Water is the natural solvent for much of the dirt that accumulates on clothing; moreover, it acts as a carrier to rid the clothing of all forms of dirt, both soluble and insoluble. A water good for the laundry should be clean, soft, clear, odorless, free from discoloration, free from iron, free from organic matter.

The very characteristic - its solvent power - that renders water valuable as a cleansing agent, or detergent, is the cause of its greatest shortcomings; for water may pass over, or through, soils that contain soluble substances of an undesirable nature. The characteristic known as hardness, possessed by some waters, is due to the presence of lime salts gathered in the way described. Hard water is not the best for laundry purposes, since lime salts decompose the soap used and form in its place an insoluble lime soap, which collects as a curd on the surface of the water. Such soap decomposition takes place as long as any lime remains in the water and the cleansing, or detergent, properties of soap are not in operation until every bit of lime has combined with soap to form lime soap. By leaving minute particles of lime soap in its pores, hard water is said to weaken a fabric. If the available supply of water is hard, then, the problem of the housekeeper is to find some means of removing lime or of reducing its ill effects.

According to the nature of the lime salts present, water is said to be either temporarily or permanently hard. Temporary hardness is caused by the presence of carbonate of lime, and such water may be softened by boiling. If the boiled water is allowed to stand, the lime settles at the bottom of the receptacle and the softened water may be drawn from the top of it. Permanent hardness is due to the presence of sulfate of lime or magnesium. Boiling has no softening effect on permanently hard water, but certain reagents are effective.

Another salt often very obnoxious in laundry water is iron. Its presence, even in very small amounts, may give a yellow tinge to clothing, owing to the deposit of minute particles of iron-rust in the pores of the fabric.

Organic matter may be present in the water used for laundry-purposes, which causes clothing washed in it to become dangerous to the wearer. It is very desirable in all the cited cases to eliminate mischievous substances.