Video: Recalling Substances That Sublime at Room Temperature and Pressure

Sublimation is the process by which a solid becomes a gas without passing through the liquid phase. Which of these substances do not sublime at room temperature and pressure? I) Carbon dioxide II) Iodine III) Water [A] I only [B] III only [C] I and II only [D] I and III only [E] I, II, and III

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Video Transcript

Sublimation is the process by which a solid becomes a gas without passing through the liquid phase. Which of these substances do not sublime at room temperature and pressure? I) Carbon dioxide, II) iodine, III) water. A) One only, B) three only, C) one and two only, D) one and three only, or E) one, two, and three.

In a solid, all the atoms, ions, or molecules are in a fixed arrangement. Whatever attractive forces exist are strong enough to hold the structure in place. If we heat up a solid, often what happens is that the thermal energy that the particles have starts to fight these attractive forces. And particles can move over one another without necessarily escaping. So we form a liquid. This process is called melting. So in a liquid, the attractive forces are still there. But the particles have enough energy to move around one another. If we heat the liquid even further, we can cause those particles to break free from those attractive forces and become a gas. This process is called evaporation.

In a gas, the thermal energy is high enough for attractive forces to be completely overcome. However, in special circumstances, we can see substances transition directly from the solid to the gas. The question has given us three substances. And it’s asking us which of these does not sublime at room temperature and pressure. That’s usually taken to be 25 degrees Celsius and one atmosphere. The solid form of carbon dioxide is known as dry ice, while the solid form of water we simply know as ice. Now in this question, we need to be particularly careful. The question tells us that sublimation is the process where a solid becomes a gas, without going through the liquid phase. So actually, the first part of this question isn’t identifying substances that sublime. But it’s finding out which of these substances is a solid at room temperature and pressure.

Dry ice is well known to be very, very cold. It’ll be a solid at negative 78.5 degrees Celsius. Containers for dry ice are usually made from thick polystyrene. This helps insulate the pellets of carbon dioxide from the surrounding air. Dry ice is well known to sublime, going directly from solid pellets into a very cold gas. But there’s something not quite right. We’ve been asked to look at whether the substance would sublime at room temperature or not. But carbon dioxide, when we take it out of a very cold freezer, is much, much below room temperature. If we let carbon dioxide warm to room temperature first, it won’t be a solid anymore. So actually, carbon dioxide is a gas at room temperature.

In this question, it’s very important to make the distinction. Room temperature and pressure doesn’t mean bringing a cold object into a normal temperature room. It means having the object itself at 25 degrees Celsius under a pressure of one atmosphere. So all our carbon dioxide would be in the gas form. And therefore, it cannot sublime. So actually, carbon dioxide is a substance that will not sublime at room temperature and pressure. So it’s definitely part of our correct answer.

So what about iodine? Iodine is actually a solid at room temperature and pressure. It will consist of I2 molecules. If you have a sample of iodine in a sealed container room temperature, after quite a while, you might start to see little crystals of iodine growing from the sides of the container. And if you heat it up a little, you might see a purple haze around the solid. And that’s iodine gas. In order to form those solid crystals, even at room temperature, the iodine solid must have turned into a gas first, traveled around the container, and turned back into a solid when it hit the walls. This is an example of sublimation.

There is a common misconception that iodine always sublimes. But actually, iodine does have a melting point of 114 degrees Celsius and a boiling point of 184 degrees Celsius. So in between these points at one atmosphere of pressure, it will actually be a liquid. But if the temperature is below the melting point, it will be a solid. And it will sublime. However, the colder it gets, the slower it will sublime. Therefore, iodine is most certainly a substance that does sublime at room temperature and pressure. So it’s not going to be part of our correct answer.

We should be able to reach our conclusion for water much more quickly. We all know that water is actually a liquid at room temperature and pressure. Water has a melting point of zero degrees Celsius. So water will not sublime at room temperature and pressure. Remember, we’re not dealing with a circumstance where we pull a cold ice cube out of the fridge. We’re talking about having water at 25 degrees Celsius to begin with. And water must be a liquid at that temperature and at one atmosphere.

This means that our answer is one and three only. Carbon dioxide and water are the substances out of the list that do not sublime at room temperature and pressure.

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