But evaporation rates also depend on how tight the bonds between those molecules are. Finally, a liquid whose molecules weigh more will tend to evaporate more slowly than one whose molecules weigh less. Molecules at a liquid's surface are attracted only "down" and "sideways" to other molecules. So it's easier to skip out at the surface than from down further in the molecular bunch.
The larger the surface, the more quickly and easily molecules make their escape. Water spilled in a pool on the floor will evaporate quickly. But the same amount of water in a long-necked bottle will take its time fleeing through the small, round surface.
Why does water evaporate more slowly than alcohol and many other liquids? Water H2O molecules are strongly bound together. These "hydrogen bonds" between the atoms of neighboring water molecules make it hard for individual molecules to break free from the bunch. So even when water has a lower molecular weight than another liquid, it may evaporate more slowly. 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.
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 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. This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. Therefore they evaporate faster.
Anonymous answered. Yes, rubbing alcohol molecules have less hydrogen bonding than water, so, even though the molecular weight of the alcohol is much higher than water, it evaporates easier.
Yes, because I researched it and did a project on it. Answer Question. Connect Connect Connect. All Topics Science.
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