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Chemistry

Acids, Bases and pH

An atom is formed of 3 main subatomic particles. Protons and neutrons are contained in the nucleus of the atom. The proton has a positive charge and the neutron doesn’t have any charge, it is neutral as its name would suggest. The third sub-particle frequently considered in chemistry is the negatively charged electron. The term proton is also used for the positively charged ion of hydrogen, as that is all you are left with when the single electron present is removed.

Acid
A molecule able to accept a pair of electrons or donate a proton (positive hydrogen ion, H+). The word acid derives from the latin word for sour and acids, which are present in many foods, do tend to taste sour. Acids are usually corrosive to metals. A lemon contains citric acid.

Base
A molecule able to donate a pair of electrons or accept a proton (H+). Bases taste bitter (and we don’t recommend trying to taste any!) and react with the oils on your hand to give a slippery feel. Potassium hydroxide is used in alkaline batteries.

Note: The definition of an acid or base involving pairs of electrons is known as the Lewis definition and in this case an acid is an electron pair acceptor, while a base is an electron pair donor. The  Brønsted-Lowry definition is more restrictive as it focusses on protons and here an acid is defined as a proton donor, while a base is a proton acceptor.

pH
The pH scale measures the acidity of a substance. It does this by looking at the concentration of protons. The more protons are present, the more acidic the solution.

The term”pH” can be split into two parts with “p” being “-log10“and the “H” representing the concentration of protons, or, more accurately, the concentration of H3O+. So if we know the concentration of protons, we can calculate the pH of a substance.

The pH scale thus ranges from 0-14, and it uses water as its midpoint. A pH below 7 means the substance investigated is acidic. A pH of above 7 means that the substance is basic. A pH of 7 means that the substance being measured is neutral, i.e. it has the same pH as water.

Compiled by Lucy Bird.