The boiling point and melting point of substances is determined by their molecular weight and the cohesion (stickiness) of the individual molecules in the substance.
Based on the cohesive molecule - molecule interactions of a substance, it may have either a high melting and boiling point or vice versa. The stronger the cohesive interactions the more the heat required for the substance's transition into liquid and gas state.
These intermolecular forces include hydrogen bonding , ionic bonding, dipole dipole, and Van Der Waals dispersion forces.