Explain why rubbing alcohol feels cold when applied to the skin.

When we think about evaporation, it's like watching a puddle of water disappear after a rainy day. It's the process by which a liquid turns into a gas. In the case of rubbing alcohol on your skin, the same thing happens—alcohol molecules gain energy and escape into the air as vapor.

During evaporation, molecules within the liquid state gain enough kinetic energy to overcome the attractive forces keeping them in the liquid, which is a type of potential energy. Imagine a busy street; the more energy people have, the less likely they are to stick together in groups—and something similar happens with alcohol molecules on your skin.

For a hands-on demonstration, you could even conduct a simple experiment. Place a small amount of water in one hand and a small amount of rubbing alcohol in the other. Notice how the alcohol feels cooler and dries faster. This is due to the more significant number of alcohol molecules gaining enough energy to evaporate compared to water molecules.

Thermal energy is essentially the total amount of kinetic energy from the movement of particles in a substance. When you feel warmth from a mug of hot cocoa or from the sun, you're experiencing the transfer of thermal energy. In our case, with rubbing alcohol, it's like the alcohol is 'stealing' this cozy warmth from your skin to fuel its transformation from a liquid into a gas.

When the alcohol absorbs this energy, your skin, losing energy, feels colder. It's similar to when you sweat during exercise. Your body releases moisture, which then takes heat from your skin to evaporate, helping you cool down. The same principle applies to the rubbing alcohol, but alcohol does it much more quickly because of its physical properties—leading us to another concept closely tied to this process, volatility.

Volatility refers to how readily a substance will vaporize—or, in other words, how easy it is for a liquid to turn into a gas at a given temperature. Rubbing alcohol, which usually contains isopropanol or ethanol, is much more volatile than water. This is why it evaporates quickly, even at room temperature.

Volatility is critical to the cooling effect you feel when applying rubbing alcohol. Because it's highly volatile, it doesn’t hang around long enough on your skin to take in heat slowly. Instead, it grabs heat fast and takes off, leaving a trail of cooler skin behind. A simple way to think about volatility is to compare scents. Perfumes, which are also volatile, quickly fill a room with fragrance, while a bowl of sugar doesn't, even though both are left open in the same space.