States of Matter

Liquefaction of Gases

Gases are difficult to transport. Due to their physical properties, it is almost impossible to transfer them from one place to another. For the same purpose, the gas is transformed into a liquid. Study of liquefaction of gases tells us about changes in the properties and structure of a gas. It also gives valuable information about the structure of matter in general. Let’s see how.

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Liquefaction is the transformation of a gaseous substance into its liquid state. This change is the outcome of change in physical conditions like temperature, pressure, and volume. Thomas Andrew was the first person to study the change of state from gases to liquids in Carbon Dioxide. It was later discovered that most real gases behave like Carbon Dioxide (CO2) and change from gases to liquids if optimum physical changes in temperature and pressure are achieved.

In his experiment on CO2, Andrews came to a conclusion that at high temperatures, despite high pressure, the gases cannot be liquefied. Also with the increase in temperature, the gases show significant deviation from the ideal behavior. In the case of carbon dioxide, at 30.98° C, the gas started changing into a liquid.

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Critical Temperature, Volume, and Pressure

Andrews in his experiment observed that above a specific temperature, the gas sample couldn’t be liquefied, howsoever high the pressure becomes.The critical temperature is the temperature at which a gas changes into liquid. With the increase in temperature, the pressure required to liquefy a gas also increases.  This temperature was the highest temperature at which a gas appears in the form of a liquid. It is the critical temperature or TC.

Critical constants play an essential role in the change of states of matter. Critical constants are critical pressure, temperature, and volume. The volume of one mole of a gas volume liquefied at critical temperature is known as the critical volume (Vc) while the pressure required to liquefy the gas at critical temperature is called as the Critical pressure  (pc).

Isotherm of Carbon Dioxide

The graph between the Pressure and Volume at a given constant temperature is the isotherm. On studying the isotherm of Carbon dioxide we get to know the different intervals of temperatures at which a gas can show signs of liquefaction:

Liquefaction of Carbon Dioxide

On studying the isotherm above we get to know the physical change of state temperature wise. Volume and pressure play a vital role in the change of state. In the above isotherm, we study the liquefaction of Carbon dioxide.

We see that the gaseous state of carbon dioxide changes to liquid at 30.98° C. The curve changes at a lower temperature, while at the higher temperature it does not show any change. At 30.98°C, the gas shows considerable deviation from the ideal gas behavior.

Equilibrium state

Here we notice that the curve at increased pressure signifies compressibility of liquid CO2 while the steep line pertains to the isotherm of the liquid, slightest of compression results in a sharp rise in the pressure, thus indicating the amount of compressibility of CO2. On attaining 21.5° Carbon dioxide behaves like a gas until point B.

The point B shows signs of liquid CO. The gas now exists in the dual form i.e both liquid and gas. At this stage compressing further does not affect the pressure on the gas, rather it results in condensation. At point C, all the COgas has condensed and further compression results in the rise in pressure.

From the above isotherm it is clear that at point A, CO2  exists in the gaseous state while at point D it exists in a liquid state. At point D the compression of the liquid CO2 is almost impossible. At point C an equilibrium state between the two states of matter is seen.

We further find that the behavior of all the gases is similar to COand this is because of the constant temperature or isothermal compression. This similar behavior shown by gases, in compression, at constant temperature is known as isothermal compression.

Gases to liquids

  • The critical temperature of the gas is the highest temperature at which the first occurrence of liquefaction of gas is seen.The critical temperature signifies the force of attraction between the molecules.The higher the critical temperature, higher is the intermolecular force of attraction and easier is the liquefaction of the gas.
  • Gases require cooling and compression both for liquefaction.

Now what gases need cooling and compression both for liquefaction? The gases which display a positive deviation from compressibility factor (Z) are permanent gases and they need both cooling and compression for the change in state.

We already know that the compressibility factor is the ratio of the original volume of a gas to the molar volume, now if the value of Z is in positive or greater than 0 then it shall need both cooling and compression for the change of state.

Understand what is Molar Volume in this Video

Effect of Compression and Cooling

Compression is the process of increasing pressure on the molecules of the gas. It brings the molecules close to each other. As soon as the molecules come in the vicinity of each other the reduced temperature slows the random movement of the molecules. This dual action of compression and cooling instigates intermolecular interactions. With the start of this intermolecular interaction, the molecules gradually and closely move toward each other leading to a change in the state.

Solved Example for you:

Q: The Critical temperature of Nis 126 K and He is 5.3 K. Which of the gases will liquify first?

Ans. N(nitrogen) liquifies easily. Intermolecular forces are high for gases which have a high critical temperature, making the change of state fast.

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