Which of the following gases is mentioned below?

Question

Examoogle · Accepted Answer

The correct answer is Option C: Carbon-dioxide gas. When carbon dioxide gas is passed through lime water (an aqueous solution of calcium hydroxide), it reacts to form an insoluble white precipitate of calcium carbonate, which turns the solution milky.

Step-by-Step Solution & Concepts:
When carbon dioxide gas is passed through lime water (an aqueous solution of calcium hydroxide), it reacts to form an insoluble white precipitate of calcium carbonate, which turns the solution milky.

Working Principle: The chemical reaction involves passing carbon dioxide ($CO_2$) through calcium hydroxide ($Ca(OH)_2$), producing calcium carbonate ($CaCO_3$), which appears as a white precipitate, along with water ($H_2^O$). The balanced chemical equation is: $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$. If excess $CO_2$ is passed, the milkiness disappears due to the formation of soluble calcium bicarbonate ($Ca(HCO_3)_2$).

Key Formulas: $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$ — Reaction of lime water with carbon dioxide; $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2(aq)$ — Reaction with excess carbon dioxide

Key Points: Calcium hydroxide solution is commonly known as lime water.; Passing carbon dioxide turns lime water milky due to the formation of insoluble calcium carbonate ($CaCO_3$).; This reaction is a standard laboratory test for the detection of carbon dioxide gas.; Passing excess carbon dioxide makes the milky solution clear again due to the formation of soluble calcium hydrogen carbonate.

Advantages: Provides a simple, visual confirmatory test for carbon dioxide gas in the laboratory.

Disadvantages: Other acidic gases like sulfur dioxide ($SO_2$) can also turn lime water milky, causing potential ambiguity if not tested properly.

Applications: Testing for the presence of carbon dioxide in chemical reactions and respiration experiments.; Industrial manufacture of sodium carbonate via the Solvay process.

Additional Info: Option A (Helium gas) is an inert noble gas and does not react with calcium hydroxide.. Option B (Oxygen gas) supports combustion but does not form a white precipitate with lime water.. Option D (Hydrogen gas) is a combustible gas that does not react with calcium hydroxide.

Answer: C is correct — Carbon dioxide gas is passed through calcium hydroxide to form calcium carbonate and water.

Exam Tip: Remember that non-metal oxides like $CO_2$ are acidic in nature and react with bases (like calcium hydroxide) to form salt and water, which is a classic neutralization-precipitation reaction often tested in RRB and SSC chemistry exams.

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