Mammoth Memory

For A-Level students or crazy people

In simple terms, an acid is a compound that has dissolved in water and has a pH of less than 7.

For A-Level…

You need to know that an acid is a compound that has dissolved in water and donated a hydrogen proton. This hydrogen proton combines with a water molecule, while the electron is released and able to roam freely. As a result, the acid will carry an electric charge.

A hydrogen atom consists of only a proton and an electron.

Hydrogen has one electron negatively charged and one proton positively charged

  diagram showing that hydrogen has one electron resized

If the electron is released to roam around in a solution, only a hydrogen proton is left. This hydrogen proton is positively charged and called an ion or, more precisely, a cation.

positively charged hydrogen proton

This hydrogen proton of H+ cation cannot exist on its own, so latches itself onto the water in the solution.


 baby hippo being gievn a piggy back ride by a sad looking bucket of water

This is why at A-Level definitions proceed as follows:

  • An acid is any substance that has donated a hydrogen proton to water
  • A base is any substance that will accept a hydrogen proton from water
  • An alkali is a base that has dissolved in water
  • A salt is a compound that is created by the reaction between an acid and a base (or alkali).
More Info