Sulfuric acid molecules donate a hydrogen ion to the chloride ion in solid sodium chloride to form molecular hydrogen chloride. The hydrogen sulfate ions remain in the mass of solid as sodium hydrogen sulfate. When this reaction is carried out in a closed system, it comes to equilibrium when only a small amount of product has been formed. If the gaseous hydrogen chloride is allowed to escape, by pumping it out, equilibrium can not be attained and the reaction eventually proceeds completely to products.