How to test for Carbon Dioxide with the use of Limewater

Carbon dioxide can be detected using a simple laboratory test. Limewater turns cloudy white when carbon dioxide is bubbled through it.

Medicine

When the old vaudeville-style medicine shows promoting various patent medicines were in full swing, limewater was often used as a part of the act. The salesperson would have an audience member blow through a straw into a glass of limewater. Since the exhaled gas is carbon dioxide, the water would turn cloudy; the huckster then announced that this reaction proved that the audience member suffered from some ailment. If too much carbon dioxide comes into contact with the cloudy limewater, it will cause the calcium carbonate precipitate to redissolve to form soluble calcium bicarbonate.

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

The huckster had a patent medicine bottle filled with vinegar or some similar acid. He then would pour some of the acid into the glass of cloudy limewater. The acid reacted with the calcium carbonate, and the water would instantly clear. This demonstrated the potent effect of the nostrum he was selling to eliminate the "disease" demonstrated by the audience member.

Chemistry

In chemistry, limewater can be used to detect the presence of carbon dioxide because limewater reacts with carbon dioxide to produce a precipitate of calcium carbonate:

Ca(OH)_{2 (aq)} + CO_{2 (g)} → CaCO_{3 (s)} + H₂O _(l)

If excess CO₂ is added, the following reaction takes place: CaCO₃ + H₂O + CO₂ --> Ca(HCO₃)₂ (colourless)

Limewater is also used in experiments involving aerobic or anaerobic respiration, to determine whether carbon dioxide was produced, by first boiling the limewater and then pouring the solute into the boiling limewater. When limewater reacts with CO₂ it becomes milky, because of the calcium carbonate, or chalk, produced.

making limewater

Limewater can be made by mixing excess calcium hydroxide with distilled water, or deionized water. The mixture needs to be shaken to ensure the solution is saturated with calcium hydroxide. It is then left to settle and the clear "saturated" solution is siphoned off the sediment.

Limewater

Lime water is the common name for saturated calcium hydroxide solution. It is sparsely soluble. Its chemical formula is Ca(OH)2. Since calcium hydroxide is only sparsely soluble, i.e. ca. 1.5 g per liter at 25 °C,[1] there is no visible distinction to clear water (homogeneous). Attentive observers will notice a slightly earthy smell. It is clearly distinguishable by the alkaline taste of the calcium hydroxide. The term lime refers to the mineral, rather than the fruit. When exposed with carbon dioxide, limewater turns into a milky solution.

While limewater is a clear solution, milk of lime on the other hand is a suspension of calcium hydroxide particles in water. These particles give it the milky aspect. It is commonly produced by reacting quicklime (calcium oxide) with an excess of water - usually 4 to 8 times the amount of water to the amount of quicklime. Reacting water with quicklime is sometimes referred to as "slaking" the lime. The calcium oxide will convert to the hydroxide according to the following reaction scheme:

 CaO + H2O → Ca(OH)2

This reaction is strongly exothermic and will generate enough heat to bring the suspension to a scalding temperature. At a ratio of 2 parts water to 1 part lime (by weight), the generated heat is sufficient to bring the suspension, i.e. the water in it, to boil.

Milk of lime is an alkaline with a pH of 12.3. It is commonly used in the chemical industry and as a neutralizing agent in municipal waste water treatment. While it has a multitude of other uses, it is best known in its (historical) use as a paint: lime wash or whitewash.