What occurs to the properties of a substance when it transforms into vapor?

When a substance transforms into vapor, it experiences a change in density due to the increase in volume as it transitions from a liquid or solid state to a gaseous state. In the vapor phase, the particles are generally much farther apart compared to when the substance is in a liquid or solid form. This expansion results in the substance becoming less dense.

The other options do not accurately describe the properties of a substance during vaporization. For instance, while some substances may become more reactive in their vapor phase, this is not a consistent or defining characteristic of vaporization. Color changes can occur with some substances when they heat or react, but this is not a universal effect tied to the transition to vapor. Lastly, the mass of the substance remains constant throughout the transition; it does not increase, as mass is conserved during phase changes. Thus, the best representation of a property change during vapor transformation is the decrease in density.