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Critical Temperature is a topic covered under CBSE Class 11 Chemistry Unit V States of Matter: Gases and Liquids. As per the latest syllabus, Unit IV-VII will carry a total weightage of 21 marks out of 70. Students can expect Short Answer Questions of 2-3 marks in the CBSE exams.
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Chemical Bonding and Molecular StructureWhat is Critical Temperature
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The critical temperature of a material is the maximum temperature at which it can be condensed while remaining in a liquid form. In other terms, a critical temperature is a temperature beyond or below which a substance cannot be liquefied from its vapor or gaseous form regardless of the amount of pressure applied to it. Tc is the symbol for it.
The French scientist Charles Cagniard de la Tour identified the critical point of a liquid in 1822. He observed that carbon dioxide could be liquefied at a temperature of 31°C when 73 atm of pressure was applied, but it could not be liquified at higher temperatures, even when pressures above 3000 atm were applied.
Dmitry Mendeleev later called the "Critical Temperature" in 1860 the maximum temperature at which compounds might exist in the liquid phase.
This is the temperature at which liquid carbon dioxide may be measured. It is gas above this temperature.
Critical volume (Vc) and critical pressure (Pc) are terms used to describe the volume of one mole of gas at a critical temperature (Tc). The critical temperature, pressure, and volume are critical constants.
Check Chapter 5 States of Matter Important Notes
Read More: Class 11 Boyle's Law
Critical Volume
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The volume occupied by one mole of gas at critical temperature and pressure is known as critical volume.
Critical Pressure
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The vapor pressure of a fluid at the critical temperature, at which distinct liquid and gas phases do not exist, is known as critical pressure.
Critical Point
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A critical point (or critical state) is the terminal point of a phase equilibrium curve in thermodynamics. The most well-known example is the liquid-vapor critical point, which is the terminal point of the pressure-temperature curve and determines the conditions for co-existing a liquid or its vapor.
The gas cannot be liquefied by pressure on its own at higher temperatures. Phase barriers vanish at the critical point, which is characterized by a critical temperature (Tc)and a critical pressure (Pc).
Graphical Representation of Critical Temperature
The critical temperature is illustrated graphically in the graph below. The graph has been plotted with pressure on the Y-axis and temperature on the X-axis, indicating that the critical temperature from the graph can be obtained from the value of the X-axis, and the value of the Y-axis, on the other hand, indicates the value of pressure required to obtain the liquid state of the substance at the critical point when the temperature is on the substance is the critical temperature.
The graph also depicts the triple point, which is the point at which the substance's temperature and pressure remain at a level that allows it to exist in all three forms of matter: solid, liquid, and gaseous.
- Only when the fluid is below its critical temperature and its pressure and volume are below the dome can liquid and gas be differentiated, since liquid and gas are in equilibrium and a surface dividing the two phases is visible.
- There is no basic method to distinguish between two states without this surface.
- At critical temperatures, liquids transition into a gaseous state invisibly and continuously, and the boundary between the two phases vanishes.
Critical Temperature, Pressure, and Volume of Some Substances
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The vapor of the material is a gas that may be liquefied by exerting pressure below the critical temperature. Carbon dioxide vapor is carbon dioxide gas that has reached its critical temperature. In the table, critical constants for several common compounds are listed.
| Substances | Critical Temperature (Tc) | Critical Pressure (Pc) | Critical Volume (Vc) |
| Ammonia (NH3) | 405.5 | 113.0 | 0.0650 |
| Carbon Dioxide (CO2) | 304.10 | 73.9 | 0.0956 |
| Nitrogen (N2) | 126.0 | 33.9 | 0.0900 |
| Water (H2O) | 647.1 | 220.6 | 0.0450 |
| Helium (He) | 5.3 | 2.29 | 0.0577 |
| Oxygen (O2) | 154.3 | 50.4 | 0.0744 |
| Chlorine (Cl) | 416.9 | 76.0 | 0.02795 |
Metals have extremely high critical temperatures (Tc) and critical pressures (Pc). Helium (noble gas) has the lowest critical temperatures (5.19K) and critical pressures of any noble gas (2.24 atm).
Read More: Dalton's Law of Partial Pressure Important Notes
Sample Questions
Ques. The quantity of intermolecular interactions between gas particles determines the critical temperature of gases. Ammonia and carbon dioxide both have critical temperatures of 405.5 K and 304.10 K, respectively. When cooling from 500 K to their critical temperature, which of these gases will liquefy first? (1 mark)
Ans. Because the critical temperature of ammonia is reached first, it will liquefy first. More cooling will be required for CO2 liquefaction.
Ques. What is critical temperature's significance? (1 mark)
Ans. The strength of the intermolecular forces of attraction that its particles are subject to can be determined by the critical temperature of a gas. A gaseous material with weak intermolecular interactions, for example, will be more difficult to liquefy than one with greater intermolecular attraction forces. As a result, the critical temperature decreases as intermolecular forces weaken.
Ques. How do a critical point and a triple point vary from each other? (1 mark)
Ans. The critical point of a material is that the substance's temperature is also the critical temperature and the pressure on the substance is also the critical stress. The triple point is where there may be a substance in all three material states, that is to say solid, liquid, and gaseous.
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