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Title: How to Make Water Boil Without Heat.
Principle(s) Investigated: The effect of atmospheric pressure on the boiling point of water.
The relationship between water vapor and temperature.
Standards : 8.7(c)Students know that substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity.
8.5(d) Students know physical processes include freezing and boiling, in which a material changes form with no chemical reaction.
Materials: 500 ml Flask, thermmometer, vacuum pump, bell jar, syringe, tap water, food color, heat source.
Procedure: 1. Fill the 500 ml flask about 1/4th with tap water (using food color will enhance the visibility of the boiling water for your students. You can use a variety of colors at different lab stations).
2. Record the temperature of the water to verify that the water is at or near room temperature. The water can be above or below room temperature, but not boiling hot before the experiment. The higher the temperature of the water, the quicker the water should boil.
3. Encase or securely place the container filled with water in a vacuum device. The vacuum should be a vacuum pump. Make sure the container is clearly visible in the vacuum pump.
4. Turn on the vacuum device. A fully encapsulated container in a vacuum device may be able to create a pure vacuum or zero pressure.
5. Observe the bubbles beginning to appear. The water will have the appearance of boiling, only there is no external heat source or steam coming from the water. If the water temperature is very warm before the experiment, then the water will boil quicker. As pressure drops, the water's boiling point lowers below 212 degrees Fahrenheit or 100 degrees Celsius.
6. Turn off the vacuum. Once the vacuum is off and pressure has been equalized, the container with water can be removed. The water will not be hot to touch even though it was boiling. As soon as the pressure is off and the pressure returns to the external environment pressure, the water should stop boiling.
7. Check the temperature. The temperature of the water should remain the same temperature it was before the experiment. The experiment was a success if the vacuum boiled water at a non-boiling temperature.
Student prior knowledge: Boiling is a type of phase transition, in which there is rapid vaporization of a liquid. Using water as an example, the boiling point of water is the temperature at which the saturated vapor pressure of water is equal to the pressure exerted on the water by the surrounding environmental pressure. When the external pressure above water is reduced (like in a vacuum chamber), the vapor pressure needed to induce boiling is also reduced, and the boiling point of water decreases.
The boiling point of water is "fixed" by its temperature, and the vapor pressure of water at that temperature. So for example, if the water is at 100 C. the boiling point is 100 C. at a pressure of 1 atm (which is equal to 760 mm of Hg). If the temperature of the water is about 25 C. the vapor pressure will be about 25 mm of Hg, which is its boiling point at 25 C. We are accustomed to thinking of the "boiling point" as the boiling point at an applied pressure of 1 atm (or 760 mm of Hg). The boiling point is the temperature of the water at a predetermined pressure.
Below is an animation depicting atmospheric pressure on earth. As you can see atmospheric pressure varies with altitude, thus the amount of oxygen in the air varies with altitude also. Standard atmospheric pressure is 1atm at Earth's surface. This would be the pressure on water by the environment. When the atmospheric pressure decreases (vacuum chamber), the boiling point of water decreases as well. When the bubbling action of boiling water occurs, these bubbles are rising and escaping because the air pressure above the water is not great enough to keep the bubbles from escaping. In outer space, with no air at all (a total vacuum), it would be impossible to keep the water together long enough to apply any heat, thus, any liquid water would begin to boil immediately.
Explanation: In this demonstration it will be shown that water can boil at a temperature below the standard temperature of 100 C because of a reduction of air pressure. In general as the pressure decreases, so does the temperature required to boil a liquid. The boiling point at the top of a mountain is lower than it is at the bottom, because the air pressure is lower at the top.
Questions & Answers: What is the relationship between water vapor, temperature and the boiling point of water?
Why do the bubbles in boiling water rise to the surface and escape?
Applications to Everyday Life:
1. People who mountain climb need to know the relaionship between air pressure and the boiling point of water. When cooking, most cooking times will increase because of the lowered air pressure at the top of a mountain. If campers want to eat well cooked meals these principles will help in the kitchen or over a camp fire.
2. Astronauts drink water on the International Space Station just as they do on earth. But in space there is a vacuum so any water pored into a drinking class would immediately boil. Space travelers have learned that they must drink water from a sealed container using a straw.
Sources: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html
http://en.wikipedia.org/wiki/Properties_of_water
http://en.wikipedia.org/wiki/Boiling_point
http://www.devillier.com/ab_zero/get_involved/resources/topic_states_of_matter.htm
Photographs:
Videos:
http://www.youtube.com/watch?v=Ks4VuXTTKmo
http://www.youtube.com/watch?v=QGSo7zcg_qA
http://www.youtube.com/watch?v=I3DA80Xb_f8
http://www.youtube.com/watch?v=XoOQNwcrDWE
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