In steam distillation, the liquid boils when the sum of the vapour pressures due to the organic liquid ($p_1$) and that due to water ($p_2$) becomes equal to the atmospheric pressure ($p$). This relationship is expressed as $p = p_1 + p_2$.

Since the organic liquid is immiscible with water, the total vapour pressure of the mixture reaches the atmospheric pressure at a temperature where the organic liquid's own vapour pressure ($p_1$) is still lower than the atmospheric pressure ($p$). Consequently, the organic liquid vaporises at a lower temperature than its actual boiling point.