# Hydrogen has been used as a rocket fuel.

Question.
Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG ? Why or why not ?

solution:
Hydrogen is a cleaner fuel than CNG. This is because it produces water on burning whereas $\mathrm{CNG}$ on burning, produces $\mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{O}$ and $\mathrm{CO}$.