Properties of Water.—When pure, water is colourless, or bluish when seen in large quantity. It should be quite inodorous. If, after keeping it for some time in a perfectly clean vessel, or if on heating it a smell is developed, the water is bad. Its taste should be pleasant and sparkling from the atmospheric gases dissolved in it. Bitterness generally indicates the presence of sulphate of magnesium (Epsom salts). Saltness is always a suspicious property, except in water obtained in the neighbourhood of salt mines or brine springs, or near the sea. It should be soft to the touch, and should dissolve soap easily. It should be bright and clear, and contain no suspended matters. Clear water is not necessarily pure, but turbid water is always to be rejected; the only exception being the brownish-tinged water from moors, which is not hurtful. In all other cases, printed matter should be legible through at least 18 inches of water in a clear glass cylinder. Thoroughly dissolved organic matter is less dangerous than suspended; the turbidity of water is therefore of great importance. But water may be bright and sparkling and apparently perfectly clear, and yet highly dangerous. The most important of the physical properties of water in regard to health are the absence of smell and turbidity, and these can be ascertained by even the most inexperienced. The chemical tests for the more important impurities are given (pages 85 to 87).
The impurities of water may be classed under four heads—gaseous, mineral, vegetable, and animal.
The gases ordinarily present in water cannot properly be regarded as impurities, inasmuch as they are always present, and greatly increase its palatableness. The dissolved nitrogen and oxygen bear to each other the proportion 1·42 to 1; where sewage contamination occurs, the oxygen will be diminished or disappear, owing to oxidation of the organic matter.
The amount of carbonic acid gas in water varies greatly. It may be considerable in chalk waters, and in contaminated well-water.
Mineral Impurities.—Mineral impurities are dissolved by water in its course through the soil, and so will vary with the character of the latter. 1. The water obtained from granitic formations contains very little mineral matter, often not more than two to six grains per gallon. Clay slate water is also generally very pure, as is the water from hard trap rocks. 2. The water from millstone grit and hard oolite is very pure, often containing only four to eight grains per gallon, chiefly calcium and magnesium sulphate and carbonate. 3. Soft sand-rock waters usually contain thirty to eighty grains per gallon of sodium salts, with a little lime and magnesia. 4. Loose sand and gravel waters vary greatly. They may be almost free from mineral matter, or the solids may be more than seventy grains per gallon, including much organic matter. 5. Waters from the lias clays vary somewhat, but commonly contain a large quantity of calcium and magnesium sulphates. 6. Chalk waters generally contain from seven to twenty grains of calcium carbonate, with smaller quantities of other salts. 7. Limestone and magnesian limestone waters differ from the last, in containing more calcium sulphate and less calcium carbonate, as well as much magnesium sulphate and carbonate in the dolomite districts. 8. Selenitic waters contain calcium sulphate in considerable quantities. 9. Clay waters usually possess the characters of water from surface wells, and are objectionable. 10. Alluvial waters generally contain a large amount of various salts, including the various calcium, magnesium, and sodium salts. 11. Artesian well water varies greatly in composition. It may contain a large amount of sodium and potassium salts, or a small quantity of iron, or calcium salts.
The commonest and most important mineral constituent of water is calcium carbonate, next to this calcium sulphate. These two salts are the chief causes of hardness of water. For practical purposes as regards use in domestic matters and in manufactures, the most important classification of waters is into hard and soft. The degree of hardness varies within wide limits—from rain-water, which has no hardness at all, to the water from new red sandstone rocks which sometimes possesses a hardness of 90 degrees; or wells in the gravel, in which it may be as much as 152 degrees.
The following classification of waters, according to the degree of hardness, beginning with the least hard and gradually increasing in hardness, is from the sixth report of the Rivers Pollution Commissioners:—1. Rain-water. 2. Upland surface. 3. Surface from cultivated land. 4. River. 5. Spring. 6. Deep-well. 7. Shallow-well water.
Calcium carbonate is the most common cause of hardness, and the hardness produced by it is remediable by boiling or chemical means. Calcium carbonate (chalk) is rendered soluble in water, by the carbonic acid contained in the latter, a double bicarbonate being thus formed. The air contained in the interstices of the soil through which water passes, often contains 250 times as much carbonic acid as ordinary air. The water, in percolating through the soil, dissolves this carbonic acid, and thus is able to take up a considerable amount of chalk.
The amount of hardness in any given water is expressed in degrees, one degree being equivalent to a grain of calcium carbonate in a gallon of water. Clarke’s soap test is employed to detect the amount of hardness. It consists of a solution of soap of a known strength. Soft water will form a lather at once with this; hard water will only form a lather after all the calcium salt is neutralised. The amount of Clarke’s solution required before a lather is produced, will give an estimate of the amount of hardness.
To Determine the Total Hardness take 70 c.c. of the water and place in a stoppered bottle. From a burette run in a sufficient quantity of the standard soap solution (of which 1 c.c. equals 1° of hardness), to produce a lather on shaking the water, which remains unbroken after standing five minutes. Thus, if 7·5 c.c. of the soap solution were required, the hardness is 6°·5, as 1 c.c. of the solution is required to produce a lather in soft water. The 6°·5 means 6·5 milligrammes of calcium carbonate in 70 c.c. or 6·5 grains in a gallon of the water.