Question

Which evaporates more quickly: \(55 \mathrm{~mL}\) of water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) in a beaker or \(55 \mathrm{~mL}\) of acetone \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\right]\) in an identical beaker under identical conditions? Is the vapor pressure of the two substances different? Explain.

Short answer

Acetone will evaporate more quickly than water because it has a higher vapor pressure at room temperature.

Step-by-step solution

Consider the nature of the liquids

Recognize that different liquids have different physical properties, such as boiling points and vapor pressures. These differences affect how quickly liquids evaporate under the same conditions.

Compare the properties of water and acetone

Acetone has a lower boiling point and a higher vapor pressure compared to water at room temperature. Vapor pressure is a measure of a liquid's tendency to evaporate, and a substance with a higher vapor pressure will evaporate more quickly.

Understand the effect of identical conditions

If water and acetone are placed in identical beakers under the same environmental conditions, the one with a higher vapor pressure (acetone) will evaporate more quickly since it requires less energy for the molecules to escape into the gas phase.

Conclude which liquid evaporates more quickly

Given the higher vapor pressure of acetone, it will evaporate more quickly than water.

Key concepts

Vapor Pressure

Understanding the concept of vapor pressure is crucial when examining the rate of evaporation of liquids. Vapor pressure refers to the pressure exerted by a vapor that is in equilibrium with its liquid or solid phase at a given temperature. Essentially, it's a measure of the tendency of a liquid's particles to escape into the gas phase. A higher vapor pressure indicates that the liquid molecules are escaping more readily, which corresponds to a higher rate of evaporation.

For instance, when considering substances like water and acetone, acetone's higher vapor pressure at room temperature suggests that its particles are more active in trying to escape the liquid phase. This activity results from weaker intermolecular forces within the acetone compared to those in water, making it easier for acetone molecules to break free and evaporate.

Factors Influencing Vapor Pressure

Several factors can affect the vapor pressure of a liquid, including:

• Temperature: As temperature increases, so does vapor pressure, because the particles gain energy and are more likely to enter the gas phase.
• Nature of the liquid: The chemical structure and intermolecular forces within a liquid influence its vapor pressure. Stronger intermolecular forces typically result in a lower vapor pressure.
• Purity of the liquid: Impurities can either raise or lower the vapor pressure, depending on their nature and interaction with the liquid.

Boiling Point

The boiling point of a liquid is another critical physical property that plays a role in the evaporation process. The boiling point is the temperature at which a liquid's vapor pressure equals the surrounding atmospheric pressure. At this juncture, the liquid begins to turn into a gas throughout the entire volume, not just at the surface.

The boiling point provides an insight into the amount of energy needed for a liquid to change into a vapor. Liquids with lower boiling points require less energy to overcome the forces holding their molecules together, allowing them to evaporate more readily than those with higher boiling points.

Comparison with Evaporation

It's important to note that evaporation differs from boiling in that it can occur at temperatures below the liquid's boiling point and happens only at the surface. Moreover, while boiling requires the liquid to reach a specific temperature, evaporation is a gradual process that can take place over a range of temperatures.

Physical Properties of Liquids

Physical properties of liquids, such as viscosity, surface tension, and heat capacity, all play roles in how a liquid behaves, including its rate of evaporation. For example, a liquid with high viscosity, which is a measure of its resistance to flow, will evaporate more slowly due to the stronger internal forces that need to be overcome for the molecules to escape.

Surface tension, the cohesive force at the surface of a liquid, also impacts evaporation. Liquids with high surface tension have molecules that are pulled together tightly at the surface, which can inhibit evaporation. Conversely, those with low surface tension, like acetone, have surfaces that are more easily disrupted, allowing molecules to transition into the gas phase more effortlessly.

Additionally, heat capacity, which quantifies the amount of heat needed to raise the temperature of a given amount of liquid by a certain degree, influences the amount of energy required for a liquid to reach its boiling point or evaporate. A liquid with a lower heat capacity can heat up faster and thus may evaporate at a quicker rate under identical conditions.