Video: Applying Knowledge of Solubility and Vapor Pressure

For statements I and II, state for each if they are true or false. I) The solubility of gases in liquids does not depend on temperature. II) The vapor pressure of a substance is independent of external pressure. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The solubility of gases in liquids does not depend on temperature. II) The vapor pressure of a substance is independent of external pressure. If both are true, state if II is a correct explanation for I.

Let’s start by looking at statement I. Solubility is the maximum amount of something that can dissolve in a solvent per quantity of solvent. The common units for solubility are grams or milligrams per 100 milliliters of solvent. Let’s imagine that we’ve dissolved some carbon dioxide in some water. And let’s imagine that the mixture is at 10 degrees Celsius. If we did the same thing at 30 degrees Celsius, we’d be able to dissolve less carbon dioxide. But why? The solubility of a gas like carbon dioxide and water is determined by the balance of factors. The intermolecular forces between carbon dioxide and water molecules and the kinetic energy of the carbon dioxide.

At 30 degrees Celsius and 10 degrees Celsius, the forces of attraction between carbon dioxide and water are about the same. But at 30 degrees, there’s more thermal energy. So the carbon dioxide molecules have more energy and find it easier to escape the water. This means that the maximum amount of CO₂ that that water can hold onto goes down as temperature increases. So statement I is actually false. The solubility of gases in liquids does indeed depend on temperature. The solubility decreases with temperature. Bear in mind, this applies to gases, not necessarily to solids in water.

What about statement II, the vapor pressure of a substance is independent of external pressure? Vapor pressure is a property of a substance. And it’s the pressure of the vapor above a pure liquid sample if it’s in a closed system, like a sealed container. Let’s imagine you’ve got a syringe. And let’s imagine there are some water inside it and this cap on the end so nothing else can get in. Just pure water, nothing dissolved inside it. And there’s no air trapped in the syringe. If you pull back on the syringe, you’re going to increase the volume inside the syringe. But you’re not going to introduce any more material. So what’s going to fill the empty space?

The answer is water vapor. Some of the liquid water will evaporate. The vapor pressure of water at 25 degrees Celsius is about 0.03 atmospheres. That’s about three percent of atmospheric pressure. It’s important to state the temperature because vapor pressure does depend on temperature. In a way, we can think about water vapor like any other gas. And its solubility will decrease as temperature increases. However, vapor pressure does not depend on external pressure. If you tuck harder on the syringe, you would increase the amount of vapor in the container. But the pressure would stay the same. This is because of the same balance between kinetic energy and intermolecular forces.

Once the concentration of the water vapor reaches a critical value, it’s as likely for water to turn from liquid to a guess as from gas to liquid. So the pressure stays constant. So statement II, the vapor pressure of a substance is independent of external pressure, is true. Since only one statement is true, we don’t need to address the second part of the question. So our final answer is that statement I is false while statement II is true.

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