A sample of solid iodine is placed in a sealed container and heated gently. The solid changes directly into a purple vapor without melting. When the vapor cools, it deposits as solid iodine on the cooler parts of the container. Which set of terms correctly describes the forward change (solid to vapor) and the reverse change (vapor to solid)?
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Correct answer
B. Sublimation and deposition
Principle or equation
Sublimation is the direct transition from solid to gas; deposition is the reverse process (gas to solid).
Why this answer is correct
Iodine undergoes sublimation when heated, and the vapor deposits back to solid on cooling, which is deposition. Evaporation and boiling are liquid-to-gas changes; melting is solid-to-liquid; condensation is gas-to-liquid.
Example
Solid CO2 (dry ice) also sublimes at room temperature, and the gas can deposit as solid on a cold surface.
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