Atmospheric Pressure

A series of free Science Lessons for 7th Grade and 8th Grade, KS3 and Checkpoint Science in preparation for GCSE and IGCSE Science.

Atmospheric Pressure
The atmosphere exerts a pressure on you, and everything around you.
Atmospheric pressure changes with altitude. The higher you go:

• the lower the weight of the air above you
  
• the lower the atmospheric pressure
  At sea level, atmospheric pressure is about 10N/cm², but it is less at the top of very high mountains.
  
  Atmospheric Pressure and Boiling
  The pressure of gas above a liquid affects the boiling point. This is called atmospheric pressure. The greater the pressure, the more energy required for liquids to boil, and the higher the boiling point.
  Higher Pressure = More Energy Required to Boil = Higher Boiling Point
  This can be visualized as air molecules colliding with the surface of the liquid and creating pressure. This pressure is transmitted throughout the liquid and makes it more difficult for bubbles to form and for boiling to take place.
  If the pressure is reduced, the liquid requires less energy to change to a gaseous phase, and boiling occurs at a lower temperature.

Vapor Pressure and Boiling
The molecules leaving a liquid through evaporation create an upward pressure as they collide with air molecules. This upward push is called the vapor pressure.
Different substances have different vapor pressures and therefore different boiling points. This is due to differing intermolecular forces between molecules.
The vapor pressure of a liquid lowers the amount of pressure exerted on the liquid by the atmosphere. As a result, liquids with high vapor pressures have lower boiling points.
Vapor pressure can be increased by heating a liquid and causing more molecules to enter the atmosphere. At the point where the vapor pressure is equal to the atmospheric pressure boiling will begin. In effect, without any external pressure the liquid molecules will be able to spread out and change from a liquid to a gaseous phase. The gas, as bubbles in the liquid, will rise to the surface and be released into the atmosphere.

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Boiling cold water In a Vacuum Chamber
Cold water at about 23 degree Celsius water is placed in a vacuum chamber and boils when the pressure in the chamber approximates the vapor pressure. A thermometer is placed inside the chamber and a decrease in temperature is observed as it boils.

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