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Seawater is a mixture of many different substances. Some of these substances can be observed when the water in seawater evaporates and leaves behind salt. Water, H2O, is a pure substance, a compound made of hydrogen and oxygen. Although water is the most abundant substance on earth, it is rarely found naturally in its pure form. Most of the time, pure water has to be created. Pure water is called distilled water or deionized water. In distilled water all of the dissolved substances mixed in water have been removed by evaporation. As water evaporates, it distills, or leaves the salt behind. The pure evaporated water is collected and condensed to form distilled water.

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Tap water is not distilled (SF Fig. 2.9 A). Most public tap water has chlorine. Chlorine is used to kill microbes in the water. Public tap water may also have other minerals intentionally dissolved in it, like fluoride, which prevents tooth decay. In some areas, tap water comes directly from wells and is untreated. The mineral composition of well water varies from place to place and gives water from different areas their distinctive tastes (SF Fig. 2.9 B).

 

Many people buy or make their own distilled water. In areas where the water is contaminated with harmful substances, due to the natural environment or a natural disaster, drinking distilled water can keep people healthy. If people do not like the taste of the water in an area due to the dissolved minerals that give it a distinctive taste, they may prefer the more neutral taste of distilled water.

 

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Image by Alyssa Gundersen

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SF Fig. 2.10. (B) Runners often rehydrate with sports drinks that are designed to replenish both water and salt.

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Image by Alyssa Gundersen

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Distilled water can be good for people, but they can have too much of a good thing. It is possible to overhydrate, or have too much water in your body. Although human bodies are mostly water, it is important to maintain the correct balance of water and other substances. When athletes sweat, they lose both water and salt (SF Fig. 2.10 A.). If an athlete drinks too much distilled or tap water after sweating heavily, they may throw off the balance of water and salts in their body. This affects the ability of their bodies to function properly. In rare cases, if an athlete has consumed too much distilled or tap water, the water and salt imbalance is too great and the athlete can die. This is the reason why serious athletes drink sports drinks that have a balance of water, salts, and sugars to keep them properly hydrated (SF Fig. 2.10 B). Most people can get the proper balance of water and salts through a normal diet and do not need to drink sports drinks.