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Which gas turns lime water milky?
A. Oxygen
B. Carbon dioxide
C. Hydrogen
D. Nitrogen
Correct Answer Is Option = Carbon dioxide
Notes / Explanation
The gas that turns lime water milky is carbon dioxide, and this reaction is one of the most well-known and useful tests in chemistry, commonly used to detect the presence of this particular gas. Lime water is a clear, colourless liquid made by dissolving a certain substance obtained from limestone in water, and it has the special property of reacting with carbon dioxide in a way that produces a visible change. When carbon dioxide gas is passed through or comes into contact with clear lime water, a chemical reaction takes place between the gas and the substance dissolved in the water, and this reaction produces a fine white solid substance that does not dissolve but instead forms tiny particles suspended throughout the liquid. These fine white particles give the previously clear liquid a cloudy, milky or chalky appearance, which is why it is said that the carbon dioxide turns the lime water milky. This change from a clear liquid to a milky one is easy to observe, and because it happens specifically with carbon dioxide, it provides a simple and reliable way of testing whether a particular gas is carbon dioxide. This test is widely used in the study of chemistry and in various practical situations. For example, it can be used to show that the gas that people breathe out contains carbon dioxide, since blowing air from the lungs through lime water causes the lime water to turn milky, demonstrating that carbon dioxide is produced by the body. It can also be used to detect the carbon dioxide that is produced in various chemical reactions and natural processes. If a large excess of carbon dioxide is passed through the milky liquid, the cloudiness may in some cases disappear again as a further reaction takes place, but the initial turning milky is the key sign used in the test. Carbon dioxide is an important gas that is present in the air, that is produced by the breathing of living things and by burning, and that is used by plants in making their food. For PPSC and other competitive examinations, the key points to remember are that the gas that turns lime water milky is carbon dioxide, that this happens because the carbon dioxide reacts with the substance dissolved in the lime water to form a fine white solid, that the change from clear to milky is used as a test for carbon dioxide, and that this test can show that carbon dioxide is present in the air breathed out by the body.
