Question

Select incorrect statement: (a) we can condense vapour simply by applying pressure (b) to liquefy a gas one must lower the temperature below \(T_{C}\) and also apply pressure (c) at \(T_{C}\), there is no distinction between liquid and vapour state hence density of the liquid is nearly equal to density of the vapour (d) However great the pressure applied, a gas cannot be liquified below it's critical temp.

Short answer

The incorrect statement is (d). Below the critical temperature, gases can indeed be liquefied by applying pressure.

Step-by-step solution

Understanding the Statements

Read and understand each statement to gauge its accuracy based on the properties of gases and critical points.

Assessing Statement (a)

Vapor can indeed be condensed into a liquid by applying pressure, assuming the temperature is below its critical temperature. This statement is generally true.

Assessing Statement (b)

To liquefy a gas, it's necessary to lower its temperature below the critical temperature (\(T_{C}\)) and apply sufficient pressure. This statement is true.

Assessing Statement (c)

At the critical temperature (\(T_{C}\)), the properties of the liquid and vapor phases of a substance become indistinguishable. The densities of the two phases become equal. This statement is true.

Assessing Statement (d)

This statement is incorrect. Above the critical temperature, no amount of applied pressure can liquefy a gas because no distinction exists between liquid and gas phases; they are in a supercritical fluid state. Below the critical temperature, applying pressure can liquefy a gas.

Selecting the Incorrect Statement

Based on the assessments, statement (d) is incorrect as it asserts that a gas cannot be liquified below its critical temperature regardless of the pressure applied, which contradicts the actual behavior of gases.

Key concepts

Liquefaction of Gases

The process of turning a gas into a liquid is known as liquefaction. This transformation happens when you cool a gas or apply pressure to it, or a combination of both. Cooling slows down the particles in a gas, decreasing their kinetic energy, which allows forces of attraction to pull the particles closer and form a liquid. Similarly, when applying pressure, you're essentially squeezing the gas particles closer together, which can also result in a liquid if the gas is below its critical temperature. It's important to note that gases can be liquefied at high pressure only if the temperature is below their critical temperature, otherwise no pressure can turn the gas into a liquid.

To use a real-world example, consider how propane is stored in tanks. Propane gas is compressed and stored as a liquid because it is below its critical temperature and under high pressure inside the tank.

Critical Temperature

Critical temperature, denoted as (\(T_{C}\)), is a term that refers to the highest temperature at which a substance can exist as a liquid, regardless of the pressure applied. Above this temperature, the substance can only exist as a gas or a supercritical fluid. In simpler terms, think of it as the 'point of no return' for gases becoming liquids. For instance, water has a critical temperature of 374°C, meaning you can't get liquid water above this temperature no matter how much you pressurize it. This concept clarifies why statement (d) from our exercise is incorrect.

Understanding critical temperature is crucial in industrial processes where they manage the state of substances for various applications such as refrigeration, propulsion systems in rockets, or even in the creation of certain materials like plastics.

Phase Transitions

Phase transitions refer to the changes of state between solids, liquids, and gases. These transitions include melting, freezing, vaporization, condensation, sublimation, and deposition. Temperatures and pressures at which these changes occur are key to managing them. For a gas to transition to a liquid, as discussed, it needs to be below its critical temperature. At the boiling point, liquid turns into gas; at the melting point, solid turns into liquid, and so forth.

These transitions are not always simple one-step processes. When considering real-world scenarios like cloud formation, rain, and natural ice, understanding how and why these transitions occur at specific temperatures and pressures is essential for predictions and applications in fields such as meteorology and environmental science.

Supercritical Fluids

A supercritical fluid is what you get when you increase the temperature and pressure of a substance above its critical point. It blurs the line between gas and liquid states. Supercritical fluids combine the best properties of both phases: they spread out like a gas but can dissolve materials like a liquid. This makes them incredibly useful in various applications, such as in supercritical fluid extraction (SFE), a key method in food and pharmaceutical processing to extract compounds without using harsh chemicals.

Caffeine extraction for decaffeinated coffee is one process that uses supercritical carbon dioxide. Because of the supercritical fluid's unique properties, it can penetrate deep into the coffee beans and selectively remove the caffeine without affecting the flavor compounds.