Question

how can the boiling piont of a liquid be raised without adding soluble impurity?

Answer 1

Insoluble impurities do not raise the boiling point of a solution.
It could be explained from Raoults law that vapour pressure of the solution is affected by the partial vapour pressure of components of a solution.
That is if the solute is not dissolved in solution than how it raise the boiling point .It won't affect it's boiling point at all.

Answer 2

Increase the pressure around the liquid or the pressure inside the container in which it is kept. it will increase the boiling point.