Metadata:Vatten, avdunstning, energi, lärarversion, svensk
How can you cool a soda at the beach?
(teacher version)
Abstract
This activity aims to show that evaporation is a chemical process that requires energy and how people can be
advantaged or disadvantaged by this.
Theory
Evaporation means that a liquid becomes a gas without boiling. When a liquid evaporates the fastest molecules leave
the liquid, the energy molecules needed for this is taken from the surroundings which may lower the temperature.
Evaporation explained at the molecular level
Phase transition when liquid water turns into water vapor through evaporation occurs by water molecules in the
liquid water moving around in all directions at different speeds and collide. In collisions may be some molecules gain
kinetic energy (kinetic energy) while others lose kinetic energy. The molecules that have a maximum kinetic energy
and therefore move quickly, leave the liquid phase in the form of water vapor. In the remaining liquid molecules lose
energy and thus move slowly, this can be read as a temperature decrease. Evaporation is thus a form of cooling.
The relative humidity controls evaporation and condensation
Evaporation takes place as long as the relative humidity is below 100%, which means that the vapor pressure above
the liquid is low, so the speed of the molecules is enough for leaving the liquid. How much water the air can contain
depends on the temperature, the warmer the air, the more water it can contain. Water molecules in the hot air has
higher speed and can bounce away from each other in collisions, rather than as the colder air trapped in each other
and form water droplets. Humidity is therefore maximal (100%) when the maximum amount of water vapor for that
temperature is reached, ie. the air is saturated.
The body sweats to cool itself
Also the human body uses evaporation to cool down. When the body gets too hot, sweat glands secrete fluid, the
liquid evaporates and thus cools the body to maintain normal body temperature. The people who do not sweat as
easily, have other ways to cool off, such as blood vessels that comes closer to the skin, which is perceived as red. Dogs
only have sweat glands between the pads, and rather than the body cools by evaporation, the dog opens the mouth
and windpipe to lose heat. Other animals have no sweat glands, such as pigs, which rolls in mud to cool the body.
Experiment
The water in the bottle is cooled by the fast molecules with high kinetic energy leaving the lining around the
bottle. The lining is cooled down and the heat conduction cooled the bottle, and finally even the water inside the
bottle. Heat from the water then headed inside the bottle to the ambient air. The water in the bottle can be kept
refrigerated for as long as the lining is wet and the humidity low enough for evaporation to happen. A large mantle
area relative to the volume is required for faster cooling, and also if the wind moves. The beaker’s thermal
conductivity also plays a role, and aluminum (metal) quickly removes the heat.
Detta verk är licensierat under en Creative Commons Erkännande 2.5 Sverige Licens. Les mere om projektet på www.dlis.eu.
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Metadata:Vatten, avdunstning, energi, lärarversion, svensk
Technical information
Only a data logger with two temperature sensors are needed in this experiment. Temperature
sensors do normally not need calibration. It is good to use as much of the coordinate system as possible, and this can
be done by selecting the auto-scale on the logger(see manual).
Equipment
In addition to the data logger with 2 temperature sensors, you need two bottles and a wet wrapping around one
bottle to perform the experiment.
Performance
Room tempered water of the same volume of is poured into the two cylinders. At the beginning of the experiment the
wet wrapper is wrapped around one of the bottles.
Questions for discussions
• Why does the curve of the bottle without wet cover, increase?
• Can the contents of the bottle with wet cover, become endlessly cold? Extend the curve how you believe it will look
like in a few hours. Why does it look like that?
• Why should I remove my bikini/swimwear after a swim?
• Why shouldn’t you wear cotton close to your body when you are sweating? Options?
• How can you cool you down when you feel overheated?
• When you are you on holiday, it might different climates on earth. It could be like your towel dries quickly or it
almost never dries, even though it’s the same temperature. Explain this phenomenon?
• In some parts of the world you perceive the temperature higher than in other parts with the same
temperature. Explain this.
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Metadata:Vatten, avdunstning, energi, lärarversion, svensk
Variations of the experiment
• What happens to the temperature when the wind blows?
• Does it matter how the bottle is designed? That is, if the bottle is tall and narrow or low and broad?
• Does it matter what material the bottle is made up of? (Glass, aluminum)
Results
Note: The graph shows clearly that the bottle with wet covers have a clear temperature decrease, while the second
bottle will be warmed to room temperature, after a while.
Detta verk är licensierat under en Creative Commons Erkännande 2.5 Sverige Licens. Les mere om projektet på www.dlis.eu.
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