A limewater test is a chemistry test that shows the presence of carbon dioxide.
When introduced to carbon dioxide, lime water (calcium hydroxide), forms into chalk or limestone, turning the lime water a milky colour. Mixing pure carbon dioxide with lime water makes the lime water milky white in moments.
Most carbon dioxide is mixed with other gases, such as oxygen in air, but it still reacts to lime water. Leaving an open container of lime water out reveals carbon dioxide in air; the lime water turns chalky in about one week.
Another way to test for carbon dioxide is by burning something in a container; the flame goes out if carbon dioxide, and not oxygen, is inside. Because several gases besides carbon dioxide put out fire, and because this test checks only for carbon dioxide in its purest form, the limewater test is more comprehensive.
Eighteenth-century Edinburgh chemistry professor Joseph Black first proved the existence of carbon dioxide and created the limewater test. He exhaled into lime water to show his students the carbon dioxide in his breath and extinguished candle flames by pouring jars of carbon dioxide onto them.
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