Pure water is the commonest compound that exists. The water we use comes to us either in the form of rain or of melted snow from the mountains. Part of it trickles or percolates through the ground and dissolves any soluble material or gases with which it comes in contact. When the water has passed into the ground and comes in contact with limestone and magnesium compounds, some of the substances are dissolved and the water becomes hard. This kind of water appears when an artesian well is drilled. Water that flows over the surface of the earth contains suspended matter or dirt and is generally called soft water.

Thus the distinction between hard and soft water depends upon the substances which they carry, and especially upon their chemical action. In soft water, soap readily lathers and the suds thus formed exert a rapid cleansing action. In hard water, soap lathers only with difficulty and often will not lather satisfactorily at all, because of the formation of lime soap, which is insoluble.

If hard water is boiled the hardness often disappears, and soap then acts as in soft water, but in some cases boiling has no effect. Such a condition is wholly due to the action of the dissolved solids upon the soap. Hard water contains either magnesium or calcium sulphates or carbonates. If carbonates only are present in the water, it is likely to become soft when it is boiled, for boiling drives out carbonic acid gas (CO2) which holds the carbonates in solution.

Thus we see that water may differ in its properties according to the influence of substances to which it has been exposed. Rain and snow water are difficult to obtain; river water may be muddy, more especially in stormy weather; artesian well water will contain in solution the minerals with which it has come in contact. The source of the purest water is a location near a mountain, or in a mountainous country. There the upland surface water has not yet come in contact with impurities, and has had little opportunity to dissolve lime or magnesia compounds.