She moved over to the chalkboard and began writing out a list, "We describe whether things are acidic, basic or neutral by using a scale called the pH scale. The pH scale ranges from zero to 14. A substance with a pH of: 0 is a very strong acid; 3 – 5 is a weak acid; 7 is neutral; 8 – 9 is a weak base; 13 – 14 is a very strong base. For example, pure water has a pH of seven and is regarded as neutral."