How do liquids cool

For example, you could compare a very narrow and tall container with a very wide and short container. Keep both containers in the sun and monitor the evaporation rates by weighing the water over time.

Do you see any difference in the evaporation rates? How does the surface area affect evaporation? Based on your results, do you think more evaporation will happen in a large lake or small pond? Extra : Test how temperature affects evaporation rates. Use two containers of the same size with the same amount of water and put one in the sun and the other in the shade.

Let the water evaporate and observe the water levels over time. How much faster does the water in the sun vaporize compared to the one in the shade? Extra : Can you find ways to prevent evaporation?

Try to design something that you can put on the water surface to prevent evaporation from happening. Does your construction decrease the evaporation rate compared to a control? Did you feel the cooling power of water and rubbing alcohol? Both liquids should feel cold on your skin. Blowing on your wet hand helps the water and alcohol to evaporate. The air flow will also support the heat transfer away from your skin. You should have noticed that your skin feels much colder when you put the rubbing alcohol on your hand compared to the water.

Both the water and the alcohol will start to evaporate once you start blowing on your hand. Compared to water, alcohol has a lower heat of evaporation. That means that for the same amount of liquid, more heat transfer occurs during the evaporation of water compared to the alcohol. However, this does not fit your observation that alcohol has a greater cooling effect than water.

The reason for that is that the amount of heat transfer also depends on the evaporation rate. As alcohol evaporates at a much faster rate compared to water due to its lower boiling temperature 82C compared to C , it is able to carry away more heat from the skin.

This means for a given amount of time, much more alcohol evaporates than water. You probably noticed this also when you did the extra activity of putting the same amount of alcohol and water outside in the sun and monitored their evaporation rates.

Other factors that influence evaporation rates are the surface area, temperature, and air flow, which you might have investigated in the extra activities. Curious about the science? The energy needed for the transformation is known as the heat of evaporation. How much energy you need depends on factors such as the type of liquid or the surrounding temperature. If it is already very hot outside, you will need less energy to vaporize a liquid; if it is very cold, you will need more.

In order to turn into a gas the molecules held together inside the liquid have to break free to get into the air. This means the hydrogen bonds holding the molecules together need to be broken. Thus, molecules that are able to form lots of hydrogen bonds among themselves are much harder to turn into a gas and have a higher heat of evaporation.

This also affects the boiling temperature of a liquid. Molecules that attract one another very strongly start to boil at higher temperatures compared with those that have weak attractions. A lower boiling point generally means a liquid will evaporate more quickly. Water, for example, with one oxygen and two hydrogen atoms, can form two hydrogen bonds per molecule. Its heat of evaporation is 2, joules per gram, or calories per gram, and it starts boiling at degrees Celsius degrees Fahrenheit.

Your body makes use of the evaporative process when sweating. Sweat, which consists of 90 percent water, starts to evaporate. The necessary heat of evaporation is extracted from the sweat itself, which leads to a heat transfer from the liquid into the gaseous state. This results in a cooling effect called evaporative cooling that helps to maintain body temperature and cools the body down when it gets too hot. The degree of cooling is dependent on the evaporation rate and heat of evaporation.

In this activity you will find out which liquid has a greater cooling power: rubbing alcohol or water. What do you think will cool more when it evaporates? Observations and results Did you feel the cooling power of water and rubbing alcohol? Both liquids should feel cold on your skin.

Blowing on your wet hand helps the water and alcohol to evaporate. And if we were to zoom in on that sweat, if we were to zoom in on that sweat, we would see the constituent water molecules and sweat is mainly H It is mainly water. Now when we talk about the temperature of something, we're talking about the average kinetic energy.

Each of the individual molecules, they all have different kinetic energies. They're all bouncing around in different ways and transferring the momentum in all different ways.

And so you can imagine a reality. Maybe this one has a fairly high kinetic energy. It's moving in that direction. This one has a lower kinetic energy. Maybe this one has a medium kinetic energy, moving in this direction. Maybe this one has a really high kinetic energy moving in that direction. And so we've already talked about how hydrogen bonds in water between the partially negative end and the partially positive ends. That's what keeps the water together as these things move past and flow past each other.

What gives the water its cohesion is these hydrogen bonds. But if all of a sudden-- remember, we're talking about the average kinetic energy-- but even if we're at room temperature, and the average kinetic energy isn't so hot, you might have individual particles, individual molecules that actually have quite a high kinetic energy and if they're in the right place, if they're near the surface and their kinetic energy is high enough to break the hydrogen bonds with neighboring water molecules, and to overcome the pressure in the atmosphere, so let's say that this is, these are just gas molecules in the atmosphere here.

But it's enough to break free and none of these things bounce into it and force it back to form hydrogen bonds. This thing could actually break free and enter and become water vapor.

If you gradually raise the temperature of a pot of water, the water boils off and turns into a gas steam. Up to a certain temperature, it is a liquid. When it reaches the boiling point, adding more heat just boils off more of the liquid, not raising the temperature.

After the liquid has boiled off, as you add more heat the gas can get a lot hotter than it could as a liquid. Evaporating liquids can cool other things, too. For instance, "evaporative cooling" is a system that is often used to cool small buildings. A fan blows the warm air in the building through a screen covered with cold water.