A propane tank is filled with a mixture of liquid and vapor. Can the content of this tank be considered a pure substance?
The correct answer and explanation is :
The content of a propane tank that is filled with a mixture of liquid and vapor cannot be considered a pure substance. A pure substance is defined as a material that has a constant composition throughout and can exist in only one phase (either solid, liquid, or gas) at a given temperature and pressure.
Explanation:
In the case of a propane tank, the contents are a mixture of liquid propane and propane vapor. Propane, when stored in a tank, typically exists as a combination of liquid and gas phases due to the pressure and temperature conditions. The liquid phase is the result of the propane being compressed into a liquid state, while the vapor phase exists above the liquid due to the vaporization of propane molecules at the tank’s temperature and pressure.
A mixture of liquid and vapor is not a pure substance for the following reasons:
- Two Phases: A pure substance must be in a single phase (either all liquid, all solid, or all gas) at a given temperature and pressure. In a tank with a propane mixture, you have both liquid propane and propane vapor. This represents a two-phase system, which is inherently not a pure substance.
- Phase Change: The transition between liquid and vapor is governed by the equilibrium between the two phases, which means the proportions of liquid and vapor change depending on the temperature and pressure. As such, the composition of the substance inside the tank can vary depending on the conditions, further disqualifying it from being a pure substance.
- Saturation Condition: When liquid and vapor coexist, they are often in a saturated state, meaning the liquid is at its boiling point for a given pressure, and the vapor is at its condensation point. This condition can lead to variations in composition based on the system’s conditions, unlike a pure substance where the composition is fixed.
In conclusion, a propane tank containing a mixture of liquid and vapor is a two-phase system and cannot be considered a pure substance because it does not have a constant composition or exist in only one phase at a given temperature and pressure.