Question

One milliliter of water is poured onto one hand, and one milliliter of acetone (fingernail-polish remover) is poured onto the other. As they evaporate, they both feel cool. Which one feels cooler and why? (Hint: Which substance is more volatile?)

Short answer

Acetone feels cooler because it is more volatile than water, leading to faster evaporation and greater heat absorption from the skin.

Step-by-step solution

Identify the property of volatility

Volatility is a property of a substance that describes how easily it vaporizes. In general, a more volatile substance will have a higher vapor pressure at a given temperature, meaning it vaporizes more readily at lower temperatures.

Compare the volatility of water and acetone

Acetone is more volatile than water because it has a lower boiling point and a higher vapor pressure at room temperature. This means that acetone will evaporate more quickly than water.

Explain the cooling effect

When a liquid evaporates, it absorbs heat from its surroundings, creating a cooling effect. Since acetone evaporates faster due to its higher volatility, it absorbs more heat energy from the skin over the same period, leading to a greater cooling sensation.

Provide the answer based on volatility

Therefore, the hand with acetone feels cooler than the one with water due to acetone's higher volatility, which causes it to absorb more heat from the skin as it evaporates.

Key concepts

Properties of Volatility

When examining the properties of volatility, you're delving into the tendency of a substance to vaporize. Volatility is a critical concept in chemistry that affects how substances interact with their environment. Crucially, volatility is not static; it varies among substances and is influenced by external conditions such as temperature. For students, understanding volatility comes down to grasping two main points: how easily a substance can transform from liquid to gas (its vaporization propensity), and the temperature at which this transformation occurs most readily.

The rate of vaporization indicates how fast a liquid enters the gaseous phase and is directly connected to the energy required to break intermolecular forces. Low intermolecular forces often mean greater volatility. These forces, which hold the molecules together in the liquid state, require energy to overcome during the phase transition to gas. Thus, substances like acetone, with weaker intermolecular bonds compared to water, exhibit higher volatility and vaporize more quickly at lower temperatures.

Vapor Pressure

Vapor pressure is a term that often comes up when discussing volatility. It represents the pressure exerted by the vapor released by a liquid (or solid) in a closed system at equilibrium. In simpler terms, it's the measure of a substance's tendency to evaporate. Vapor pressure is determined by the kinetic energy of molecules in a substance; those with sufficient kinetic energy can escape into the vapor phase. This pressure increases with temperature because heat energizes more molecules to break free from the liquid.

This concept ties in with the textbook exercise where acetone, by virtue of its higher vapor pressure compared to water, evaporates more readily from the skin. Therefore, it's essential to recognize that a high vapor pressure implies a liquid can vaporize quickly at lower temperatures, which is a hallmark of high volatility.

Evaporation and Cooling Effect

Let's turn our attention to the evaporation and cooling effect, a phenomenon that's not only relevant to chemistry but also to daily experiences such as feeling cooler after a swim. Evaporation is a natural method of cooling; it requires energy in the form of heat to convert liquid into vapor. This energy is typically absorbed from the material's surroundings, which, in the case of acetone on skin, is the skin itself.

When a volatile liquid like acetone evaporates, it takes more heat away from the surface it's in contact with compared to a less volatile liquid like water. Therefore, the cooling effect felt is more pronounced. This illustrates why when you apply a substance like acetone or alcohol to your skin, the rapid evaporation causes a significant chilling sensation. This understanding can help you visualize molecular interactions and the resulting sensations on a macroscopic scale, as in the textbook exercise.

Comparing Volatility of Substances

The comparison of the volatility of substances can offer much insight into their physical properties and their behavior in various situations. For instance, acetone and water behave quite differently due to their respective volatility. When comparing volatile substances, we consider their intermolecular bonds, molecular size, and shape, which all play roles in a substance's boiling point and vapor pressure.

Substances with weak intermolecular forces, such as hydrogen bonds, Van der Waals forces, or dipole-dipole interactions, tend to be more volatile. Hence, acetone, with weaker intermolecular forces than water, is more volatile and exhibits a lower boiling point and a higher vapor pressure. This comparative understanding of volatility is essential, not only in explaining the cooling effects as experienced in our textbook problem but also in practical applications such as perfumery, where more volatile compounds diffuse scent more readily, or in the development of fuels that require specific evaporation rates.