L5 - heating Gases

objectives

Factors affecting Gases

• Unlike a solid or liquid, a gas does not necessarily expand when heated.
• That is because its volume depends on the container it is in.
• When dealing with a fixed mass of gas, there are always three factors to consider:
  • pressure
  • volume
  • temperature
• Depending on the circumstances, a change in temperature can produce a change in pressure, or volume, or both.

How pressure changes with temperature (at constant volume)

• As the temperature of the air rises, so does the pressure. This is because the molecules move faster. There is a greater change in momentum when they hit the sides of the flask, so a greater force:
• The cylinders used for storing gas are strong enough to withstand any extra pressure due to normal rises in temperature.
• It is dangerous to throw aerosol cans on bonfires because they might burst.
• However, that is mainly because more of the liquid propellant in the can turns to gas.

Comparing expansions of solids, liquids, and gases

• At constant pressure, gases expand much more than liquids which, in turn, expand more than solids.
• For example, for the same volume of material and the same rise in temperature (starting at room temperature):
  • Water expands 7 times as much as steel
  • Air (at constant pressure) expands 16 times as much as water.
• It is the strength of the attractions between the particles (molecules, for example) that makes the difference. In a solid, the attractions are very strong.
• If the temperature rises and the particles move faster, this has very little effect on their separation because they are so tightly held together.
• In a liquid, the attractions are weaker, so the expansion is greater.
• In a gas, the attractions are extremely weak, so the expansion is much more.