Evaporative Cooling with Liquids
Have you ever wondered why we sweat when our environment is getting hot or when we exercise? Sweating is a life-saving strategy that cools our body down and maintains our body temperature. Without sweating, our body cannot regulate our temperature, which can lead to overheating or even heatstroke. But why does sweating have a cooling effect? The answer is evaporative cooling. Turning a liquid such as sweat from its liquid state into a gas requires energy. This energy is taken from our body, or sweat, in the form of heat. The resulting heat transfer leads to the desired cooling effect. In this activity, you can observe this cooling power in action—ready to get cool?
The process of changing a liquid from its liquid state into a gaseous state is called evaporation. Every liquid can be turned into a gas if enough energy is added to the liquid in the form of heat. The energy that is needed to transform a liquid into a gas is known as the heat of evaporation. How much energy you need depends on factors like the type of liquid or the outside temperature. If it is already very hot outside, you will need less energy to vaporize a liquid, and 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 that the hydrogen bonds holding the molecules together need to be broken. Thus, molecules that are able to form lots of hydrogen bonds between 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 each other very strongly start to boil at higher temperatures compared to molecules that have weak attractions. A lower boiling point generally means that a liquid will evaporate quicker. Water, for example, with one oxygen atom and two hydrogen atoms, can form two hydrogen bonds per molecule. Its heat of evaporation is 2260 Joules per gram, or 541 calories per gram, and it starts boiling at 100C (212F).
Your body makes use of the evaporation process when sweating. We usually start to sweat when our body temperature reaches about 37C (or 98.6F). Sweat, which consists of 90% water, then 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?
- Rubbing alcohol
- Small cups or bowls (2)
- Your hands
- Pipet or medical dropper
- Fill one small cup or bowl with one tablespoon of water
- Fill the second small cup or bowl with one tablespoon of rubbing alcohol
- Suck up some water from the first bowl using the pipet or medical dropper.
- Carefully drop one or two drops on the back of our hand and spread the liquid with your fingers. When the water touches your skin, how does it feel?
- Blow softly over the skin area that you just covered with water. Does your skin feel any different when blowing on the water? Can you sense a difference in temperature while blowing? How does it feel?
- Rinse your pipet with some rubbing alcohol and then suck up some of the alcohol with your pipet.
- Drop one or two drops on the back of your other hand and spread the liquid with your fingers. Does the alcohol feel different when it touches your skin? How?
- Again, blow over the area on your hand where you put the alcohol. What sensation do you feel this time? Does your hand feel warmer or colder compared to water when blowing on the liquid? Can you think of a reason why?
Extra: Find out how fast rubbing alcohol and water evaporate. Put the same amount of water and rubbing alcohol in two different cups and place them both in the sun. Observe how long it takes for the liquids to completely evaporate (depending on how warm it is, this might take some time). Which liquid vaporizes faster? You can even determine the evaporation rate by weighing the cups in the beginning and throughout your experiment to find out how much water is lost due to evaporation.
Extra: Evaporation also happens in lakes, the ocean, and reservoirs, and contributes to significant drinking water loss worldwide. Find out how the surface area of a body of water influences evaporation rates. Put the same amount of water in different sized containers to create a different water surface area for each of them. 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?
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. 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 100C), 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.
Ask an Expert
- Flush down the unused rubbing alcohol in the sink with plenty of cold water. Wash your hands with soap and clean your work area.
- How Sweat Works, from How Stuff Works
- Specific heat, heat of vaporization, and density of water, from Khan Academy
- Perspiration Cooling of Body, from HyperPhysics
- Heat of vaporization of water and ethanol, from Khan Academy at Youtube
- Just Keep Cool - How Evaporation Affects Heating and Cooling, from Science Buddies
- Science Activities for All Ages!, from Science Buddies