Acidic solutions turn litmus paper red, turn a universal indicator red or yellow, taste tart or sour, conduct electricity, react with metals, and neutralize bases. When an acid and a metal react, hydrogen is released; when an acid and a base react, an ionic compound called a salt is formed.
An acidic solution has a pH lower than 7. The stronger the solution's pH, the stronger the acid is. The strength of an acid refers to the proportion of hydrogen ions in the solution, not the concentration of acid to water. The proportion of hydrogen ions determines how many protons the acid can donate in a chemical reaction.
There are two types of acids: monoprotic acids and polyprotic acids. Monoprotic acids are able to donate one proton; this category includes hydrochloric acid, nitiric acid and acetic acid. Polyprotic acids donate more than one proton to a reaction. The polyprotic acids include sulfuric acid, carbonic acid and phosphoric acid.
Superacids are part of a special subset of the polyprotic acids. A superacid is an acid stronger than undiluted sulfuric acid. These acids are strong enough to react with water, leaving behind crystalline precipitates in the solution. Perchloric acid is an example of a superacid.