What is the difference between a base and an alkali?

A base is a substance that can accept protons or donate a pair of electrons, while an alkali is a base that dissolves in water.

In more detail, a base is a substance that has the ability to accept protons (H+ ions) or donate a pair of valence electrons. The concept of bases comes from the Brønsted-Lowry theory and the Lewis theory. According to the Brønsted-Lowry theory, a base is a proton acceptor. On the other hand, the Lewis theory defines a base as an electron pair donor. Bases can be found in various states of matter, including solid, liquid, and gaseous, and they are not necessarily soluble in water.

An alkali, however, is a specific type of base that is soluble in water. This means that when an alkali is added to water, it dissolves, increasing the concentration of hydroxide ions (OH-) in the solution. This results in a pH greater than 7, making the solution alkaline. Alkalis are often metal hydroxides, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). When these substances dissolve in water, they dissociate into their constituent ions, contributing to the increase in hydroxide ion concentration.

In summary, while all alkalis are bases, not all bases are alkalis. The key difference lies in their solubility in water. Understanding these differences is crucial in the study of chemistry, as it helps to predict the behaviour of substances in different environments and reactions.