What is the normal transition temperature, T_trs?

The normal transition temperature, referred to as T_trs, specifically denotes the temperature at which two phases of a substance are in equilibrium at 1 atm pressure. This concept is crucial in phase diagrams where different states—such as solid, liquid, and gas—interact under varied conditions. At T_trs, one can observe the balance between the phases, meaning that the rate of formation of one phase equals the rate of transformation into the other phase.

For example, consider the transition from solid to liquid (melting) or from liquid to gas (vaporization); at T_trs, these transitions are reversible, and both phases can coexist stably when the pressure is maintained at 1 atm. This definition is fundamental in thermodynamics and provides a clear understanding of how substances behave under standardized conditions.

The other options reflect different temperature-related phenomena but do not accurately capture the definition of T_trs. Options discussing equilibrium of chemical reactions, melting, or condensing cover broader processes rather than the specific transition temperature between phases at a standard atmospheric pressure.