A reaction that can (and does!) proceed in both directions at the same time, i.e. while the reactants react together to form the products, the products are busy reacting to form the reactants. In practice, a balance (called an equilibrium) between products and reactants is reached. soluble : Describing a chemical that can form a solution in a particular liquid. For example, sugar is soluble in water. The opposite of soluble is "insoluble".
A reversible reaction is a chemical reaction that may proceed in both the forward and reverse directions. In other words, the reactant and product of one reaction may reverse roles, without adding chemicals.