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.
How Does It Work. Batteries are comprised of two different metals suspended in an acidic solution. With the Fruit-Power Battery, the two metals are zinc and copper. The zinc is in the galvanization on the nails, and the pennies are actually copper-plated zinc. The acid comes from the citric acid inside each lemon. The two metal components are electrodes, the parts of a battery where electrical current enters and leaves the battery. With a zinc and copper setup, the electron flow is out of the penny (copper) and into the nail (zinc) through the acidic juice inside the lemon.