Answer is: reversible reaction.
The concentrations of reactants and products in an equilibrium mixture are determined by the equilibrium constant K.
When writing chemical equations for reversible reactions, two arrows are used, for example: NH₄Cl(s) ⇆ NH₃(g) + HCl(g) or
CuSO₄(s) + 5H₂O(l) ⇆ CuSO₄·5H₂O(s).
