Red cabbage extract will change colour depending on acidity and act as an indicator of a potential chemical change. Acidity is measured on a pH scale where pH stands for “potential for Hydrogen”. Acidity is caused by hydrogen atoms that have lost their electrons and are roaming free in water [H+]. The scale goes from 0 to 14. The colour scale represents the concentration of these positively charged hydrogen ions [H+]. Acid, or low pH, gives foods a sour taste, and base or high pH is usually bitter.