Question

Critical temperature of gas is the temperature (1) below which it cannot be liquefied (2) at which it solidifies (3) above which it cannot be liquefied (4) above which it can no longer remain in the gaseous state

Short answer

Option 3: above which it cannot be liquefied.

Step-by-step solution

Identify the Meaning of Critical Temperature

The critical temperature of a gas is a specific property related to the phase transitions of gases.

Understand the Nature of Liquefaction

Liquefaction is the process of transforming a gas into a liquid, typically by cooling or compressing the gas. A gas can only be liquefied if it is below its critical temperature.

Link Critical Temperature to Liquefaction

Based on the meaning of the critical temperature, this temperature marks the highest temperature at which a gas can be converted into a liquid, regardless of the pressure applied.

Analyze the Given Options

Evaluate the given options to find the one that correctly defines the critical temperature: (1) below which it cannot be liquefied (2) at which it solidifies (3) above which it cannot be liquefied (4) above which it can no longer remain in the gaseous state.

Conclusion

Option (3) above which it cannot be liquefied accurately describes the critical temperature.

Key concepts

Phase Transitions

Phase transitions refer to the changing of a substance from one state of matter to another, such as from solid to liquid or liquid to gas. This change happens because of energy transfer, such as heating or cooling. For example, water boils at 100°C, transforming from liquid to gas.
Phase transitions are essential in understanding how materials behave under different temperatures and pressures. These changes are due to alterations in the energy and arrangement of particles. The critical temperature is a key parameter in phase transitions because it defines a threshold above which a gas cannot become liquid, regardless of pressure.

Liquefaction

Liquefaction is the process of turning a gas into a liquid. This can occur through cooling or compressing the gas. For instance, when water vapor condenses into liquid water on a cold glass, it is undergoing liquefaction.
A gas can be liquefied by lowering its temperature or increasing its pressure. However, if the gas is above its critical temperature, liquefaction cannot occur, no matter how much pressure is applied. So, understanding the critical temperature is important for processes like refrigeration and gas storage. It sets the limit for when a gas can be converted into a liquid form.

Properties of Gases

Gases exhibit unique properties that differentiate them from solids and liquids.
Firstly, gases have no fixed shape and take the shape of their container. They also have low density compared to solids and liquids.

Gases are compressible, meaning their volume can change when pressure is applied. This property makes gases behave differently under various conditions. For example, the breathable air we use gets compressed into cylinders for scuba diving.
The critical temperature is key in understanding these properties because it determines the upper limit at which gas can be liquefied. Below this temperature, gas molecules move slower and can be compressed into a liquid. Above this temperature, their high energy makes liquefaction impossible.

Thermodynamics

Thermodynamics is the study of energy transfer and transformations. It includes concepts like temperature, heat, and work, which are crucial for understanding how energy changes affect matter.
One critical idea in thermodynamics is the critical temperature, where a gas can no longer be liquefied, no matter how much pressure is applied. This directly relates to the energy of gas molecules. When the temperature is above the critical level, the energy between molecules is too high to form liquid.
Thermodynamics also covers laws like the conservation of energy and the increase of entropy. These laws help us predict how energy moves and changes, providing a framework for many scientific and engineering processes.