A fundamental category of many compounds whose water-based solutions have a sour taste, turn blue litmus paper red, and can combine with metals to form salts. Acids have a pH lower than 7 (strong acids have a pH of less than 3). Specific types of acids include: Arrhenius acid: any chemical that increases the number of free hydrogen ions (H+) when they're added to a water-based solution. The more free hydrogens produced, the stronger the acid.Brönsted or Brönsted-Lowry acid: any chemical that acts as a proton donor in a chemical reaction.Lewis acid: any chemical that accepts two electrons to form a covalent bond during a chemical reaction.