Key concepts
Physics
Evaporation
Heat transfer
Temperature
Introduction
Have you ever wondered why we sweat when our environment is hot or when we exercise? Sweating is a life-saving strategy that cools the body down and maintains its temperature. Without sweating, the body cannot regulate its 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?
Background
The process of changing a liquid into its 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 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.
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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 2,260 joules per gram, or 541 calories per gram, and it starts boiling at 100 degrees Celsius (212 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?
Materials
Rubbing alcohol
Water
Two small cups or bowls
Tablespoon
Pipet or medical dropper
Preparation
Fill one small cup or bowl with one tablespoon of water.
Fill the second small cup or bowl with one tablespoon of rubbing alcohol.
Procedure
Suck up some water from the first bowl (water) 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 the same quantity of liquid 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? Does your hand feel warmer or cooler compared with 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 (small) 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.
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 airflow will also support the heat transfer away from your skin. You should have noticed that your skin feels much cooler when you put the rubbing alcohol on your hand compared with the water. The water and the alcohol will start to evaporate once you start blowing on your hand. Compared with 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 with the alcohol.
This does not fit your observation that alcohol has a greater cooling effect than water, however. 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 with water due to its lower boiling temperature (82 compared to 100 degrees 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 airflow.
Cleanup
Flush any unused rubbing alcohol down the sink with plenty of cold water. Wash your hands with soap, and clean your work area.
More to explore Just Keep Cool—How Evaporation Affects Heating and Cooling, from Science Buddies
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
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
