~Conditions under which reversible reactions in solution are complete.~ The equilibrium between substances in solution may be disturbed and the reaction caused to go on in one direction to completion in either of three ways. 1. _A gas may be formed which escapes from the solution._ When sodium nitrate and sulphuric acid are brought together in solution all four ions, Na^{+}, NO_{3}^{-}, H^{+}, SO_{4}^{--}, are formed. These ions are free to rearrange themselves in various combinations. For example, the H^{+} and the NO_{3}^{-} ions will reach the equilibrium H^{+} + NO_{3}^{-} <--> HNO_{3}. If the experiment is performed with very little water present, as is the case in the preparation of nitric acid, the equilibrium will be reached when most of the H^{+} and the NO_{3}^{-} ions have combined to form undissociated HNO_{3}. Finally, if the mixture is now heated above the boiling point of nitric acid, the acid distills away as fast as it is formed. More and more H^{+} and NO_{3}^{-} ions will then combine, and the process will continue until one or the other of them has all been removed from the solution. The substance remaining is sodium acid sulphate (NaHSO_{4}), and the reaction can therefore be expressed by the equation NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}. 2. _An insoluble solid may be formed._ When hydrochloric acid (HCl) and silver nitrate (AgNO_{3}) are brought together in solution the following ions will be present: H^{+}, Cl^{-}, Ag^{+}, NO_{3}^{-}. The ions Ag^{+} and Cl^{-} will then set up the equilibrium Ag^{+} + Cl^{-} <--> AgCl. But silver chloride (AgCl) is almost completely insoluble in water, and as soon as a very little of it has formed the solution becomes supersaturated, and the excess of the salt precipitates. More silver and chlorine ions then unite, and this continues until practically all of the silver or the chlorine ions have been removed from the solution. We then say that the following reaction is complete: AgNO_{3} + HCl = AgCl + HNO_{3}. 3. _Two different ions may form undissociated molecules._ In the neutralization of sodium hydroxide by hydrochloric acid the ions H^{+} and OH^{-} come to the equilibrium H^{+} + OH^{-} <--> H_{2}O. But since water is almost entirely undissociated, equilibrium can only be reached when there are very few hydroxyl or hydrogen ions present. Consequently the two ions keep uniting until one or the other of them is practically removed from the solution. When this occurs the neutralization expressed in the following equation is complete: NaOH + HCl = H_{2}O + NaCl.