In chemistry, a base is a substance that can accept hydrogen ions (H+) or donate electron pairs. There are three common definitions of bases in chemistry: Arrhenius bases, Brønsted bases, and Lewis bases). All definitions agree that bases are substances that react with acids. Some key features of bases include:

• Slippery feel: Bases are slippery to the touch.
• Bitter taste: Bases taste bitter.
• Color change: Bases change the color of indicators, such as turning red litmus paper blue.
• React with acids: Bases react with acids to form salts and water in a neutralization reaction.

Examples of bases include sodium hydroxide, calcium carbonate, and potassium oxide. Bases can be classified as alkalis if they are soluble in water. Common bases include metal oxides, metal hydroxides, and ammonium hydroxide.

The Brønsted-Lowry theory defines a base as a substance that can accept hydrogen cations (H+) or protons). This includes aqueous hydroxides since OH- does react with H+ to form water, so that Arrhenius bases are a subset of Brønsted bases. However, there are also other Brønsted bases which accept protons, such as aqueous solutions of ammonia (NH3) or its organic derivatives (amines) ).

Lewis bases are defined as molecules that have the ability to accept an electron pair bond by entering another atoms valence shell through its possession of one electron pair).

In summary, bases are an important class of chemical compounds that have a variety of properties and uses in chemistry.