13.12: Sublimation
- Page ID
- 53811
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How did early settlers do their laundry in the winter?
Of course, they could wash the clothes inside, but where to dry them? There were no dryers available, so the clothes had to be hung up to dry. In the winter, you would expect that ice would form on the clothes, but that didn’t happen. Even in freezing weather, the water might freeze initially, but would eventually go off as a vapor – from solid ice directly to the gas stage. There are still families today who have to rely on this cumbersome process to get their laundry done.
Sublimation
Vapor Pressure of a Solid
"Sublimation" surveys the vapor pressure of a liquid and its dependence upon temperature. Solids also have a vapor pressure, though it is generally much less than that of a liquid. A snow bank will gradually disappear even if the temperature stays below \(0^\text{o} \text{C}\). The snow does not melt, but instead passes directly from the solid state to the vapor state. Sublimation is the change of state from a solid to a gas, without passing through the liquid state.
Examples of Sublimation
Sublimation of Iodine
When iodine is heated sublimation can be readily observed. Iodine's vapor is a distinctive purple color and has a very strong scent, making it easy to detect. The video below shows the sublimation and deposition of iodine. The beaker containing iodine is covered with a round-bottom flask that contains ice. When the iodine is heated it sublimes. When iodine vapors cool, deposition occurs. Deposition is the change of state from a gas to a solid.
Dry Ice
"Dry ice", or solid carbon dioxide, is a substance that sublimes at atmospheric pressures. Dry ice is very cold \(\left( -78^\text{o} \text{C} \right)\) and so is used as a coolant for goods, such as ice cream, that must remain frozen during shipment. Because the dry ice sublimes rather than melts, there is no liquid mess associated with its change of state as it warms. As you may have seen in demonstrations, dry ice merely transforms from a solid to a cold, steamy looking gas.
Ferrocene
Ferrocene (an iron-containing compound) is usually purified by sublimation. A sample of crude ferrocene is gently heated causing it to sublime. When the ferrocene is cooled, reddish ferrocene crystals deposited on the outside of that tube as shown in the figure below.
Summary
- Sublimation is the change of state from a solid to a gas, without passing through the liquid state.
- Deposition is the change of state from a gas to a solid.
- Carbon dioxide is an example of a material that easily undergoes sublimation.
Review
- Define sublimation.
- What is another name for solid carbon dioxide?
- How is ferrocene purified?
- Name a substance which undergoes sublimation.