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Chemistry Students Consider Unusual Phase Changes

September 18, 2022

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Mr. Watt's Chemistry II students got to go outside this week to observe solid iodine changing straight from a solid to a gas and vice versa (sublimation and deposition) when heated and cooled. The cool part about it is that iodine changes from a grayish solid to a purple gas when it is heated. Though it could melt if heated more gradually, the melting point and boiling point of iodine are close enough together that a regular flame will cause rapid sublimation. The interesting part (as shown in the video) is that deposition also occurred when the iodine gas came into contact with the relatively cooler glass funnel. The phase changes were almost interchangeable during the demonstration. 

While the phase changes sublimation and deposition are somewhat less familiar to the average citizen, they can happen with any pure substance (element or compound). Many people are familiar with the smell of mothballs, which are made of the pure substance paradichlorobenzene and sublime just like iodine. Have you ever noticed that they just disappear over time? Ice cubes in our freezers do the same thing. If it snows and the temperature doesn't get above freezing for a while, deep snow in like fashion.