Which of the following factors does not affect the vapor pressure of a liquid? (A) Concentration of solutions, (B) volume of the liquid, (C) temperature, (D) intermolecular forces.
Our goal in this question is to identify which of the answer choices does not have an effect on the vapor pressure of a liquid. The vapor pressure of a liquid is the equilibrium pressure exerted by the vapor on the surface of the liquid phase. The vapor above the surface of the liquid is produced by the process of evaporation. In a closed container at a specific temperature, the amount of vapor above the surface of the liquid remains constant over time.
Let’s imagine water is inside this closed container. The amount of vapor, and therefore the vapor pressure, remains constant over time, because the rate of evaporation is equal to the rate of condensation. If a factor or change disrupts this equilibrium by increasing or decreasing the evaporation rate, then a new equilibrium will need to be established. And the vapor pressure could be larger or smaller. To solve this problem, we will need to read each answer choice carefully and determine if the factor impacts how easily molecules in a liquid can escape the surface to form a vapor.
Intermolecular forces determine the strength of the forces of attraction between molecules in a liquid. Evaporation can only occur if the intermolecular forces between molecules in the liquid are overcome and the liquid molecules escape from the surface of the liquid to become vapor. A liquid with stronger intermolecular forces will not evaporate as easily as a liquid with weaker intermolecular forces. This means that liquids with stronger intermolecular forces tend to have lower vapor pressure than liquids with weaker intermolecular forces. Intermolecular forces are a factor that affect the vapor pressure of a liquid. Therefore, we can eliminate answer choice (D).
Now, let’s consider answer choice (C). Increasing the temperature of a liquid increases the kinetic energy of the molecules in the liquid. This means the rate of evaporation will increase because more molecules in the liquid phase will absorb enough energy to overcome the intermolecular forces that hold them together. The vapor pressure of the liquid will increase with temperature because the rate of evaporation is increasing. Because temperature is a factor that affects the vapor pressure of a liquid, we can eliminate answer choice (C).
Now, let’s discuss answer choice (A) concentration of solutions. When a solute is added to a liquid, such as by dissolving sugar in water, the evaporation process slows down. This is because the dissolved solute particles interfere with the evaporation process. If the concentration of the solution is increased by adding more solute, then the vapor pressure will decrease further. Because the concentration of a solution does affect the vapor pressure of a liquid, we can eliminate answer choice (A).
Finally, let’s discuss answer choice (B) the volume of the liquid. If we were to add more liquid to a container already holding a liquid, the rate of evaporation would probably not change. This is because we have not changed the area of the surface of the liquid, which is where evaporation takes place. If we increase the volume and the area of the surface of the liquid, then the rate of evaporation would increase. However, the rate of condensation would also increase due to the larger surface area. Because the rate of evaporation and the rate of condensation change by the same degree, the vapor pressure of the liquid does not change.
The factor which does not affect the vapor pressure of the liquid is, therefore, the volume of the liquid, or answer choice (B).