The speed of which a solid changes from a solid to a gas is called ______________.

• A. Melting rate
• B. Evaporation rate
• C. Sublimation rate
• D. Deposition rate

Answer: (C)

Sublimation rate is the speed at which a solid changes directly into a gas without becoming liquid. This direct solid-to-gas transition happens when the solid’s molecules gain enough energy to overcome intermolecular forces and escape into the surrounding gas phase, typically under conditions where the ambient pressure is not high enough to allow condensation back into a liquid.

This makes sublimation rate the correct term for this process. It differs from melting rate (solid to liquid) and evaporation rate (liquid to gas), and from deposition rate (gas to solid). A familiar example is dry ice, which sublimes at room temperature, turning directly into carbon dioxide gas. The rate of sublimation increases with higher temperature, lower pressure, and greater surface area. In hazmat scenarios, sublimation is important because some solids release hazardous vapors as they sublimate, even without melting, and the rate can significantly affect vapor exposure and chamber atmosphere considerations.