39. Explain why sweating cools you down in terms of molecular kinetic energy.
40. You take a pizza out of the oven and let it cool briefly. When you bite into it, the crust is not too hot but the cheese burns your mouth. Explain why this happens in terms of specific heat capacity.
39. KE = 1/2*m*v^2.
As the more energetic molecules of water (from the sweat) leave due to evaporation, the average energy of the remaining molecules is decreased and lower energy means lower T. Another way of saying the same thing is that energy to help the molecules escape (evaporate) must come from someplace and that place is from the sweat itself. That lowers the temperature of the remaining water.
40. Specific heat of something is how much energy it takes to heat it up or the energy lost in cooling it. In general terms, low specific heat materials heat up fast and cool down fast. An example is a piece of metal--just a little heat added is enough to get it hot enough to burn your finger. Conversely, just a little time is all it needs to cool down so the metal can be picked up safely. For high specific heat materials (water is an example), just a little added heat hardly raises the temperature at all so a lot of heat is required to heat a pot of water BUT a lot of time must go by before it is cool enough to drink. Think of coffee.
All of that is a preamble. The crust has a low specific heat; cheese has a high specific heat.
39. When you sweat, the process of evaporation plays a key role in cooling down your body. Sweating involves the release of water from your sweat glands onto the surface of your skin. The water on your skin then evaporates into the air, absorbing energy in the process.
In terms of molecular kinetic energy, the water molecules on your skin have different kinetic energies. Some molecules have higher kinetic energy, which means they move faster and have more energy. When these high-energy water molecules escape into the air as vapor, they effectively take away some of the heat from your body.
The evaporation of sweat cools you down because it requires energy to break the intermolecular forces between water molecules. This energy is obtained from the surroundings, mainly your skin and body. As a result, the remaining water on your skin and the underlying tissues have lower kinetic energy, resulting in a cooling effect.
40. The difference in temperature experienced when biting into a pizza, where the crust is not too hot but the cheese burns your mouth, can be explained using the concept of specific heat capacity.
Specific heat capacity refers to the amount of heat energy required to raise the temperature of a substance per unit mass. Different substances have different specific heat capacities, meaning they can absorb different amounts of heat energy before their temperature changes.
In the case of a pizza, the crust and cheese are likely made of different materials with different specific heat capacities. The crust, typically made of bread or dough, has a higher specific heat capacity compared to the cheese. This means that the crust requires more heat energy to raise its temperature compared to the cheese.
When the pizza comes out of the oven, the crust has absorbed most of the heat energy during baking, resulting in a higher temperature. As a result, the crust cools down more slowly compared to the cheese. By the time you take a bite, the crust has had more time to release some of its heat energy, making it feel less hot. On the other hand, the cheese, having a lower specific heat capacity, retains more heat energy and remains at a higher temperature, causing it to burn your mouth when you bite into it.