Yes, it is possible to determine whether a substance is a gas or a liquid based on temperature and pressure alone, without directly measuring its mass or volume. This can be accomplished by using the phase diagram of the substance.
A phase diagram is a graphical representation that shows the relationship between pressure, temperature, and the different phases (solid, liquid, and gas) of a substance. On a phase diagram, the regions corresponding to gas, liquid, and solid states are typically delineated.
For most substances, the phase diagram displays a line separating the gas and liquid regions, known as the vaporization or boiling curve. Along this curve, the substance can exist in equilibrium as both a gas and a liquid.
To determine whether a substance is a gas or a liquid at a given temperature and pressure:
Locate the point on the phase diagram that corresponds to the given temperature and pressure values.
If the point is located in the region above the boiling curve, it indicates that the substance is in the gas phase at those conditions.
If the point is located in the region below the boiling curve, it indicates that the substance is in the liquid phase at those conditions.
It's important to note that this method assumes that the substance in question behaves according to typical phase behavior and follows the phase diagram pattern for its specific conditions. Deviations from this behavior, such as the presence of supercritical fluids or unusual substances, may require additional considerations.
Therefore, by using temperature and pressure information in conjunction with the phase diagram, you can determine whether a substance exists as a gas or a liquid without directly measuring its mass or volume.