Content Curator
Critical Pressure of a substance is the pressure applied to liquefy the substance at its critical temperature. It is also defined as the pressure at the critical point. Here in this article, let’s discuss critical pressure, partial pressure, liquefaction of gases, triple point and some important questions.
Read Also : Three States of matter
Partial Pressure
Partial Pressure is defined as the pressure exerted by each of the gases within the container filled with more than one gas.
The pressure that is exerted by one gas of the mixture of gasses if it occupies the same volume on its own is known as Partial pressure. Every gas exerts some amount of pressure in a mixture. The total pressure of all the ideal gas is the sum of partial pressures of individual gases in the mixture. This can be expressed by the following equation:
Vx / Vtotal = Px / Ptotal = Nx / Ntotal
Where,
Vx is the partial pressure of a particular gas.
Px is the partial pressure of the gas x.
Vtotal is the total volume of the mixture.
Nx is the amount of gaseous substance.
Ptotal is the total pressure of the mixture.
Ntotal is the total amount of substance in a mixture.
Critical Pressure
Critical Pressure of a substance is the pressure that corresponds to the critical point of the substance.
“The minimum pressure that must be applied to liquefy a gas at its critical temperature is called critical pressure.” The critical pressure of a substance is also denoted as PC.
The relation between critical pressure and Van der Waals equation is given as:
PC= a/ 27b2
Critical Pressure and States of Matter
There are three states of matter; solid, liquid and gas. They all are separated by phase boundaries. These phase boundaries are influenced by two factors– temperature and pressure. So, changing the temperature-pressure combinations can help pinpoint a phase boundary.
In the case of solids, as we know, the particles inside any object are extremely close and tightly bound. In the case of liquids, the particles are able to move around freely though there is some force of attraction between the particles which contains them in a particular form( fluids).
In the case of gases, the particles of any gas are apart from each other and there is a very little amount of force holding the gas particles together.
Critical Temperature (TC)
The characteristic temperature of a gas above which the gas cannot be liquefied by applying pressure is called critical temperature. The relation between critical temperature and Van der Waals equation can be given as
TC= 8a/ 27RB
Critical Volume(VC)
The volume occupied by 1 Mole of gas at its critical pressure and critical temperature is called the Critical Volume. The relation between critical volume and Van der Waals constants is
Vc = 3b
Critical Pressure: Liquefaction of Gases
The phenomenon of converting gas into liquid by lowering the temperature or by increasing the pressure or both is known as liquefaction of gases.
Gases like CO2, HCl, NH3, Cl2, and SO2 get liquefied at room temperature when sufficiently high pressure is applied. Such gases are called temporary gases.
Gases like He, H2, N2, O2, CO and CH4 don't liquefy at room temperature no matter how much high pressure may be applied. These gases are called permanent gases”.
Critical Pressures of Some Common Substances
| Substances | Critical Pressure (PC) (In Bar) |
| Air | 37.858 |
| Argon | 48.7 |
| Carbon di-oxide | 73.8 |
| Chlorine | 77.1 |
| Helium | 2.3 |
| Hydrogen | 13.0 |
| Nitrogen | 34.0 |
| Water | 220.5 |
Critical Pressure: Triple Point
Every matter has three common states which are solid, liquid, and gas. The physical state of matter is due to two primary factors, temperature, and pressure. Each substance has a different phase boundary, and by adjusting the temperature-pressure combinations, the phase boundary of any substance can be marked.
Some substances can exist in all three states of matter. The point at which a substance can exist in all three states is called the Triple Point.
Critical Pressure: Triple Point of Water
Water exists in three states; solid(ice), liquid(water), gas(water vapour). The triple point of water is defined as the temperature and pressure at which liquid water, solid ice and water vapour can coexist in a stable equilibrium.
The thermal stable equilibrium is at 293.1600 K and partial vapour pressure of 611.657 Pascals. Even if the total pressure of a system is above the triple point of water, provided that the partial pressure of the water vapour is 611.657 Pa, then the system can still be brought to the triple point of water.
Critical Pressure of Fluid
The critical pressure of a fluid can be defined as the vapour pressure of the fluid at its critical temperature. Above the critical point, distinct gas and liquid phases do not exist. While attaing the critical temperature of a substance, the properties of the gases and liquid phases are almost similar, resulting in only one phase.
Below graph indicates the triple point and critical point of any substance.
Critical Pressure: Things to Remember
- Critical Pressure is defined as the minimum pressure that is needed to be applied to liquefy a gas at its critical temperature.
- It is also defined as the pressure that corresponds to the critical point of the substance.
- The phenomenon by which the gas is converted into a liquid by lowering the temperature or by increasing the pressure or both is known as the liquefaction of gases.
- The triple point is the point at which a substance can exist in all three states.
- The temperature above which the gas cannot be liquefied by applying pressure is called critical temperature and the volume occupied by 1 Mole of gas at its critical pressure and critical temperature is called the Critical Volume.
Critical Pressure: Important Questions
Ques: What is the Critical Pressure of a Fluid? (1 Mark)
Ans: The critical pressure of a fluid can be defined as the vapor pressure of the fluid at its critical temperature (above which point, distinct gas and liquid phases do not exist). While approaching the critical temperature of a substance, the properties of the gaseous and liquid phases are known to become the same, resulting in only one phase.
Ques: What is meant by the term ‘critical point’ in thermodynamics? (1 Mark)
Ans: In thermodynamics, the term ‘critical state’ or ‘critical point’ is used to denote the endpoint of a phase equilibrium curve. For example, the critical point between the liquid phase and the vapour phase defines the conditions under which a liquid substance can coexist with its vapour. At a high enough temperature point (above the critical temperature), a given gas cannot be liquefied by simply applying pressure to it.
Ques: What is the Critical Point of Water? (1 Mark)
Ans: The critical point of water occurs at a temperature of 647 Kelvin (which is equal to 374 degrees celsius and 705 degrees fahrenheit) and a pressure of 22.064 MPa (which is roughly equal to 218 atmospheres of pressure).
Ques: What is the importance of critical temperature? (1 Mark)
Ans: The critical temperature of a gas shows the strength of the intermolecular forces of attraction that its particles are subject to. For example, a gaseous substance with relatively weak intermolecular forces will be harder to liquefy than a gaseous substance featuring stronger intermolecular forces of attraction. Therefore, the weaker the intermolecular forces, the lower the critical temperature.
Ques: What happens at temperatures above the critical temperature? (1 Mark)
Ans: At temperatures that are higher than the critical temperature of a substance, the molecules of the substance are known to have too much kinetic energy for the intermolecular forces of attraction to bind them together in the liquid phase. Thus, above the critical temperature, the substance cannot be liquefied.
Ques: What is Triple Point? (1 Mark)
Ans: Matter has three common states: namely solid, liquid, and gas. The physical states of these substances are affected by two primary factors: temperature, and pressure. Each substance has a phase boundary, and by adjusting the temperature-pressure combinations, we can find the phase boundary of any substance. Some substances have a triple point, at which the substance can exist in all three states of matter.
Ques: What is Critical State? (1 Mark)
Ans: In thermodynamics, the critical state is the endpoint of a phase equilibrium curve of any substance. The critical state is the endpoint of the pressure-temperature curve that determines when liquid and its vapour can coexist. We know that, at higher temperatures, gas cannot be liquefied by pressure alone. Therefore, the critical temperature and a critical pressure PC, the phase boundaries vanish.
Ques: What is Critical Pressure? (2 Marks)
Ans: The critical pressure of a substance is the pressure that must be applied in order to liquefy that substance at its critical temperature. It is the pressure corresponding to the critical point of the substance. It is defined as the point on the temperature and pressure scale in which a substance can exist both as liquid and vapour. Above the temperatures that exceed the critical temperature, the substance cannot be liquified no matter how much pressure is applied. Critical pressure is denoted by 'PC.'
For example, the critical pressure of chlorine (symbol: Cl) corresponds to 76 atm or 7,700 kiloPascals.
Ques: What is Liquefaction of Gases? (2 Marks)
Ans: Gas enters the liquefaction state when the intermolecular forces of attraction become so high that they bind the gas molecules together, forming the liquid state. By increasing the pressure on the molecules, the intermolecular forces of attraction can be increased. Hence, increasing and decreasing the amount of temperature and pressure in the liquefaction of gases is extremely important. As the temperature of gases increases, so makes the difficult for them to liquify. Thus, they require more pressure to complete the process.
Ques: What is Critical Pressure of Water? (2 Marks)
Ans: The critical pressure of water is the vapour pressure at its critical temperature, which is above the point at which the distinct liquid and gaseous phases do not exist. When the temperature of water approaches the critical temperature, then the gaseous and liquid phases' properties become same. It results in the occurrence of only one phase. It is a well-known fact the critical point of water occurs at a temperature point of 647 Kelvin. It is equal to 374 degrees Celsius and 705 degrees Fahrenheit. The critical pressure is equal to 22.064 MPa, which is approximately equal to 218 atmospheres of pressure.
Ques: What is the Importance of a Critical Temperature? (2 Marks)
Ans: The critical temperature refers to the strength of intermolecular forces of attraction. It is so important because it tells us about the liquefaction of solids. The weaker the intermolecular force, the more difficult will it be to liquefy the gas. Example: Weak intermolecular forces are present in hydrogen and helium both. Therefore, their critical temperature is low and they are difficult to liquefy. Whereas Carbon dioxide and ammonia, both have strong intramolecular forces of attraction; hence they can be easily liquefied because their critical temperature is higher than their room temperature.
Ques: What is the Difference Between the Critical Point and the Triple Point? (2 Marks)
Ans: Critical Point: The critical point of a substance is the point when the temperature of the substance is also the critical temperature (i.e., the highest temperature above which the substance cannot be liquified) and the pressure acting on the substance is also the critical pressure (i.e., the pressure at which a substance is liquified).
Critical point refers to the coexistence of two phases of the same substance. It is the point at which saturated liquid and saturated vapour lines meet on a phase diagram.
Triple Point: Triple point is the point when a substance can exist in all three states of matter, i.e., solid, liquid, and gaseous. In other words, it is the temperature and pressure of a substance when it can exist in all three states (solid, liquid, and gaseous).
The triple point determines the coexistence of three phases of the same substance. On a phase diagram, the point at which the sublimation line and fusion line, and vaporization meet is known as the triple point.
Ques: Give Critical Pressure of these substances:
Ammonia, Carbon Dioxide, Nitrogen, Water, Helium, Chlorine and Lithium. (3 Marks)
Ans:
| Substance | Critical Pressure (PC) |
| Ammonia (NH3) | 111.3 atm |
| Carbon Dioxide (CO2) | 72.8 atm |
| Water (H2O) | 217.7 atm |
| Nitrogen (N2) | 33.5 atm |
| Lithium (Li) | 652 atm |
| Chlorine (Cl) | 76.0 atm |
| Helium (He) | 2.24 atm |
Comments