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Updated: July 1, 2025
If to a water you suspect to be hard from presence of carbonate of lime or carbonate of iron in solution in carbonic acid, i.e. as bicarbonates, you add some clear lime-water, and a white precipitate is produced, you have a proof of carbonate of lime hardness. If the precipitate is brownish, you may have, also, carbonate of iron. I will now mention a very delicate test for iron.
We may briefly state that the chief object is to precipitate the bicarbonates of lime and magnesia held in solution by the water, and so get rid of what is known as the temporary hardness. To accomplish this, strong lime water is introduced in a clear state to the water to be softened, the quantity being regulated according to the amount of bicarbonates in solution.
The immediate effect of this is that a proportion of the carbonic acid of the latter combines with the invisible lime of the clear lime water, forming a chalky precipitate, while the loss of this proportion of carbonic acid also reduces the invisible bicarbonates into visible carbonates.
Softening by carbonate of soda. The hardness of water, as already explained, being principally due to the presence in solution of bicarbonates and sulphates of lime and magnesia, can be reduced by addition of carbonate of soda, which decomposes these salts slowly in cold water but quickly in hot, forming insoluble compounds of lime and magnesia, which are slowly precipitated as a fine mud, leaving the water charged, however, with a solution of bicarbonate and sulphate of soda.
During ebullition the bicarbonates, which are soluble, become carbonates, which are insoluble, giving off their carbonic acid as gas, rendering by the precipitate produced, but not allowed in a boiler time to settle the water muddy, but incapable of decomposing soap.
Hence clear lime-water forms a good test for the presence of bicarbonates of lime or iron in a water. But water may be hard from the presence of other salts, other lime salts. For example, certain parts of the earth contain a great deal of gypsum, or natural sulphate of lime, and this is soluble to some extent in water.
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