Even carbon dioxide can dissolve in the groundwater, causing the water to be naturally carbonated. This water is sometimes sold as mineral water. Groundwater can be heated by magma below the Earth's surface. The heated water can create hot springs, springs with water that is naturally hot (Figure 13.23). Some hot springs are used as natural hot tubs and are considered therapeutic and spiritual by some people. However, hot springs can be dangerous, too.
We used a level tablespoon of salt in a cup of water. The batteries we have shown here are more correctly called cells. An actual battery is made up of two or more cells. The zinc-air cell produces about three quarters of a volt.
Cathodes can be made of manganese dioxide, mercuric oxide, nickel oxyhydroxide, lead dioxide or lithium oxide. Potassium hydroxide is the electrolyte used in most battery types, but some batteries use ammonium or zinc chloride, thionyl chloride, sulfuric acid or lithiated metal oxides. The exact combination varies by battery type.