1) Nature
Carbon dioxide is slightly acidic. It turns blue litmus paper red. During rainy season, blue litmus paper kept open in the laboratory slowly turns red, due to the presence of carbon dioxide in air.
2) Stability
It is very stable gas at ordinary temperature and pressure.
3) Combustibility
Carbon dioxide is neither combustible, not a supporter of combustion. A burning splinter or a burning candle, gets put off (Fig.11.9), but metals like potassium, sodium, magnesium etc. continue to burn in carbon dioxide.
The following experiment is performed to observe the burning of magnesium in carbon dioxide.
Experiment 5
Prepare and collect a jar of carbon dioxide. Ignite a ribbon of magnesium, and introduce it in the jar of carbon dioxide. The magnesium ribbon continues to burn in carbon dioxide. Deposits of carbon can be seen on the inner sides of the jar (Fig.11.10).
4) Reaction with water
Carbon dioxide dissolves in water only to a very small extent. This small quantity of the gas combines with the water to form carbonic acid.
Form this point of view carbon dioxide can be considered as the acid anhydride of carbonic acid which is a dibasic acid. Soda water is highly concentrated solution of carbon dioxide.
5) Action with basic oxides
Carbon dioxide reacts with basic oxides such as potassium oxide, sodium oxide and calcium oxide to form carbonates.
6) Action with alkalis
Carbon dioxide is readily absorbed by alkalis such as sodium hydroxide and potassium hydroxide, to form their respective carbonates.
The above reaction of carbon dioxide with potassium hydroxide is used to purify air. When air is passed through a solution of potassium hydroxide, it absorbs carbon dioxide.
Hence, air, which is first passed through aqueous potassium hydroxide, and then passed through limewater, does not turn the latter milky, as all the carbon dioxide is absorbed by the potassium hydroxide (Fig.11.11).
In the case of limewater or calcium hydroxide solution, the ability to absorb carbon dioxide in very poor. This is because, calcium hydroxide is only sparingly soluble in water. However, carbon dioxide is able to turn the limewater milky. This milkiness is due to the formation of white insoluble calcium carbonate.
But if carbon dioxide is passed for a longer time, it reacts further with the calcium carbonate and water to from calcium hydrogen carbonate. This salt is soluble in water. Hence the milkiness gradually disappears and the solution becomes colourless.
However, it this colourless solution is boiled, the bicarbonate once again forms the insoluble calcium carbonate. As a result, the milkiness reappears.
Calcium hydrogen carbonate can exist only in the form of solution. It cannot be separated from the solution, as it decomposes to give the carbonate, carbon dioxide and water.
7) Action on heated carbon
When carbon dioxide is passed over red-hot carbon in the form of coke charcoal, etc., the carbon dioxide loses one of its two atoms of oxygen. As a result, carbon dioxide gets reduced and becomes carbon monoxide. At the same time the hot carbon also gets converted to carbon monoxide (Fig.11.12).
