Question

When a person has a severe fever, one therapy to reduce the fever is an "alcohol rub." Explain how the evaporation of alcohol from the person's skin removes heat energy from the body.

Short answer

During an alcohol rub, the evaporation of alcohol from the skin removes heat energy from the body, helping to reduce fever. This occurs because alcohol has a relatively low boiling point, which allows it to evaporate quickly when applied to the warmer skin surface. As the alcohol evaporates, it absorbs heat energy from the skin, transforming from a liquid to a vapor and effectively removing heat from the body. This process can temporarily lower the body's temperature and provide relief from fever, but it should be used in conjunction with medical advice and proper treatment.

Step-by-step solution

Understand Evaporation

Evaporation is the process by which a liquid changes its state to a vapor or gas. This occurs when the molecules of the liquid gain enough heat energy to overcome the intermolecular forces holding them together. As a result, they move apart and become less dense, transforming into a vapor or gas state.

Examine the Variables Involved

In the case of the alcohol rub, three main factors are involved: the surface of the skin, the alcohol, and heat energy. The alcohol is applied to the skin, and the heat energy is exchanged between the skin (which is warmer due to the fever) and the alcohol.

Explain the Process of Evaporation in an Alcohol Rub

When alcohol is applied to the skin, it readily evaporates because of its relatively low boiling point (around 78.4°C or 173.12°F for ethanol). The heat energy required to evaporate the alcohol comes from the surface of the skin, making this a heat exchange process. As the alcohol gains heat energy from the skin, it evaporates, changing from a liquid to a vapor and removing heat from the skin.

Describe Heat Removal from the Body

The overall effect of the evaporation of alcohol from the skin is that heat energy is removed from the body, helping to lower the temperature of the skin. Since the human body primarily dissipates heat through the skin, this process can help to effectively reduce fever. By lowering the body's overall temperature, the alcohol rub can provide relief from a severe fever.

Understand the Benefits and Limitations of Alcohol Rubs

While alcohol rubs can help to reduce fever in some cases, they only provide temporary relief, and they should not be relied upon as a standalone treatment for fever. It is essential to consult a medical professional for appropriate treatment and monitoring.

Key concepts

Evaporation Process

Evaporation is a fundamental physical phenomenon where a liquid turns into a gas or vapor. This process occurs when molecules at the surface of the liquid gain sufficient energy to break free from the bonds that hold them in the liquid state. These molecules then escape into the air, leaving behind a slightly cooler liquid as the more energetic molecules are lost. Evaporation happens naturally at all temperatures, not just at the boiling point, but the rate of evaporation increases with temperature.

Contrary to boiling, evaporation is a surface phenomenon and doesn't occur throughout the liquid. It can be observed in our everyday life, for example, when a puddle of water dries up after the rain or when clothes dry after being hung out. In the context of an “alcohol rub” used to reduce fever, the alcohol applied to the skin evaporates quickly, taking away heat energy from the surface, thus cooling the body.

Heat Energy Transfer

Heat energy transfer is a core concept in thermodynamics that involves the movement of thermal energy from one place to another or from one substance to another. There are three primary modes of heat transfer: conduction, convection, and radiation. During evaporation, heat energy is transferred from the skin to the alcohol through a process akin to conduction.

The skin, which is warmer due to the fever, contains thermal energy. When alcohol is applied to the skin, this heat energy is absorbed by the alcohol, causing its molecules to move more rapidly and eventually evaporate. The loss of these high-energy molecules from the alcohol results in a net cooling effect on the skin, as the remaining liquid alcohol has a lower average kinetic energy.

Alcohol Rub Therapy

Alcohol rub therapy is a traditional method utilized to reduce fever by applying alcohol, typically ethanol or isopropanol, directly to the skin. Because these types of alcohol have a lower boiling point compared to water, they evaporate more quickly at body temperature. As the alcohol evaporates, it absorbs heat from the skin due to the heat energy needed for the phase transition from liquid to gas.

While alcohol rubs can provide a cooling effect and temporary relief from fever, it's important to note that they do not address the underlying cause of the fever. Furthermore, they can be dangerous if misused, especially in young children, and can lead to alcohol poisoning if the fumes are inhaled or the alcohol is absorbed through the skin. Therefore, modern medical advice often leans towards caution or against the use of alcohol rubs for fever reduction.

Boiling Point of Substances

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. This temperature is specific to each substance and can be influenced by atmospheric pressure; for instance, water boils at 100°C (212°F) at sea level but will boil at a lower temperature at higher altitudes where air pressure is lower.

The boiling point is also a reflection of intermolecular forces at play. A substance with strong intermolecular forces will have a higher boiling point because more energy is required to overcome these forces and allow the molecules to escape into the vapor phase. Alcohols, like ethanol, have a lower boiling point than water, which is why they evaporate faster when used in an alcohol rub treatment for fever.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles: atoms, molecules, or ions. These forces govern many properties of substances, including melting and boiling points, vapor pressure, and surface tension. In liquid alcohol, the major intermolecular forces are hydrogen bonds and Van der Waals forces.

Alcohols have the unique ability to form hydrogen bonds due to the presence of the hydroxyl group (-OH). While hydrogen bonds are stronger than Van der Waals forces, they are still weaker than the covalent bonds that hold atoms together within a molecule. The relatively weak intermolecular forces in alcohols contribute to their lower boiling points compared to water. This property plays a significant role in the effectiveness of alcohol rubs in fever therapy, as the alcohol's fast evaporation rate is a direct result of its weak intermolecular forces coupled with the added heat energy from the feverish body.