The pH scale is used by scientists to measure how strong or weak an acid or alkali is. The scale runs from pH0 to pH14 with the strong acids having the lowest numbers on the scale and the strong alkalis having the highest numbers. pH7 is in the middle and is called neutral i.e. it’s neither acid nor alkali. Strong acids and alkalis can be very corrosive and dangerous. Acids and alkalis react in a process called neutralisation to give the products of a salt and water. This reaction reduces the corrosive nature of acids and alkalis, which is why vinegar is put on a bee sting. Acids also undergo neutralisation reactions with bases, which are solid. Alkalis are bases that dissolve in water. The process of neutralisation is used by some cleaning products including toothpaste.