Define the term boiling point - NSC Physical Sciences - Question 3 - 2016 - Paper 2

The relationship between the strength of intermolecular forces and boiling point is direct; as the strength of these forces increases, the boiling point also increases. This occurs because stronger intermolecular forces require more energy to overcome, thus raising the temperature needed for the substance to transition from liquid to gas.

In compound A (Propane), the dominant intermolecular force is London dispersion forces, which are relatively weak. In contrast, compound B (Propan-2-one) exhibits both London forces and dipole-dipole interactions, making its intermolecular forces stronger. This results in a higher boiling point for compound B compared to compound A.

Compounds C (Propan-1-ol) and D (Propanoic acid) both involve hydrogen bonding due to the presence of an -OH group. However, propanoic acid also has a carboxylic acid functional group, allowing it to form stronger hydrogen bonds than propan-1-ol, leading to a significantly higher boiling point for compound D.

Compound B (Propan-2-one) is a liquid at room temperature. Its boiling point of 56 °C indicates that it is below the boiling point at standard atmospheric pressure, thus it remains in the liquid state.