An acid is a molecule that creates H+ and H3O+ ions in water (according to the Arrhenius definition), that donates H+ ions (protons) in a chemical reaction (according to the Bronsted-Lowry definition), or that accepts an electron pair in a reaction(according to the Lewis definition).
Acids taste sour (although, in a laboratory, never taste an acid; they can be highly corrosive) and turn blue litmus paper red; they have pH values of less than 7. Acids are covalent compounds, but when dissolved in water, they will conduct electricity. When an Arrhenius acid is put into water, it will lose its H+, a process called aqueous dissociation. Strong acids are those that completely give up their protons in solution, whereas weak acids reach an equilibrium concentration of H+ in solution. Many organic molecules that play important roles in biological processes are also acids, such as amino acids and carboxylic acids.