What is an acid? what is a base?

An acid can simply be defined (Arrhenius) as a compound that produces hydrogen ions when dissolved in solution. This production of hydrogen ions lead to reduction of pH of the solution. On the other hand, a base can be defined as a compound that produces hydroxyl ions when dissolved in solution. This production of hydroxyl ions lead to increase in pH of the solution.

Another definition  (Bronsted-Lowery) describes an acid as a compound that donates a proton and a base as a compound that accepts a proton. According to this definition, the solvent has a role in accepting the proton from an acid or in donating the proton to the base. Water as a solvent can either accept or donate a proton depending on the dissolved compound. This type of solvents are known as amphiprotic. Moreover, protophillic solvents (e.g. ammonia) are solvents that only accept protons from dissolved compounds. Protogenic solvents (e.g. acetic acid) are solvents that only donate protons to dissolved compounds. Aprotic solvents (e.g. hexane) neither accept nor donates protons.

Another definition (Lewis) describes an acid as a compound that accepts an electron pair from the solvent (or the reaction) and a base as a compound that provides an electron pair for the solvent or (for the reaction). This definition is more suitable for describing acid - base reactions in organic and inoraganic chemistry.