Boiling
point of water |
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Water
will boil at a temperature of 100C. This temperature however, depends on
the air pressure of the atmosphere. At
low air pressure the water boils at temperatures significantly less than
100C. At sea level the boiling point of water is 100C but on Mount Everest,
where the air pressure is significantly less, water will boil at 60C. This
is not really of great concern for us, but out in space where there is no
air, it can actually make the astronauts blood boil.
Our atmosphere acts
as a safety blanket around the Earth. It: |
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Demonstrating
the link between air pressure and boiling point of a liquid is easy. Set
up the syringe as shown on the left. Place hot water from the tap in the
syringe. Push the end of the syringe into a stopper and raise the plunger.
The liquid will start to boil. Raising the plunger lowers the air pressure inside the
syringe and thus allows the water to boil at a less than 100C . Click to see how this works. |
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1) Explain why placing an ice cube on the glass reduces the air pressure. Use the words: water vapour, condensation, molecule, vacuum. 2) How would the world be different if the atmospheric pressure was half of what it is now on the surface of the Earth? |
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3) Consider the animation on the right. It shows how the vapour pressure of water increases as heat is applied. It increases until it equals the pressure at the surface of the liquid at which point it starts to boil. At 47 mmHg the boiling temperature of water is 37 oC. During a training mission the cabin containing the crew suffered sudden depressurization. The astronaut complained that the saliva in his mouth started to boil. However the water in his blood did not start to boil even though his body temperature was high enough to cause the water to boil. Explain why.
Hint. Think of how the body maintains blood pressure. |