What Is Critical Pressure Definition Formula and Role in Phase Diagrams
What is the Critical Pressure?
The pressure that corresponds to the critical point of the substance is termed as the critical pressure of that substance. It is defined as the point on the temperature and pressure scale in which a liquid substance can coexist with its vapour. For temperatures that exceed the critical temperature, it cannot be liquified no matter the amount of pressure applied. Hence, we can also define critical temperature as the pressure applied to a substance to liquefy it at its critical temperature. Critical pressure is denoted by 'PC.'
Liquefaction of Gases
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 difficulty of it to liquify. Thus, they require more pressure to complete the process.
Triple Point
The matter has three common states, namely solid, liquid, and gas. The physical states of these substances are influenced by two primary factors, temperature, and pressure. Each substance has a phase boundary, and therefore by adjusting the temperature-pressure combinations, we can pinpoint the phase boundary of any substance. Some substances have a triple point at which the substance can exist in all three states of matter.
Critical State
In thermodynamics, the critical state is said to be 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.
Critical Pressures of Some Common Substances
The critical pressure of water is 217.7 atm or 22,060 kiloPascals.
The critical pressure of ammonia (chemical formula: NH3) is approximately 111.3 atm or 11,280 kiloPascals.
The critical pressure of chlorine (symbol: Cl) corresponds to 76 atm or 7,700 kiloPascals.
The critical pressure of Helium (symbol: He) is 2.24 atm or 227 kiloPascals.
The critical pressure of nitrogen corresponds to 33.5 atm or 3390 kiloPascals.
The critical pressure of Methane is 45.79 atm or 4,640 kiloPascal.
The critical pressure of Oxygen corresponds to 49.8 atm or 5,050 kiloPascal
The critical pressure of sulfur corresponds to 207 am or 21,000 kiloPascal.
Gold has a critical pressure of about 5,000 atm or 510,000 kiloPascal.
Mercury's critical pressure is 1,720 atm or 174, 000 kiloPascal.
Critical Pressure of Water
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The critical pressure of water is defined as the vapour pressure at its critical temperature, which is above the point at which the distinct gas and liquid phases do not exist. When the temperature of water approaches the critical temperature, then the gaseous and liquid phases' properties become the 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.
FAQs on Critical Pressure in Thermodynamics and Phase Equilibria
1. What is critical pressure in chemistry?
The critical pressure is the minimum pressure required to liquefy a gas at its critical temperature. At this pressure, the gas and liquid phases become indistinguishable, forming a supercritical fluid. It is a characteristic property of every substance and depends on intermolecular forces. For example, carbon dioxide (CO2) has a critical pressure of about 7.38 MPa.
2. What is the definition of critical pressure?
The critical pressure (Pc) is defined as the pressure needed to convert a gas into a liquid at its critical temperature (Tc). Above this pressure and temperature, distinct liquid and gas phases do not exist. It is a key term in phase equilibria, real gases, and thermodynamics.
3. What is the formula for critical pressure from the van der Waals equation?
From the van der Waals equation, the critical pressure is given by Pc = a / (27b2). Here:
- a = measure of intermolecular attractive forces
- b = measure of molecular volume
This formula applies to real gases described by the equation:
(P + a/n2V2)(V − nb) = nRT.
4. What are the units of critical pressure?
The SI unit of critical pressure is the pascal (Pa). Common units include:
- Pa (SI unit)
- kPa or MPa
- atm (atmosphere)
- bar
For example, the critical pressure of water (H2O) is about 22.1 MPa.
5. How is critical pressure related to critical temperature?
The critical pressure is the pressure required to liquefy a gas at its critical temperature, beyond which liquid and gas phases cannot be distinguished. Both values define the critical point of a substance. At this point:
- The density of liquid equals the density of gas.
- The surface tension becomes zero.
- A supercritical fluid is formed above Pc and Tc.
6. What happens to a gas above its critical pressure?
Above its critical pressure (and critical temperature), a substance exists as a supercritical fluid rather than a distinct liquid or gas. In this state:
- There is no clear phase boundary.
- It has liquid-like density.
- It has gas-like diffusivity.
Supercritical CO2 is widely used in extraction processes and green chemistry.
7. How do intermolecular forces affect critical pressure?
Stronger intermolecular forces generally lead to a lower critical pressure because gases are easier to liquefy. Substances with strong hydrogen bonding or dipole–dipole interactions require less pressure to condense. For example:
- H2O has strong hydrogen bonding.
- Noble gases have weak London dispersion forces and usually higher critical pressures relative to their size.
8. What is the difference between critical pressure and vapor pressure?
The critical pressure is the pressure needed to liquefy a gas at its critical temperature, whereas vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. Key differences:
- Critical pressure occurs only at the critical point.
- Vapor pressure varies with temperature.
- Vapor pressure exists below the critical temperature only.
9. Why can a gas not be liquefied above its critical temperature?
A gas cannot be liquefied above its critical temperature because intermolecular attractions are insufficient to overcome molecular kinetic energy, regardless of pressure applied. Even if pressure exceeds the critical pressure, the substance forms a supercritical fluid instead of a true liquid. This explains why both temperature and pressure are required for gas liquefaction.
10. Can you give an example of critical pressure for common substances?
Yes, each substance has a characteristic critical pressure that defines its critical point. Examples include:
- CO2: 7.38 MPa
- H2O: 22.1 MPa
- N2: 3.39 MPa
- O2: 5.04 MPa
These values are important in thermodynamics, real gas behavior, and industrial liquefaction processes.
