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.
The most obvious difference between Li-ion and NiMH batteries is the material used to store power. Lithium-ion batteries are made of carbon and highly reactive lithium, which can store a lot of energy. Nickel metal hydride batteries use hydrogen to store energy, with nickel and another metal (such as titanium) keeping a lid on the hydrogen ions.