The key factor that determines the strength of an acid is its dissociation constant, often expressed as pKa. This value reflects how readily an acid donates protons (H⁺ ions) to a solution. A lower pKa indicates a stronger acid, as it signifies greater dissociation in water and a higher concentration of hydrogen ions. The strength of an acid can also affect its reactivity with other substances, influencing chemical reactions and biological processes. Understanding acid strength is crucial in fields like chemistry and biochemistry, where pH levels play a significant role.