A substance is observed to change directly from a solid to a gas when heated at a pressure below its triple point. Which term best describes this change of state?
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Correct answer
B. Sublimation
Principle or equation
Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the liquid phase, occurring when the pressure is below the triple point.
Why this answer is correct
At pressures below the triple point, the liquid phase is not stable, so heating a solid can cause it to vaporize directly. This process is called sublimation. Melting is solid to liquid, evaporation is liquid to gas, and condensation is gas to liquid.
Example
Solid CO2 (dry ice) at atmospheric pressure sublimes directly to CO2 gas without melting.
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