When water is ionized, it can act as either an acid or a base.

An acid is a chemical species or substance that donates protons or hydrogen ions and/or accepts electrons. An example is hydrochloric acid.

A base is a chemical species or substance that donates electrons or hydroxide ions or that accepts protons.

Water undergoes various types of chemical reactions. One of the most important chemical properties of water is its ability to behave both as an acid (a proton donor) and a base (a proton acceptor).

In other words water is amphoteric. An amphoteric compound is a molecule or ion that can react as an acid as well as a base.

With hydrogen chloride, water accepts a proton to form a hydronium ion (H3O+), while with ammonia, water acts as a proton donor and is converted to a hydroxide ion (OH-).