Question

Use your knowledge of changes of state to explain the following: a. Why is a spray, such as ethyl chloride, used to numb a sports injury during a game? b. Why does water in a wide, flat, shallow dish evaporate more quickly than the same amount of water in a tall, narrow glass? c. Why does a sandwich on a plate dry out faster than a sandwich in plastic wrap?

Short answer

Ethyl chloride numbs by rapidly evaporating and cooling. Water in a wide dish evaporates faster due to more surface area. A sandwich dries out faster on a plate because it is more exposed to air.

Step-by-step solution

Explain why ethyl chloride is used to numb a sports injury

Ethyl chloride is used to numb a sports injury due to its volatile nature. When sprayed on the skin, it evaporates quickly, absorbing heat from the skin and the surrounding area. This rapid evaporation causes a cooling effect, lowering the temperature of the skin and underlying tissues, which helps to numb the pain.

Explain why water evaporates faster in a wide, flat dish

Water in a wide, flat, shallow dish evaporates faster than in a tall, narrow glass because a greater surface area is exposed to the air. Evaporation occurs at the surface of the liquid, so more surface area leads to more molecules escaping into the air, increasing the evaporation rate.

Explain why a sandwich dries out faster on a plate

A sandwich on a plate dries out faster than a sandwich in plastic wrap because the plastic wrap limits the exposure to air. The limited air exposure in the wrapped sandwich reduces the rate of moisture loss, while the unwrapped sandwich has a larger surface area exposed to the air, leading to faster evaporation of moisture.

Key concepts

Evaporation

Evaporation is the process where liquid turns into vapor. This happens when the molecules in the liquid gain enough energy to break free from the surface and enter the air. When ethyl chloride spray is applied to a sports injury, it evaporates quickly because ethyl chloride is volatile. Volatile substances transition to a gas phase easily at lower temperatures. As ethyl chloride evaporates, it absorbs heat from the skin, causing a cooling effect. This is why it's used to numb pain during sports injuries. The faster evaporation absorbs more heat, providing a faster cooling effect and immediate relief.

Heat Absorption

Heat absorption plays a crucial role in state changes. During evaporation, heat from the surrounding environment is absorbed. This is a form of heat transfer known as endothermic reaction. Taking the example of ethyl chloride, as it evaporates, it absorbs heat from the skin.
This happens because the energy required to turn liquid ethyl chloride into vapor has to come from somewhere; in this case, it’s taken from the skin. This lowers the temperature of the skin and numbs it.
Another example is water in different containers. If heat is constantly absorbed by the water surface, it will lead to quicker evaporation.

Surface Area

The surface area significantly affects evaporation rates. Imagine water in two different containers: a wide, flat dish and a tall, narrow glass.
In a wide, flat dish, more water surface is exposed to the air. Because evaporation happens at the surface, a larger exposed surface means more water molecules can escape into the air. This leads to faster evaporation.
Conversely, in a tall, narrow glass, the surface area is smaller. Hence, fewer water molecules escape into the air, slowing down the rate of evaporation. Therefore, surface area is directly proportional to the evaporation rate.

Moisture Loss

Moisture loss is a common issue in everyday scenarios, like drying out food. When a sandwich is left on a plate, it's exposed to air from all sides. This larger surface area exposure increases the rate of moisture loss due to faster evaporation. The sandwich loses its moisture quickly and dries out.
On the other hand, if you wrap a sandwich in plastic, the plastic acts as a barrier. It limits the exposure to air, reducing the surface area available for evaporation. This leads to slower moisture loss, keeping the sandwich fresher for longer. Proper packaging can significantly slow down the moisture loss in food items.