Boiling point of water
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 astronaut’s blood boil.

Our atmosphere acts as a safety blanket around the Earth. It:
- protects us from U.V. radiation from the sun, via the ozone layer.
- moderates the temperature of Earth’s surface.
- it keeps water from boiling at very low temperatures.
- provides oxygen for us to breathe.
The astronaut’s space suit actually acts like an atmosphere protecting the astronaut from the dangers of space.

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.

Reducing the air pressure inside the flask is easy. The pressure drops when an ice cube is placed on top.
Another demonstration can be conducted by boiling water in a flask. Allow the water to boil for a little while so that most of the air is driven out of the flask. Stopper the flask and turn it upside down on the bench. Place an ice cube on its base and watch the water inside the flask start to boil. The ice cube causes the air pressure inside the flask to drop.

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?

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.