Acids and bases are two classes of substances with contrasting properties. Vinegar contains acetic acid and lemons contain citric acid, while baking soda (sodium hydrogen carbonate) and soap are bases. The pH scale, from 0 to 14, measures how acidic or basic a watery liquid is: 7 is neutral, below 7 is acidic, above 7 is basic, and each step is a ten-fold change. An acid-base indicator changes color with pH. The dye from red cabbage is red in acid, purple when neutral, and green to yellow in base, so it can sort the liquids on the Bench.