Carbon dioxide

Carbon dioxide is one of the oxides of carbon. The gas is present in the air at a level of approximately 0.03% by volume. It is also found dissolved in water. The gas is one of the by-products of all decaying organic matter. Without carbon dioxide there is no life on earth. It is used by all plants in the process of photosynthesis and both plants and animals evolve carbon dioxide in respiration.

Laboratory preparation of carbon dioxide

Carbon dioxide is prepared in the laboratory by the action of dilute hydrochloric acid on marble (calcium carbonate).

When dilute hydrochloric acid is poured on marble chips, effervescence occurs. Dilute hydrochloric acid reacts with the marble chips to give calcium chloride, water and carbon dioxide.

$\text{CaCO}_{3(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{CaCl}_{2(aq)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)}$

The properties of carbon dioxide

Analyse the properties of carbon dioxide

Physical properties

1. Carbon dioxide is a colourless and odourless gas.
2. It is denser than air.
3. When the gas is cooled to −78°C, it turns straight into the solid (it sublimes). Sublimation is the change of a gas straight into a solid or change of a solid straight into a gas. Solid carbon dioxide is called dry ice. It sublimes when it is heated or exposed to air.
4. It has a melting point of −199°C and boiling point of −91.5°C.
5. Carbon dioxide does not support combustion. This is why it is used in fire extinguishers.

Chemical properties

Reaction of carbon dioxide with lime water (Test for carbon dioxide)

When a little carbon dioxide gas is bubbled into lime water (calcium hydroxide solution), the solution turns milky. This is due to the formation of a white precipitate of insoluble calcium carbonate.

$\text{Ca(OH)}_{2(aq)} + \text{CO}_{2(g)} \rightarrow \text{CaCO}_{3(s)} + \text{H}_2\text{O}_{(l)}$

This is a confirmatory test for the presence of carbon dioxide. The test serves to distinguish carbon dioxide from any other gas.

Reaction of carbon dioxide with magnesium

When a burning magnesium ribbon is lower into a gas jar containing carbon dioxide gas, it continues to burn for a short time with a spluttering flame. A white ash of magnesium oxide and black specks of carbon are formed. The black specks of carbon can be seen on the sides of the gas jar.

$2\text{Mg}_{(s)} + \text{CO}_{2(g)} \rightarrow 2\text{MgO}_{(s)} + \text{C}_{(s)}$

This clearly shows that carbon dioxide contains carbon and oxygen.

Reaction of carbon dioxide with water

Carbon dioxide reacts with water to form a weak carbonic acid. When carbon dioxide is bubbled into water, it dissolves to form a weakly acidic solution of carbonic acid:

$\text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)} \rightleftharpoons \text{H}_2\text{CO}_{3(aq)}$

The solution turns a blue litmus paper pink. This indicates that the solution is slightly acidic and hence too weak to turn the blue litmus paper to red (as strong acids do). The solution has no effect on red litmus paper.

Uses of carbon dioxide

1. Fire extinguisher: Carbon dioxide is inert (i.e. it does not burn). It is denser than air and does not support combustion.
2. Manufacture of aerated (fizzy) drinks: Soda water and mineral water contain carbon dioxide dissolved under pressure.
3. Refrigeration: Carbon dioxide is used for refrigeration purposes (i.e. in the deep-freezing of foods).
4. Manufacture of sodium carbonate by the Solvay Process: Carbon dioxide is used in the manufacture of anhydrous sodium carbonate in the Solvay Process.
5. Manufacture of baking soda: Carbon dioxide is used in making baking soda (sodium bicarbonate). Baking soda is prepared by passing carbon dioxide into cold concentrated sodium hydroxide solution: $\text{CO}_{2(g)} + 2\text{NaOH}_{(aq)} \rightarrow \text{Na}_2\text{CO}_{3(aq)} + \text{H}_2\text{O}_{(l)}$
6. Rain making: When pieces of dry ice (solid carbon dioxide) are dropped into clouds, the temperature of the clouds is lowered to such an extent that rain precipitates out.
7. Photosynthesis: Plants use carbon dioxide from the air to manufacture their own food through the process of photosynthesis.