The pH scale is used to measure how acidic or basic a substance is. The scale goes from 0 to 14. A substance with a pH of 0 through 6 is considered an acid, while a substance with a pH of 8 through 14 is considered a base. A pH of 7 is neutral. Uncontaminated, pure water is neutral. The closer the number is to 7, the weaker the substance is. The farther the number is from 7, the stronger the substance is.
A substance with a pH of 1 or 2 would be a strong acid. A substance with a pH of 5 or 6 would be a weak acid.
A substance with a pH of 13 or 15 would be a strong base. A substance with a pH of 8 or 9 would be a weak base.
An example of a strong acid is vinegar. The pH of vinegar is 2.4. An example of a weak acid is citric acid. The pH of citric acid is 4.8.