following experiment beforehand. Take three hard-boiled eggs, still in their shells, which will take the place of teeth, as it is not advisable to experiment on teeth in this way. Place one in a cup of fluoride mouthwash and another in a cup of toothpaste, and leave the third as it is. The egg in toothpaste needs longer than the other two and will benefit from being left there for a couple of days. The egg in the mouthwash needs only a few minutes.
Put vinegar into three cups. Remove the eggs from the mouthwash and the toothpaste. Rinse and dry all three eggs and add one to each cup of vinegar. Label the cups. Place a teaspoon over each egg to hold it under the vinegar.
Observe how bubbles form on the surface of the untreated egg as the acid attacks the eggshell. The acid will start to attack the surface of the other two eggs, but this will take longer - seven to 10 hours.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn