Test for carbon dioxide using limewater

Chemical analysis • Identification of common gases

Flashcards

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Safety consideration for limewater

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Limewater is a basic solution and may irritate skin and eyes, so handle with care and avoid contact.

Key concepts

What you'll likely be quizzed about

Definition of carbon dioxide

Carbon dioxide is a colourless, odourless gas with formula CO2. Carbon dioxide is slightly soluble in water and reacts with basic water solutions to form carbonate species.

Limewater and its role

Limewater is an aqueous solution of calcium hydroxide, Ca(OH)2, which is clear when prepared correctly. Limewater provides calcium ions that react with dissolved carbon dioxide to produce an insoluble white precipitate of calcium carbonate, which causes cloudiness.

Standard test procedure

Collect the gas sample in a delivery tube and bubble it through a sample of limewater in a test tube or gas jar. Continuous bubbling forces the gas to dissolve and react with the limewater. A steady formation of a milky or cloudy suspension indicates the production of calcium carbonate and therefore the presence of carbon dioxide.

Chemical equations and explanation

Carbon dioxide reacts with calcium hydroxide according to the equation: CO2 + Ca(OH)2 → CaCO3(s) + H2O. Continued addition of CO2 causes the white calcium carbonate to dissolve by forming soluble calcium hydrogen carbonate: CaCO3(s) + CO2 + H2O → Ca(HCO3)2(aq). The first reaction causes cloudiness; the second reaction causes the cloudiness to clear if CO2 is in excess.

Limitations and possible false positives

Very low concentrations of carbon dioxide may not produce a visible cloud, causing a false negative. Other acidic gases or impurities can also cause turbidity or colour changes in limewater, causing false positives. High concentrations of CO2 can clear the cloudiness after initial precipitation, so timing and observation method affect the result.

Key notes

Important points to keep in mind

Limewater is a Ca(OH)2 solution that turns cloudy when CO2 is present due to CaCO3 formation.

Cloudiness indicates CO2; prolonged bubbling can clear the cloudiness by forming Ca(HCO3)2.

Low CO2 concentration may not produce visible cloudiness; interpret negative results cautiously.

Other acidic gases or impurities can cause turbidity; confirmatory tests may be necessary.

Use fresh, clear limewater and bubble gas through a delivery tube for reliable results.

Record timing of cloud formation because immediate precipitation followed by clearing indicates excess CO2.