Fact 1: Boiling means change from a liquid into a gas by adding heat energy.

Fact 2: Propane can be either a liquid or a gas.

Given the two facts above, answer the question "what is added to propane to change it through the boiling process" with the following options: 
- vapors
 - energy
 - pH
 - heat
 - fats
 - SO2
 - fusion
 - oxygen


 

    heat     