Video: Identifying the Statement That Is Necessarily True of a Nonionic Substance with a Low Boiling Point in a Set of Statements

Which of the following is necessarily true of a non-ionic substance with a low boiling point? [A] It has a small vapor pressure. [B] It has weak intermolecular attractive forces. [C] It has a high freezing point. [D] It has a high heat of vaporization. [E] It will be a liquid at very low temperatures.

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Video Transcript

Which of the following is necessarily true of a nonionic substance with a low boiling point? A) It has a small vapor pressure. B) It has weak intermolecular attractive forces. C) It has a high freezing point. D) It has a high heat of vaporization. Or E) it will be a liquid at very low temperatures.

The boiling point, of course, is the temperature at which a substance in the liquid phase changes into the gas phase. If we compare a substance that’s a liquid to a substance that’s a gas, the particles in the liquid are much closer together than the particles of a gas. When a substance boils, it’s because some of the particles that are liquid have enough energy to escape the liquid and become a gas. This means that the process of boiling requires energy so that we can put enough energy into the liquid so that they can escape the liquid phase and become a gas.

When enough energy has been put into the liquid so that the particles that have been able to escape into the gas phase are in equilibrium with the particles that are still in the liquid phase, we say that that substance has reached its boiling point. This means that at the boiling point, gas particles are condensing to turn into a liquid at the same rate as liquid particles are escaping into the gas phase. There are many factors that can affect how easy it is for a liquid to escape into the gas phase and therefore reach the boiling point.

In this question, we want to know what would make a substance have a low boiling point. So, let’s look through the answer choices to figure out which one of these would affect the boiling point that way.

The first answer choice is about the vapor pressure of the substance, specifically the substance having a small vapor pressure. The vapor pressure is the pressure that’s exerted by the particles of gas that have evaporated from the liquid. So, how does vapor pressure affect the boiling point? Well, let’s say that we have an open container of liquid. The atmospheric pressure — that is the pressure from the atmosphere — will be pushing down on the liquid, which will make it harder for the liquid to escape into the gas phase. In other words, the pressure from the atmosphere makes it harder for the liquid to escape into the gas phase and therefore reach its boiling point. When a substance reaches its boiling point, the vapor pressure of the gas is equal to the atmospheric pressure that’s pushing down on the liquid.

So, if a substance has a very small vapor pressure, the atmospheric pressure will make it difficult for the liquid particles to escape into the gas phase. And it will take a longer time to reach the boiling point, where the vapor pressure and the atmospheric pressure are equal. In other words, having a small vapor pressure corresponds to the substance having a higher boiling point because we need to put more energy into the liquid in order to allow more liquid particles to escape into the gas phase so that the vapor pressure and the atmospheric pressure can become equal. We’re looking for something that would cause the substance to have a low boiling point, not a high boiling point. So, answer choice A is not correct.

Our next answer choice is about intermolecular forces between particles. As we discussed previously, when a substance is a liquid, the particles are much closer together than when the substance is a gas. Separating these liquid particles to move them into the gas phase requires energy. Intermolecular forces are attractive forces between particles, so if a substance has weak intermolecular forces, it’s going to be easier to separate the particles. And since the boiling point is reached when enough liquid particles have been separated and escaped into the gas phase, if it’s easier to separate the particles, the substance will have a lower boiling point.

So, it looks like we found our answer. If a substance has weak intermolecular attractive forces, it’s going to be easier to separate the particles. And therefore, it will be easier to reach the boiling point. So, the substance will have a low boiling point. Let’s take a look through our other answer choices.

Answer choice C is having a high freezing point, which is, of course, the point where a solid goes into a liquid. If the substance has a high temperature for when it turns from a solid into a liquid, we wouldn’t necessarily expect there to be a low temperature for the liquid turning into a gas. So, answer choice C is incorrect.

Answer choice D is having a high heat of vaporization. The heat of vaporization is the energy that’s needed to go from a liquid to a gas. So, if a substance has a high heat of vaporization, that means it would need a lot of energy to go from a liquid to a gas. Needing a lot of energy to go from a liquid to a gas would correspond to a high boiling point, not a low one. So, answer choice D isn’t correct either.

Our final answer choice says that the substance will be a liquid at a very low temperature. This one might be tempting to pick because if a substance is a liquid at a low temperature, it might turn into a gas at a low temperature as well. But we’re looking for something that’s necessarily true. There are many factors that affect how easily a substance will turn from a liquid to a gas. So, answer choice E is not correct.

As we suspected, answer choice B is the correct answer. A substance that has weak intermolecular attractive forces will have a low boiling point.

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