The basicity of HCl is zero because it is a strong acid. Basicity is a measure of the ability of a compound to accept protons (H+ ions). A strong acid, like HCl, completely dissociates in water, releasing all of its protons. This means that there are no protons left for the compound to accept, so its basicity is zero.

In contrast, a strong base, like NaOH, completely dissociates in water, releasing all of its hydroxide ions (OH- ions). This means that there are plenty of hydroxide ions available to accept protons, so its basicity is high.

The basicity of a compound is typically measured on a scale from 0 to 14, where 0 is the lowest basicity (strongest acidity) and 14 is the highest basicity (strongest alkalinity). HCl has a basicity of 0, while NaOH has a basicity of 14.