Temporary hardness consists of the carbonates of alkaline earths held in solution by an excess of carbonic acid. Lime combines with 1 molecule of carbon dioxide to form the ordinary normal carbonate (chalk), which is practically insoluble in water. When, however, excess of carbonic acid is present, hydric calcic carbonate (bicarbonate) which is fairly soluble is produced. This is easily demonstrated by passing carbon dioxide into somewhat diluted lime-water, which at first becomes turbid from precipitated chalk, but soon clears by formation of soluble hydric carbonate. If the solution be now boiled, the hydric carbonate is decomposed, and the excess of carbonic acid is driven off as CO₂, and the chalk again precipitated. The reactions are represented by the following equations:-- Ca(OH)₂ + CO₂ = CaCO₃ + OH₂. (1) { CaCO₃ CaCO₃ + CO₂ + OH₂ = { (2) { H₂CO₃ Magnesia forms soluble double carbonates in a similar manner, but on continued boiling gradually loses the whole of its carbonic acid, and is precipitated as magnesium hydrate, Mg(OH)₂. One of the most important reactions in connection with temporary hardness is that caused by the addition of calcium hydrate (slaked lime), which forms the basis of Clark’s softening process. When an equivalent amount of lime is added to a solution of hydric calcic carbonate, it displaces the water of the “half-bound” carbonic acid, forming a second molecule of calcium carbonate, which is precipitated together with that originally present, as is represented in the following equation:-- CaCO₃ } } + Ca(OH)₂ = 2CaCO₃ + 2OH₂. (3) H₂CO₃ } Hydric magnesium carbonate is also precipitated by lime, but the reaction is somewhat different, the magnesia being removed as hydrate as follows:-- { MgCO₃ { + 2Ca(OH)₂ = 2CaCO₃ + 2OH₂ + Mg(OH)₂. (4) { H₂CO₃ It will be noted that 2 equivalents of lime are required to precipitate 1 of magnesia. Two molecules of sodium hydrate (NaOH) or potassium hydrate (KOH) may be substituted for 1 of Ca(OH)₂ with similar results, and in some cases it is practically advantageous to use the former, as the sodium carbonate formed in precipitating the temporary hardness reacts again on the permanent, throwing down the lime and magnesia as carbonates. (See p. 101.) [Illustration: FIG. 19.--Plan of Archbutt and Deeley’s Apparatus.]