Take $5 \mathrm{ml}$ of dilute hydrochloric acid in a test tube and add some zinc granules into it. Bring a burning matchstick at the mouth of the test tube. We can hear a pop sound. Which is this gas that burns with a pop sound?

$\mathrm{Zn}+2\mathrm{HCl}\to {\mathrm{ZnCl}}_2+{\mathrm{H}}_2$

What are the reactants and products in this reaction?

Look at the graph which shows the amount of heat energy produced by burning $1 \mathrm{g}$ of different fuels.

Which fuel has the highest calorific value among these?