Problem 83 - Chemistry: An Atoms-Focused Approach

Question

Clean Skin Disposable wipes used to clean the skin prior to getting an immunization shot contain ethanol. After wiping your arm, your skin feels cold. Why?

Step-by-Step Solution

Answer

Answer: The skin feels cold after using ethanol-containing disposable wipes due to the process of evaporative cooling. As ethanol evaporates from the skin's surface, it absorbs heat from the skin, reducing the temperature of the skin's surface and leading to the cooling sensation.

Step 1: Identifying the components involved in the process

The main components involved in this process are: 1. The skin 2. The disposable wipes containing ethanol

Step 2: Understanding how ethanol evaporates

Ethanol evaporates faster than water due to its lower boiling point (78.37°C or 173.07°F). When the wipe is used to clean the skin, some ethanol is left on the surface of the skin.

Step 3: Linking evaporation to the cooling sensation

Evaporation is a cooling process. When ethanol evaporates from the skin's surface, it absorbs heat from the surrounding area (i.e., the skin) in order to undergo the phase transition from a liquid to a gaseous state. This heat absorption reduces the temperature of the skin's surface, leading to the feeling of cold.

Step 4: Highlighting the concept of evaporative cooling

The phenomenon that the student should focus on is called evaporative cooling. Evaporative cooling occurs when a liquid absorbs heat from its environment during the process of evaporation. In this case, ethanol absorbs heat from the skin during its evaporation, leading to the cooling sensation experienced on the skin.

Key Concepts

Ethanol Evaporation

Ethanol is a common alcohol that is used in many applications, including sanitizers and cleaning wipes. One of the distinct characteristics of ethanol is its relatively low boiling point of about 78.37°C (173.07°F). This means that ethanol can easily transition into a gaseous state at room temperature.
When you apply a wipe containing ethanol to the skin, left over ethanol molecules on the surface of your skin begin to gain energy from the surrounding environment. As these molecules acquire sufficient energy, they start to escape from the liquid state to the air.

As ethanol molecules leave the liquid phase, they take away energy in the form of heat, which leads to a decrease in temperature on the skin's surface. The faster evaporation rate compared to water makes ethanol very effective in creating a cooling effect and explains why your skin feels cooler after using ethanol-based wipes.

Phase Transition

When a substance changes from one phase to another, such as from liquid to gas, it undergoes a process called a phase transition. This process is essential in understanding many natural and industrial phenomena, including evaporative cooling.
In the case of ethanol, the liquid state transforms into a gaseous state as it evaporates from the skin. During evaporation, the ethanol molecules have to overcome intermolecular forces that keep them in the liquid phase. This requires energy, typically absorbed as heat from the surroundings.

Phase transitions like evaporation are dynamic and highly influenced by the environment, such as temperature and surface area. As more molecules transition to the gas phase, less liquid ethanol remains on the skin, continuously leading to a cooler feeling because of ongoing heat absorption from the skin. The energy absorbed during this transition results in the lowered temperature experienced during and after the phase transition.

Cooling Process

The cooling sensation you experience on your skin after using wipes containing ethanol is an example of evaporative cooling. This specific type of cooling process involves a liquid taking in heat energy from its surroundings to transition into a vapor.
When the liquid (in this case, ethanol) evaporates, it pulls energy in the form of heat away from your skin surface. This is because evaporation is an endothermic process—it requires energy to break the bonds of the molecules in the liquid, allowing them to become gas.

This phenomenon not only explains why your skin feels cooler but also highlights the practical use of ethanol in products designed to evaporate quickly, leaving no residue.

• Evaporative cooling is vital in regulating temperature environments naturally, such as the way sweating helps to cool the body.
• It's also a principle used in technology, such as cooling systems that rely on the evaporation of a liquid to dissipate heat.

By understanding the cooling process, one gains insight into everyday occurrences and the scientific principles behind them.