To determine the appropriate technique for purifying solid substances that transition directly from a solid to a vapor state without becoming a liquid, we must examine the characteristics of the options provided:
- Crystallization: This method is used to purify substances that are dissolved in a liquid, allowing them to form pure solid crystals as the solution cools. It involves a liquid phase, thus, it is not suitable for substances transitioning directly to vapor.
- Sublimation: This technique is specifically designed for substances that go directly from solid to vapor without passing through a liquid state. It effectively separates sublimable solids from non-sublimable impurities. This aligns perfectly with the behavior described in the question.
- Distillation: This involves the conversion of a liquid into vapor (and back to liquid), making it unsuitable for substances that do not liquefy.
- Chromatography: A method for separating components based on differential adsorption, not directly applicable to simple solid-to-vapor transitions.
Thus, the correct technique for the purification of such solid substances is Sublimation. This method takes advantage of the unique property of certain substances to vaporize directly into gas from a solid state and then re-deposit as a purified solid when conditions allow.