A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from reversible process in thermodynamics.
Weak acids and bases undertake reversible reactions. For example, carbonic acid: H2CO3 (l) + H2O(l) ⇌ HCO−3 (aq) + H3O+(aq).
The concentrations of reactants and products in an equilibrium mixture are determined by the analytical concentrations of the reagents (A and B or C and D) and the equilibrium constant, K. The magnitude of the equilibrium constant depends on the Gibbs free energy change for the reaction.[2] So, when the free energy change is large (more than about 30 kJ mol−1), then the equilibrium constant is large (log K > 3) and the concentrations of the reactants at equilibrium are very small. Such a reaction is sometimes considered to be an irreversible reaction, although in reality small amounts of the reactants are still expected to be present in the reacting system. A truly irreversible chemical reaction is usually achieved when one of the products exits the reacting system, for example, as does carbon dioxide (volatile) in the reaction
The answer about muscles is extensors
According to the theory of vitalism all the living substances are special and are distinct from the non-living substances, that is, they comprise a unique entity within themselves. This soul or entity distinguishes them from the non-living components.
However, the theory was rejected when urea was taken out successfully from a non-living matter by Wohler, proving the fact that the biological constituents can be extracted from a non-living source. The modern scientists now believe that the living things are just more composite than the non-living substances.
