An acid is a substance which has a high concentration of H+ ions. Acids have a pH of between 1 and 7. A base is a substance with a high concentration of OH- ions. Bases have a pH of between 8 and 14. The pH scale is defined by Wikipedia as the negative decimal logarithm of hydrogen ion activity in a solution; in laymen’s terms, a substance with less hydrogen ions (H+) and more hydroxide ions (OH-) is said to be basic, or alkaline. A substance with more hydrogen ions and less hydroxide ions is said to acidic. Water, an almost neutral substance, has a pH of approximately 7. When an acid and a base are combined, the main resultant bi-product is water: HOH. Both acids and bases have the potential to be corrosive.
Examples of very corrosive bases include sodium hydroxide (also known as caustic soda or lye) with a pH of 13 (out of a possible 14) and potassium hydroxide (ph 13.5). Highly acidic substances include sulphuric acid; used in car batteries with a pH of 3 and hydroiotic acid (closely monitored acid used for making methamphetamine) with a pH of 1.