How do you tell if an acid is a strong acid or weak acid?

Answer 1

Different acids will have varying levels of strength - the most obvious way to see this is to look at the pH of the acid. That is the lower the pH, the stronger the acid.

The reason behind this is due to the degree of ionisation of the acid in aqueous solution. For example, hydrochloric acid is a strong acid (complete ionisation) whilst acetic acid is weak as it does not ionise completely.

Answer 2

A strong acid is one that completely dissociates in water to form ions, resulting in high conductivity and completely ionized molecules. A weak acid only partially dissociates in water, resulting in low conductivity and mostly intact molecules remaining. You can determine the strength of an acid by measuring its pH or conductivity in water.