Explain, why `o`-hydroxybenzaldehyde is a liquid at room temperature while `p`-hydroxybenzaldehyde is a high melting solid?

Explanation

In - hydroxybenzaldehyde, there is intramolecular hydrogen bonding which further prevents association of the molecules. However, in p - hydroxy benzaldehyde, there is intermolecular hydrogen bonding and therefore, there is association among the molecules :

Thus, because of larger association, p - hydroxy benzaldehyde is a solid at room temperature but o - hydroxybenzaldehyde is a liquid because of less association.