Zeroth Law And Thermal Equilibrium

As stated in NCERT Class 11, Chapter 12 (Thermodynamics), the Zeroth Law of Thermodynamics is the basis for defining temperature. It states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. Here P is in equilibrium with Q, and Q is in equilibrium with R; therefore P must be in thermal equilibrium with R, which means there is no net heat flow between them and they share the same temperature. This transitive property is precisely why a thermometer (the third body Q) can be used to compare two bodies. The option claiming P is always hotter is wrong because equilibrium implies equal temperature, not a hierarchy. The option about continuous heat flow forever is wrong because thermal equilibrium means zero net heat exchange. The option that the relation depends on mass is wrong because temperature equality in equilibrium is independent of mass; mass affects heat capacity, not the equilibrium temperature condition. A consistency check confirms the conclusion: equal temperature gives zero temperature difference, so the second law's requirement of no spontaneous heat flow is satisfied.