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 lesson, we are focusing on identification of carbon dioxide gas.
How is Carbon Dioxide Gas Formed?
Carbonates react with dilute acids to produce salt, water and carbon dioxide gas.
Example:
Dilute hydrochloric acid reacts with limestone (calcium carbonate) to form calcium chloride (salt), water and carbon dioxide gas is formed.
Chemical Equation:
2HCl + CaCO3 → CaCl2 + H2O + CO2
Test for Carbon Dioxide Gas
Step 1: Observation during gas formation
When carbon dioxide is produced in a reaction, you will observe effervescence, which is formation of bubbles in the solution. This happens because carbon dioxide gas, which is slightly soluble in water, escapes from the solution.
Step 2: Limewater Test
To confirm the gas is carbon dioxide, we pass the gas through limewater (calcium hydroxide solution) using a delivery tube.
Lime water is calcium hydroxide, which has alkaline properties.
Carbon dioxide gas is an acidic gas. Hence, carbon dioxide gas will react with calcium hydroxide, to form a salt and water.
Chemical Equation:
Ca(OH)2 + CO2 → CaCO3 + H2O
A white precipitate is formed. This white solid is calcium carbonate, which is insoluble in water. The solution turns milky.
What Happens When Excess Carbon Dioxide is Passed?
If you allow more gas to pass through the milky limewater, the white precipitate dissolves and the solution becomes colourless again.
What is happening here?
The insoluble calcium carbonate reacts with more carbon dioxide and water to form calcium bicarbonate, also called calcium hydrogencarbonate.
Chemical Equation:
CaCO3 + H2O + CO2 → 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 – How to Identify Carbon Dioxide Gas
Carbon dioxide is a colourless and odourless gas.
It is produced when carbonates react with dilute acids (effervescence is observed).
When bubbled through limewater, it forms a white precipitate (solution turns milky).
With excess carbon dioxide, the white precipitate dissolves and the solution becomes clear again.
This two-stage limewater test is a reliable way to identify carbon dioxide gas in the laboratory.
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