In aqueous solutions, acids ionise to produce hydrogen ions (H⁺). An aqueous solution just means ‘dissolved in water’, so water is the solvent. The strength of an acid depends on how much it ionises in water.
In aqueous solutions, strong acids completely ionise to form H⁺ ions, making the solution highly acidic. Below are some examples of strong acids:
As you can see, all of the acid molecules fully ionise in water.
In aqueous solutions, weak acids only partially ionise, so the solution is less acidic than with strong acids. Below are some examples of weak acids:
We can identify a weak acid by the reversible reaction symbol (⇌). This symbol means that the reaction is in a state of equilibrium, where the amount of product and reactant is constant. This means that only a small proportion of the acid molecules ionise in water, resulting in a relatively low concentration of H+ ions.
Concentration is a measure of how much solute is dissolved in a solution. A higher concentration means there are more solute particles in a given volume of the solution. On the other hand, a lower concentration means there are fewer solute particles.
Concentration can also change if more solvent is added to a solution. Adding more solvent to a solution dilutes it by reducing the concentration of solute. In contrast, if no additional solvent is added and more solute is added instead, the solution becomes more concentrated.
In aqueous solutions, the concentration of hydrogen ions (H⁺) can be measured using the pH level.
Strong acids fully ionise, producing a higher concentration of H⁺ ions than weak acids, which only partially ionise. Whereas, weak acids are only partially ionised. Therefore, the more concentrated the solution of an acid, the lower the pH level will be.
As the pH scale decreases by one unit, the concentration of H⁺ ions increases by a factor of 10. For example, a solution with a pH of 1 has a concentration that is 10 times greater than a solution with a pH of 2, and 100 times greater than a solution with a pH of 3.