The acidity or alkalinity of a substance is measured using a quantity known as pH. Technically, the pH of a substance is a measure of hydrogen ion concentration within a solution. Despite the microscopic definition of pH, it can be measured using macroscopic items such as pH paper.
The pH scale varies from 0 to 15 with low numbers representing acidity and high numbers representing alkalinity. When pH paper is dipped into a solution it turns a certain color depending upon the acidity or alkalinity. Water has a neutral pH of around 7 and turns pH paper green. Acidic solutions turn pH paper red and alkaline solutions lead to a purple color.