In certain chemical reactions, gases are formed. Unlike gases like chlorine, which has greenish yellow colour with pungent smell, most gases are colourless and odourless. How do we identify these gases? In this video, we are focusing on identification of carbon dioxide gas.
How do we form carbon dioxide gas?
Carbonates react with dilute acids to produce salt, water and carbon dioxide gas.
For example, when dilute hydrochloric acid reacts with limestone (calcium carbonate), calcium chloride (salt), water and carbon dioxide gas is formed.
2HCl + CaCO3 → CaCl2 + H2O + CO2
Testing and observations for identifying Carbon dioxide gas
In a reaction that forms carbon dioxide gas, you will observe effervescence, which is formation of bubbles in the solution. This is observed because the slightly soluble carbon dioxide gas escapes from the solution formed.
We bubble this gas into the limewater through a delivery tube. Lime water is calcium hydroxide, which has alkaline properties. If you recall, carbon dioxide gas is actually an acidic gas. Hence, carbon dioxide gas will react with calcium hydroxide, to form a salt and water.
Ca(OH)2 + CO2 → CaCO3 + H2O
The white precipitate that you see in the limewater is actually calcium carbonate, which is an insoluble carbonate.
What happens if you allow more carbon dioxide gas to pass through?
If you allow more gas to pass through, you will realise the white precipitate actually dissolves. What is happening here?
When calcium carbonate further reacts with carbon dioxide gas, calcium bicarbonate, also called calcium hydrogencarbonate is formed.
CaCO3 + H2O + CO2 à Ca(HCO3)2
This compound is soluble in water, and it forms colourless solution. Hence, you will notice the white precipitate disappearing if you allow more carbon dioxide gas to bubble through.
Summary
Like many other gases, carbon dioxide gas is colourless and odourless.
When it is formed, we should observe effervescence. We bubble the gas through limewater, which is an alkali. If white precipitate is formed, we confirm the gas is carbon dioxide, which is an acidic gas. The white precipitate is insoluble calcium carbonate, which is the product formed from an acid and alkali reaction.
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