What is a fuel?

• A. A substance that releases heat energy when it burns
• B. A substance that cools when it evaporates
• C. A substance that conducts electricity
• D. A substance that dissolves in water

Answer: (A)

A fuel is a substance that stores chemical energy and releases it as heat when it burns in oxygen. The defining idea is energy released through combustion, which is why fuels are used to provide heat or power engines. When the fuel reacts with oxygen, the chemical bonds break and new bonds form in the products, releasing energy as heat (and often light). That’s why this option is the best description of a fuel. The other statements describe different properties—cooling when it evaporates is an endothermic process, conducting electricity is a property of some materials, and dissolving in water is about solubility—not about releasing energy by burning.