The stength of an acid or a base is measured with the help of the universal indicator. It an be in the form of a paper or solution. When it is dipped in an acid, base or any such substance, its colour changes from red to green or green to blue. There always is a chart attached to it, giving the values of each colour change. Acids usualy rank from 1 to 6(<7) and bases from 8 to 14(>7), while neutral substances are always on number 7.