# Why are gases considered liquids

So far we have only discussed pressure in solids and liquids. In contrast to gases, solids and liquids are incompressible. We have already considered this fact in the experiment with the water devil. We have also found that gas has neither shape nor volume elasticity. This was due to the fact that the molecules of a gas, in contrast to the solid and liquid states of aggregation, have a relatively large distance. The intermolecular forces are therefore extremely small in gases. In the borderline case of the so-called ideal gas, these forces are completely neglected.

As a model of an ideal gas, one can imagine a closed volume in which elastic balls perform random movements without touching each other. Of course, the balls are not always evenly distributed over the room. If one imagines the pressure in such a way that the balls hit the vessel wall, this uneven distribution results in statistical fluctuations in the force that a trapped gas exerts on a piston. The pressure propagation in gases, just like the pressure propagation in liquids, takes place in the same way in all directions. Since in the ideal case no repulsive forces act between the molecules of a gas, gases can be compressed very strongly compared to liquids. The compressibility is very high, but they no longer have any elasticity in volume. Experience shows that you still have to apply a not inconsiderable pressure to change the volume of a gas. Empirically one finds for ideal gases that at constant temperature double the pressure has to be applied in order to halve the volume. Of course, this only applies if the amount of gas is kept constant. This law becomes Boyle-Mariotte's Law or even just Boyle's law called.

A detailed examination of this law is given later in the kinetic gas theory performed.
With this law we can state the compressibility of a gas: In general, we had the formula for compression
= derived.
From Boyle-Mariott's law
V = p0 V.0
follows
.
This applies

This means that the compressibility of ideal gases at low pressures is very high. The greater the pressure, the lower the compressibility. To prove the air pressure, spectacular experiments were carried out as early as the 17th century. Otto von Guericke (1602 - 1686) made one attempt: the attempt ofMagdeburg hemispheres. In this attempt before the Reichstag in 1654 Otto von Guericke placed two hemispheres loosely on top of one another. The hemispheres were then pumped out. A rope was attached to each half of the ball, and 8 horses were pulling the rope on each side. The strength of the 16 horses was not enough to pull the emptied hemispheres apart against the air pressure. We want to show a similar experiment in the lecture:
Since we have no horses available for the lecture, we have to conduct the experiment with professors. Of course, we use smaller hemispheres to keep the proportions. But even our professors fail to pull the hemispheres apart against the air pressure.
The air pressure was first quantified by Toricelli in 1643. We carry out the following experiment for the quantitative investigation of the air pressure:

For this experiment we use a tube that is closed with a lockable piston. The pipe can be pumped out by pulling out the piston. Instead of pulling the piston with a rope, we now use a spring balance. This means that it is possible to read directly which force is acting, and it can be calculated which pressure is prevailing. Even without measuring exact values, one can observe that the compression of the piston is easier the more volume is still enclosed. If the piston is pushed very far into the tube, further compression is very difficult to achieve.