The anions ionized in aqueous solution are all hydroxide ions (current theory holds that substances that can absorb protons during ionization are alkaline, and those whose anions are all OH- are alkalis, collectively referred to as bases). Reacts with acid to form salt and water. Typical bases include amines (including ammonia, chemical formula: NH3·H2O), caustic soda (sodium hydroxide, chemical formula: NaOH), hydrated lime [calcium hydroxide, chemical formula: Ca(OH)2], etc. The broader concept of a base refers to a substance that donates electrons or accepts a proton.

Chemical definition: A compound in which all anions are hydroxide ions (OH-) when ionized. (The pH value of some salt substance solutions is also greater than 7, but it is not an alkali, such as: soda ash (sodium carbonate) Na2CO3, baking soda (sodium bicarbonate) NaHCO3, etc.)