What is the process called in which a substance changes directly from the solid state to the gaseous state without passing through the liquid state?

Introduction / Context:
Matter can change from one physical state to another when energy is absorbed or released. Common changes include melting, freezing, evaporation, and condensation. Some substances, however, can change directly from solid to gas without becoming liquid in between. Recognising the correct name for this process is important in basic physics and chemistry, especially when studying phase changes and phase diagrams.

Given Data / Assumptions:
• The change described is from solid directly to gaseous state. • There is no intermediate liquid state in this process. • Examples include substances like dry ice (solid carbon dioxide) and naphthalene under appropriate conditions.

Concept / Approach:
The direct transition from solid to gas is known as sublimation. During sublimation, molecules at the surface of a solid gain enough energy to break free from the solid structure and enter the gaseous phase, bypassing the liquid state. This usually occurs when the substance has a relatively high vapour pressure at temperatures below its melting point or when pressure conditions favour the gas phase. Evaporation is the change from liquid to gas, condensation is gas to liquid, distillation is a separation technique based on different boiling points, and fusion (melting) is solid to liquid.

Step-by-Step Solution:
Step 1: Identify the initial and final states: solid to gas. Step 2: Recall that the term for solid to liquid is melting or fusion and for liquid to gas is evaporation or boiling. Step 3: Recognise that the special case where solid directly becomes gas is called sublimation. Step 4: Compare with other terms: condensation is gas to liquid, distillation is a purification method, and evaporation is liquid to gas. Step 5: Therefore, the correct term for direct solid to gas change is sublimation.

Verification / Alternative check:
Consider dry ice (solid CO2) at atmospheric pressure. It does not melt into liquid CO2 under normal conditions; instead, it directly forms CO2 gas, producing the foggy effect commonly seen in stage performances. This is a textbook example of sublimation. Similarly, iodine crystals can slowly turn into iodine vapour, and naphthalene balls gradually disappear from cupboards as they sublimate. These familiar examples confirm that the process is known as sublimation.

Why Other Options Are Wrong:
Option a (Evaporation): This is the process where a liquid changes to gas at temperatures below its boiling point; it does not start from the solid state. Option b (Condensation): This is gas changing to liquid, the opposite direction of what is described. Option d (Distillation): This is a separation or purification technique involving boiling and condensation, not a fundamental phase change name. Option e (Fusion): Fusion or melting is the change from solid to liquid, not directly to gas.

Common Pitfalls:
Learners sometimes confuse sublimation with evaporation because both involve a transition to the gaseous state. The key difference is the starting state: sublimation starts from a solid, while evaporation starts from a liquid. It is also important not to mix up condensation (gas to liquid) with freezing (liquid to solid). Paying attention to the initial and final phases in the question helps keep the terms straight.

Final Answer:
The direct conversion of a solid into a gas without passing through the liquid state is called Sublimation.