Which of the following statements is correct for helium? (1) It has a positive Joule-Thomson coefficient above \(40 \mathrm{~K}\) (2) Its spontaneous expansion causes it to warm up. (3) It has to be compressed before it can liqufy. (4) It has a negative Joule-Thomson coefficient above \(40 \mathrm{~K}\). (a) \(1,2,3\) (b) \(1,2,4\) (c) \(1,3,4\) (d) \(2,3,4\)

The process of liquefying gases is a crucial element in various industries, particularly in the production of cryogenic fluids and the storage and transport of gases.

Liquefying a gas involves cooling it below its boiling point at atmospheric pressure or applying pressure to increase the boiling point. The latter method, which includes compression and cooling, is essential for gases that remain gaseous at very low temperatures, like helium.

Significantly, the variables impacting a gas's ability to liquefy are eloquently described by the thermodynamic principles of pressure, volume, and temperature relationships. For instance, as per statement 3 from the exercise, helium does require compression before it can liquefy, due to its extremely low boiling point under standard atmospheric conditions.

• For most gases, liquefaction is achievable through the application of the Joule-Thomson effect, wherein a temperature change occurs upon the expansion of the gas without any heat exchange with the environment.
• However, Helium, along with hydrogen, is an exception due to its inversion temperature characteristics, which we will explore in the context of its unique properties in the following section.

Helium is a noble gas known for its inertness and unique set of properties which differentiate it from other gases. A primary feature of helium is its extraordinarily low boiling and melting points.

This gas does not solidify at standard atmospheric pressure, no matter how low the temperature drops, which is an important trait to consider when discussing the Joule-Thomson effect and the liquefaction process.

The Joule-Thomson Effect in Helium

Unlike most gases, helium's behavior under the Joule-Thomson effect is uncommon. When most gases expand, they cool due to a positive Joule-Thomson coefficient, but helium, above 40 K, has a negative Joule-Thomson coefficient. This means it actually warms up, as correctly stated in statement 2 of the given exercise.

• Understanding Helium's behavior is critical in applications that require precise temperature control, such as MRI machines, where liquid helium is used to achieve the necessary superconducting state.

Thermodynamics is a branch of physics that deals with the relationships between heat and other forms of energy.

In the context of the liquefaction of gases and the Joule-Thomson effect, thermodynamics provides a foundation for understanding how energy is transferred and transformed. The core laws of thermodynamics dictate how these processes work and why certain gases behave differently under varying conditions.

First Law of Thermodynamics

This law, also known as the law of energy conservation, highlights that energy cannot be created or destroyed. When considering the Joule-Thomson effect, the first law implies that the temperature change is due to the internal energy change of the gas during expansion or compression.

Second Law of Thermodynamics

It states that entropy, or disorder, always increases in an isolated system. The liquefaction of gases represents a decrease in entropy which requires an external force, such as compression and cooling, to be applied. This aligns with statement 3, as helium must be forced into a more ordered state (liquid) from a less ordered state (gas).