A propane tank is filled with a mixture of liquid and vapor propane. Can the contents of this tank be considered a pure substance? Explain.
The correct answer and explanation is :
Yes, the contents of the propane tank can be considered a pure substance, but with some important considerations.
A pure substance is defined as a material that has a uniform and unchanging composition, with all the particles being of the same type. Pure substances can exist in various phases, such as solids, liquids, or gases, and these phases can coexist under certain conditions. For a substance to be pure, it must have a constant composition throughout, meaning all of its molecules are chemically identical.
In the case of a propane tank that contains a mixture of liquid and vapor propane, the substance inside the tank is still propane (C₃H₈) throughout, regardless of whether it is in the liquid or vapor phase. Propane is a single compound, meaning it is made up entirely of propane molecules. Even though it exists in two phases (liquid and vapor), the substance itself does not change in composition; it remains propane.
The reason this situation still qualifies as a pure substance is that the mixture of liquid and vapor is in equilibrium. At a given temperature and pressure, propane exists in a liquid-vapor equilibrium, with some propane molecules in the liquid phase and others in the gas phase. The ratio of liquid to vapor depends on the specific conditions (temperature and pressure), but the material inside the tank is still composed entirely of propane molecules, with no other substances mixed in. This phase separation doesn’t alter the fact that the material is one pure chemical substance, propane.
However, it’s crucial to note that for the tank to maintain this equilibrium, the temperature and pressure must be within the correct range for propane. If the tank were to be subjected to extreme conditions that caused chemical changes (such as breaking down into other compounds), it would no longer be a pure substance. But under normal conditions, the propane in the tank is still considered a pure substance.