Question

A doctor sprays a liquid on your skin to numb it before injecting a painkiller. You notice that the liquid evaporates quickly. You also notice that your skin feels cold. Explain why your skin feels cold.

Short answer

The skin feels cold because as the liquid sprayed on it evaporates, it absorbs heat from the skin. This heat absorption process, which is required for the liquid to change its state from a liquid to a gas, results in the cooling sensation felt on the skin. This is an example of evaporation being an endothermic process.

Step-by-step solution

Identify the process

Firstly, it's crucial to understand that the process taking place on the skin when the liquid is sprayed is 'evaporation'. Evaporation is the transformation of a liquid into a gas. In this case, the heat energy for evaporation is taken from the skin, causing it to feel cool.

Understand heat transfer and cooling

Evaporation is an endothermic process, meaning it absorbs heat. When the liquid sprayed on the skin evaporates, it absorbs heat energy from the skin to gain enough energy to change its state from a liquid to a gas. This heat that is absorbed from the skin lowers the skin's temperature, making it feel cool.

Relate to everyday experience

This is similar to when we sweat on a hot day. When our sweat (which is essentially water) evaporates, it pulls heat away from our skin, making us feel cooler. This is our body's natural way of regulating temperature and preventing overheating.

Key concepts

Understanding Endothermic Processes

When we experience a feeling of cold after a liquid on our skin evaporates, we are actually feeling the effects of an endothermic process. An endothermic process is a type of chemical reaction or physical change that absorbs heat energy from its surroundings. This is a fundamental concept in thermodynamics and plays a vital role in various natural and technological processes.

In the case of the liquid used by a doctor, the substance evaporates, taking in heat from the skin as part of its phase transition. It's similar to how melting ice absorbs heat, leaving the surroundings cooler. The energy absorbed is necessary to overcome molecular forces and allow the liquid particles to disperse into the air as a gas.

One way to grasp this idea further is by imagining a sponge absorbing water. Here, the sponge represents the liquid that is being evaporated, and the water signifies the heat energy. Just as the sponge gets heavier with water, the liquid requires more energy to transform into gas, which it absorbs from the skin.

Heat Transfer Explained

The concept of heat transfer is all about the movement of thermal energy from one thing to another. It's a core aspect of understanding why evaporating liquids cause cooling. Heat can be transferred in three ways: conduction, convection, and radiation. In our example, when the liquid on your skin begins to evaporate, heat transfer occurs through conduction, where energy moves from the warmer skin to the cooler liquid.

Imagine holding an ice cube; your hand feels cold because the heat from your hand transfers to the ice cube, causing it to melt. Similarly, as the liquid on your skin evaporates, it pulls heat from your skin, resulting in a cool sensation. It's important to realize that heat doesn't get 'used up' but rather gets moved from one place (your skin) to somewhere else (the evaporating liquid).

Practical Takeaway

• Heat always flows from something at a higher temperature to something at a lower temperature until thermal equilibrium is reached.
• During evaporation, the liquid's temperature doesn't necessarily increase; instead, its molecules with the highest kinetic energy escape as gas, leading to a drop in temperature.

State Change of Matter

The cooling effect you feel when a liquid on your skin evaporates is closely related to the state change of matter, namely from liquid to gas. Matter typically exists in three states: solid, liquid, and gas. These states are dependent on temperature and pressure conditions. When a substance undergoes a state change, it alters its physical form but not its chemical composition.

For instance, water can exist as ice (solid), liquid water, or steam (gas), depending on the temperature. When a liquid turns into a gas, as in evaporation, the particles must gain enough energy to break the intermolecular forces holding them in the liquid state. This energy uptake leads to a sensation of cooling, just like when water evaporates from damp clothes, leaving them cooler than before.

Simply put, during a state transition, if energy must be absorbed from the environment (like from your skin), the process cools the surroundings. Conversely, when a state change releases energy, like condensation (gas to liquid), it warms the surroundings.