17. The heat of fusion of a substance is the quantity of heat A. absorbed when a liquid freezes B. absorbed when a solid melts C. required to fuse the substance into water D. absorbed when water evaporates

The answer is **B. absorbed when a solid melts**.<br /><br />Here's why:<br /><br />* **Heat of fusion** (also called latent heat of fusion) is the amount of heat energy required to change a substance from a solid to a liquid at its melting point. This process requires an input of energy, meaning heat is absorbed.<br /><br />Let's look at why the other options are incorrect:<br /><br />* **A. absorbed when a liquid freezes:** Freezing is the opposite process of melting. Heat is *released* when a liquid freezes, not absorbed.<br />* **C. required to fuse the substance into water:** Fusion refers to the melting process, not specifically combining with water. While some substances might dissolve in water, this isn't the definition of heat of fusion.<br />* **D. absorbed when water evaporates:** This describes the heat of vaporization, the energy needed to change a liquid to a gas.<br />