Hess's law states that the total heat change of a reaction is the same, regardless of the number of steps or the pathway taken from reactants to products. This means that the heat change is determined only by the initial and final states of the system, not by the specific route taken during the reaction.
In practical terms, if a reaction can occur via multiple pathways, the total enthalpy change will always be the same as long as the starting and ending points are unchanged. This principle is rooted in the conservation of energy, which emphasizes that energy does not change simply because the reaction occurs in a different manner. The idea that the heat change is path-independent is what makes Hess's law a powerful tool for calculating enthalpy changes for complex reactions from known data of simpler steps.
The other options do not accurately capture the essence of Hess's law. While reactions do depend on conditions such as temperature, the central takeaway of Hess's law specifically emphasizes the independence from the reaction path taken, focusing on the initial and final states.