Question

Acetic acid was added to a solid X kept in a test tube. A colourless, odourless gas Y was evolved. The gas was passed through lime water, which turned milky. It was concluded that:

• A. Solid X is sodium hydroxide and the gas Y is \(CO_2\).
• B. Solid X is sodium bicarbonate and the gas Y is \(CO_2\).
• C. Solid X is sodium acetate and the gas Y is \(CO_2\).
• D. Solid X is sodium bicarbonate and the gas Y is \(SO_2\).

Hint:

Whenever any acid reacts with a metal carbonate or metal hydrogen carbonate, carbon dioxide gas (\(CO_2\)) is always evolved.
The milkiness test of lime water is the definitive test for confirming the presence of \(CO_2\) gas.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question describes a chemical reaction where acetic acid is mixed with an unknown solid substance \(X\) in a test tube.
This reaction produces a colorless and odorless gas \(Y\).
When gas \(Y\) is bubbled through lime water, the lime water turns milky, which is a classic qualitative analytical test for the detection of carbon dioxide gas.

Step 2: Key Formula or Approach:
Carboxylic acids, like acetic acid (\(CH_3COOH\)), react with metal carbonates or metal bicarbonates to produce a salt, water, and carbon dioxide gas.
The balanced chemical equation for the reaction of acetic acid with sodium bicarbonate (\(NaHCO_3\)) is:
\[ CH_3COOH\text{(aq)} + NaHCO_3\text{(s)} \to CH_3COONa\text{(aq)} + H_2O\text{(l)} + CO_2\text{(g)}\uparrow \]
The balanced chemical equation for the reaction of the evolved gas with lime water (calcium hydroxide solution) is:
\[ Ca(OH)_2\text{(aq)} + CO_2\text{(g)} \to CaCO_3\text{(s)}\downarrow + H_2O\text{(l)} \]

Step 3: Detailed Explanation:

• Acetic acid is a weak organic acid that contains a carboxyl group.

• When it is added to sodium bicarbonate (solid \(X\)), a rapid reaction occurs, causing effervescence due to the evolution of carbon dioxide gas (gas \(Y\)).

• Carbon dioxide is a colorless and odorless gas.

• When this carbon dioxide gas is passed through lime water (aqueous solution of calcium hydroxide), it reacts to form calcium carbonate (\(CaCO_3\)), which is an insoluble white precipitate.

• The suspension of this fine white precipitate suspended in water gives the lime water its characteristic milky appearance.

• Option (A) is incorrect because sodium hydroxide (\(NaOH\)) is a strong base, and its reaction with acetic acid is a simple neutralization reaction that yields sodium acetate and water without producing any gas.

• Option (C) is incorrect because sodium acetate is already the salt of the reaction and does not react with acetic acid to release gas.

• Option (D) is incorrect because the reaction of sodium bicarbonate with an acid cannot produce sulfur dioxide (\(SO_2\)), as there is no sulfur source present in the reactants.


Step 4: Final Answer:
Therefore, the solid \(X\) is sodium bicarbonate and the gas \(Y\) is \(CO_2\).