The pH scale measures how acidic or alkaline a solution is. At GCSE it runs from 0 to 14.
A neutral solution has a pH of 7. Pure water is neutral. Acidic solutions have a pH lower than 7, and the lower the number, the more acidic the solution. Alkaline solutions have a pH higher than 7, and the higher the number, the more alkaline the solution.
- Dilute hydrochloric acid: about pH 1
- Pure water: pH 7
- Dilute sodium hydroxide solution: about pH 13
Universal indicator gives an approximate pH: it goes red or orange in strong acids, green when neutral, and blue or purple in strong alkalis. A pH meter or probe gives a more accurate number.
