Physics

Sound

Gases Can be Liquefied by Applying Pressure and Lowering Temperature

When a high pressure is applied to a gas, it gets compressed (into a small volume), and when we also lower its temperature, it gets liquefied. So, we can also say that gases can be liquefied (turned into liquids) by compression and cooling. This happens as follows.
There is a lot of space between the particles of a gas. We can reduce the spaces (or distances) between the particles of a gas by enclosing it in a cylinder and compressing it by pushing in the piston [see Figures 49(a) and (&)]. If enough force (or pressure) is applied to the piston, the gas is highly compressed (into a small volume). The particles of gas get so close together that they start attracting each other sufficiently to form a liquid [see Figure 49(c)]. And we say that the gas has liquefied. When a gas is compressed too much, then heat is produced due to compression. So, while applying pressure to liquefy gases, it is necessary to cool them to take away the heat produced during compression. Cooling lowers the temperature of compressed gas and helps in liquefying it. From this discussion we conclude that gases can be liquefied by applying pressure and lowering temperature. In other words, gases can be liquefied by compression and cooling. A gas must be cooled below a certain temperature before liquefaction by pressure can occur. Cooling is usually done by pouring water over the coils carrying the compressed gas. Ammonia gas can be liquefied by applying high pressure and lowering the temperature. That is, ammonia gas can be liquefied by compression and cooling.
We have just studied that increasing the pressure and lowering the temperature can change the state of matter from a gas to a liquid.Even decreasing the pressure and raising the temperature can change the state of matter. This will become clear from the following example.Solid carbon dioxide (dry ice) is stored under high pressure. This is because on decreasing the pressure on solid carbon dioxide, it gets converted directly into carbon dioxide gas. For example, when a slab of solid carbon dioxide is kept exposed to air, then the pressure on it is reduced to normal atmospheric pressure (1 atmosphere), its temperature rises, and it starts changing into carbon dioxide gas.Thus, the conversion of solid carbon dioxide into carbon dioxide gas is a change of state (from solid to gas) which is caused by the decrease in pressure and higher atmospheric temperature. Solid carbon dioxide is a white solid called dry ice. Solid carbon dioxide is an extremely cold substance. It is used to 'deep freeze' food and to keep ice-cream cold. Since solid carbon dioxide directly changes into carbon dioxide gas (or sublimes), and does not melt to produce a liquid (like ordinary ice), it is called dry ice. Dry ice can produce much lower temperatures than that produced by ordinary ice. So, it is much more effective for cooling purposes than ordinary ice.

Notes

Sound - Notes

1. 3. Humidity of Air