A student is investigating the boiling and freezing point of a liquid, He observes the liquid in the apparatus while he gently heats the flask with a Bunsen burner flame. The liquid and its vapour are colorless and transparent. 

1. Suggest one observation that will tell the student when the reading on the thermometer shows the boiling point of the liquid. 

2. Explain why the temperature does not rise above its boiling point even though the flask is still being heated. Use the word thermal energy in your answer.

3. What effect on the boiling point is produced by a soluble non-volatile impurity, such as table salt, and an insoluble foreign substance, such as sand?