In fact, measuring of temperature itself requires the thermometer to be at the same state as the system it measures. Equilibrium states are reached for physical as well as chemical reactions. Equilibrium is dynamic in the sense that changes continue, but the net change is zero. Reversible Chemical Reactions Heat transfer, vapourization, melting, and other phase changes are physical changes. These changes are reversible and you have already experienced them.
Many chemical reactions are also reversible.
The law of mass action is universal, applicable under any circumstance. However, for reactions that are complete, the result may not be very useful. The mass action law states that if the system is at equilibrium at a given temperature, then the following ratio is a constant. This is the ideal law of chemical equilibrium or law of mass action . The units for K depend upon the units used for concentrations.