Water contains a variety of chemical, physical, and biological substances which are either dissolved or suspended in it. From the moment it condenses as rain, water dissolves the chemical compounds of its surroundings as it falls through the atmosphere, runs over ground surfaces, and percolates through the soil. Water also contains living organisms which react with its physical and chemical elements. For these reasons, water must often be treated before it is suitable for use. Water containing certain chemical or microscopic organisms may be harmful to some industrial process while being perfectly adequate for others. Disease-causing (pathogenic) microorganisms in water can render it dangerous of human consumption. Groundwater from limestone areas may be very high in calcium carbonates, making it hard and thus requiring softening before use.
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