Question

Read the following statements and identify the incorrect statement. (a) Volume of one mole of a gas at critical temperature is called molar volume. (b) Pressure of a gas at critical temperature is called critical pressure. (c) The critical temperature, pressure and volume are called critical constants. (d) Critical temperature is the highest temperature at which a gas can exist as liquid, above this temperature it is a gas.

Short answer

The incorrect statement is (a) because 'molar volume' refers to the volume of one mole of a substance at any specified conditions, not necessarily at critical temperature.

Step-by-step solution

Examine Statement A

Review the definition of molar volume and its relation to gases at different temperatures, including the critical temperature. Molar volume usually refers to the volume occupied by one mole of a substance, which can be at standard temperature and pressure (STP) or any other specified conditions, but not specifically at the critical temperature.

Examine Statement B

Check the definition of critical pressure. Critical pressure is defined as the pressure of a gas or vapor in its critical state, which is in line with statement (b).

Examine Statement C

Look at the terminology used to describe critical states. The term 'critical constants' is not widely accepted or used. Nevertheless, critical temperature, pressure, and volume are typically understood to be the set of conditions that define the critical point for a substance.

Examine Statement D

Analyze the concept of critical temperature. Critical temperature is indeed the highest temperature at which a liquid can exist; beyond this temperature, it becomes a gas, and it cannot be liquefied by pressure alone. This agrees with statement (d).

Identify the Incorrect Statement

After evaluating each statement against known scientific definitions and concepts, identify the statement that is incorrect. Statement A is incorrect because while it does refer to the volume of a gas, the term 'molar volume' does not specifically refer to the volume of one mole of a gas at critical temperature.

Key concepts

Molar Volume

When diving into the concept of molar volume, we touch upon a critical point of understanding gases and their behavior. In essence, molar volume is the volume occupied by one mole of a substance. Typically, we consider this volume at standard temperature and pressure (STP), which is 0 degrees Celsius and 1 atmosphere of pressure. To visualize this, imagine filling a balloon with exactly one mole of any gas at STP; the balloon's size would describe the gas's molar volume.

However, molar volume can change with different conditions. That's because gases are compressible and expand or contract with temperature and pressure variations. The scenario changes significantly when we observe a gas at its critical temperature, which leads us to examining critical phenomena in chemistry.

Critical Pressure

Critical pressure plays a significant role in understanding a substance's transition between phases. It is the pressure required to liquefy a gas at its critical temperature. At this juncture, the distinction between the liquid and gas phases becomes indistinguishable, as they merge into one supercritical fluid. This pressure is unique to each substance, forming a part of its 'personality' or characteristic behavior.

For instance, water has a critical pressure of about 217.7 atmospheres. This means that if you try to liquefy steam (above 100 degrees Celsius), you would have to apply a pressure of 217.7 atmospheres if the steam is at the critical temperature of 374 degrees Celsius. Below this temperature, lower pressures could condense the steam, but above the critical temperature, no amount of pressure can bring water back to its liquid state.

Critical Temperature

The critical temperature is a thermal ceiling for the liquid phase of a substance. Above this temperature, the substance cannot exist as a liquid, regardless of the pressure applied. This is the highest temperature at which a substance's liquid and gas phases can coexist in equilibrium.

Understanding critical temperature requires grasping that at this state, the properties of the gas and liquid phase converge, meaning surface tension disappears and the phases are no longer distinguishable — a condition known as the critical point. For water, the critical temperature is around 374 degrees Celsius. Past this critical temperature, even compressing the water vapor to high pressures will not cause it to turn into liquid; it becomes a supercritical fluid, a hybrid state with properties of both gas and liquid. This concept is particularly important in industrial applications such as supercritical fluid extraction.