# What are four important properties of gas

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II. The gas laws

### Ideal gases

Definition of an ideal gas:

Fig.20

1. Rigid spheres are seen as a model for the gas particles under consideration (atoms, molecules), i.e. the gas particles should not be deformable.
2. The collisions of the particles with one another and with the wall should be completely elastic, i.e. no energy whatsoever is lost.

3. The available space should be infinitely large, i.e. the gas can expand infinitely.

Fig 22
4. The diameter of the particles should be infinitely small, i.e. the gas consists of particles whose diameter is very small in contrast to their distance from one another and in relation to the size of the room.

5. The particles have no interactions with one another (no attraction or repulsion), which means that they are electrically absolutely neutral (no dipoles).

• These prerequisites can be roughly achieved by taking a gas whose density is very small and whose temperature is so high that its molecules move almost like spherical particles that can collide elastically, but otherwise do not exert any forces on one another.
• The general gas equation applies to an ideal gas.
• Real gases do not behave like ideal gases.
• When heated by 1 C, the ideal gas expands by the 273rd part of its volume that it has at 0 C. If it cannot expand, its pressure increases (see Gay-Lussac's law).
• All ideal gases contain the same number of molecules under the same conditions (same pressure, same temperature and same volume) (Avogadro's hypothesis).
• One mole of each ideal gas takes up the space of 22.4 liters under normal conditions (temperature = 0 C and pressure = 1.013 bar).
• The state of an ideal gas is described by volume, pressure and temperature. The experiments of many scientists on the effects of changes in these three variables have led to the establishment of the ideal gas laws (see Boyle-Mariotte's gas law and Gay-Lussac 's gas law).