What does a low pKa indicate about a substance?

A low pKa value signifies that a substance is more acidic. The pKa is the negative logarithm of the acid dissociation constant (Ka) and serves as a measure of the strength of an acid in solution. When the pKa is low, it indicates that the acid readily donates protons (H⁺ ions) to the solution, demonstrating a stronger acidic character.

In other words, acids with lower pKa values are more likely to dissociate in water, leading to higher concentrations of hydrogen ions and thus a lower pH, which is characteristic of acidic solutions. This relationship highlights the inherent nature of the acid; the stronger the acid, the more easily it donates protons, thereby resulting in a smaller pKa value. Preferably, this helps distinguish strong acids, which typically have low pKa values, from weak acids that have higher pKa values and are less willing to donate protons.