Understanding Pure Substances in Thermodynamics

In thermodynamics and chemistry, a pure substance is defined as a substance that has a fixed chemical composition throughout. This means that its composition does not vary from one point to another. A pure substance can exist in one or more phases (solid, liquid, or gas), but the chemical composition must be the same in all phases.

Let's analyze each option provided to determine which one is NOT a pure substance:

Analyzing the Options

We will evaluate each option based on the definition of a pure substance:

1. Mixture of air (gas phase) and liquid air: Air is primarily a mixture of nitrogen (\(\text{N}_2\)) and oxygen (\(\text{O}_2\)), along with small amounts of other gases like argon (\(\text{Ar}\)). Gaseous air is a mixture. Liquid air is also a mixture. However, when air is cooled to liquefy, the components with higher boiling points (like oxygen) condense more readily than those with lower boiling points (like nitrogen). As a result, the composition of liquid air is different from the composition of gaseous air in equilibrium with it (liquid air is typically richer in oxygen than the gaseous phase). Since the chemical composition is not the same in both the gas and liquid phases, a mixture of air (gas phase) and liquid air is NOT a pure substance.

2. Combustion product of a fuel: The products of combustion of a fuel typically consist of several different chemical compounds such as carbon dioxide (\(\text{CO}_2\)), water vapor (\(\text{H}_2\text{O}\)), nitrogen (\(\text{N}_2\)) (from the air used), unburnt fuel, carbon monoxide (\(\text{CO}\)), and possibly other substances like ash or sulfur oxides depending on the fuel. This is clearly a mixture of different chemical species. Therefore, it is NOT a pure substance.

3. Atmospheric air: Atmospheric air is a mixture of various gases, mainly nitrogen (\(\text{N}_2\)) and oxygen (\(\text{O}_2\)). While its composition is relatively constant geographically and at lower altitudes, it is still a mixture of different chemical substances. Therefore, strictly speaking, atmospheric air is NOT a pure substance. However, for many engineering calculations, especially in thermodynamics, air is often treated as a single substance with average properties or as an ideal gas mixture with a fixed composition, simplifying analyses.

4. Vapours of ammonia: Ammonia (\(\text{NH}_3\)) is a single chemical compound with a fixed chemical formula. Its vapor phase is simply ammonia in gaseous form. A single chemical compound in any phase or combination of phases (where applicable) is considered a pure substance as long as the composition is uniform. Therefore, vapors of ammonia IS a pure substance.

Comparing the Options and Identifying the Non-Pure Substance

Based on the analysis:

• Vapours of ammonia (Option 4) is a pure substance.
• Combustion product of a fuel (Option 2) and Atmospheric air (Option 3) are mixtures, and therefore, strictly speaking, not pure substances.
• Mixture of air (gas phase) and liquid air (Option 1) is a mixture where the composition differs significantly between the two phases present. This is a defining characteristic of something that is NOT a pure substance when multiple phases are involved.

While options 2 and 3 are also not pure substances, option 1 presents a scenario (multi-phase mixture with varying composition) that is a classic example used to illustrate the definition of a pure substance. Option 1 most definitively fails the requirement of having a uniform composition across all phases present.

Therefore, the option that is NOT a pure substance is the mixture of air (gas phase) and liquid air.

Revision Table: Key Concepts

Additional Information on Pure Substances and Mixtures

Understanding the distinction between pure substances and mixtures is fundamental in various fields like chemistry, thermodynamics, and engineering. Here are some additional points:

• Homogeneity: A pure substance must be chemically homogeneous throughout its volume. This doesn't mean it must be in a single phase; a mixture of ice and liquid water is a pure substance because both phases are \(\text{H}_2\text{O}\).
• Variable Composition: The key reason a mixture is not a pure substance is the potential for variable composition. Even if a mixture is currently homogeneous (like atmospheric air in a room), its composition can often be easily changed by physical means (e.g., separating oxygen and nitrogen from air by liquefaction and distillation).
• Ideal Gas Assumption: In many thermodynamic problems, air is treated as an ideal gas mixture with constant composition. This simplifies calculations, but it doesn't change the fact that air is fundamentally a mixture of different gases.
• Multiple Phases of Mixtures: When a mixture exists in multiple phases, the composition usually differs between the phases. For example, boiling seawater produces water vapor (nearly pure water) leaving behind liquid brine (higher salt concentration). Similarly, condensing air results in liquid air with a different N₂/O₂ ratio than the original gaseous air. This compositional difference across phases is why multi-phase mixtures are definitively not pure substances.