Problem 66 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When \(16.8 \mathrm{~g}\) of white solid \(X\) were heated, \(4.4 \mathrm{~g}\) of acid gas \(A\) that turned lime water milky was driven off together with \(1.8 \mathrm{~g}\) of a gas \(B\) which condensed to a colourless liquid. The solid that remained, \(Y_{1}\) dissolved in water to give an alkaline solution, which with excess barium chloride solution gave a white precipitate \(Z\). The precipitate effervesced with acid, giving off carbon dioxide. Identify \(A, B\) and \(Y\) and write down the equation for the thermal decomposition of \(X\).

Step-by-Step Solution

Answer

A is \( CO_2 \), B is \( H_2O \), Y is \( CaO \). Equation: \( CaCO_3 \rightarrow CO_2 + CaO \).

Step 1: Identify gas A

The gas A turns lime water milky, indicating the presence of carbon dioxide \( CO_2 \). When \( CO_2 \) is bubbled through lime water \(( Ca(OH)_2 )\), it forms calcium carbonate \(( CaCO_3 )\), which causes the milkiness.

Step 2: Identify gas B

The gas B condenses into a colorless liquid, which suggests it is water vapor \( H_2O \). The absence of color points toward a simple compound, making \( H_2O \) the most likely candidate.

Step 3: Determine the identity of solid Y

The solid \( Y_1 \) dissolves in water to form an alkaline solution that reacts with barium chloride to produce a white precipitate, indicating the presence of a carbonate. Thus, \( Y_1 \) is likely calcium oxide \(( CaO )\), which dissolves and reacts to form calcium hydroxide \(( Ca(OH)_2 )\).

Step 4: Identify the precipitate Z

The reaction of \( Ca(OH)_2 \) with excess barium chloride produces barium carbonate \(( BaCO_3 )\), which effervesces with acid due to the release of carbon dioxide, confirming the identity of Z as \( BaCO_3 \).

Step 5: Propose the decomposition reaction for X

Given the identities of the gases and solid, \( X \) is likely calcium carbonate \(( CaCO_3 )\). The thermal decomposition equation is: \[ CaCO_3 \rightarrow CO_2 + H_2O + CaO \] which simplifies to \[ CaCO_3 \rightarrow CO_2 + CaO \].

Key Concepts

Calcium Carbonate

Calcium carbonate, symbolized as \( CaCO_3 \), is a commonly found compound in rocks. It is the main component of agricultural lime and is also used in various applications, ranging from industrial to household uses. Calcium carbonate is chalky white in appearance.

When subjected to heat, calcium carbonate undergoes a thermal decomposition reaction, which is a type of chemical reaction where a compound breaks down into simpler substances due to heat. In this context, calcium carbonate breaks down into calcium oxide \( CaO \) and carbon dioxide \( CO_2 \). This decomposition is represented as: \[ CaCO_3 \rightarrow CaO + CO_2 \] Understanding this reaction is important because it is a fundamental process used in many industrial applications, such as cement manufacturing.

Carbon Dioxide Formation

The formation of carbon dioxide \( CO_2 \) signifies an important aspect of the thermal decomposition of calcium carbonate. During the breakdown, one of the products is carbon dioxide, a colorless gas with significant roles in various environmental and physiological processes.

Its formation can be easily observed through a simple test: when carbon dioxide is bubbled through lime water, it reacts to form calcium carbonate, which causes the solution to turn milky. This is a classic laboratory method to confirm the presence of carbon dioxide, highlighting its chemical reactivity and importance in chemical identification. The turning of lime water milky acts as a clear indicator of carbon dioxide presence, making it a valuable tool for chemists in identifying this gas.

Chemical Identification

Identifying chemical substances involves understanding their unique properties. In this exercise, gas \( A \) was identified as carbon dioxide due to its ability to turn lime water milky. This reaction is one of the key indicators of carbon dioxide presence.

The other gas, B, condensed into colorless liquid, identified as water vapor \( H_2O \), due to its transparency and common occurrence during decomposition reactions involving carbonates. This exemplifies the process of chemical identification where knowing the properties and typical reactions of compounds allows for accurate determination of substances.

• Gas A: Identified as \( CO_2 \) due to the lime water test.
• Gas B: Identified as \( H_2O \) owing to its colorless liquid state at room temperature after condensation.

Solid \( Y_1 \), which dissolved to give an alkaline solution, was identified as calcium oxide \( CaO \). The formation of a white precipitate with barium chloride confirms the presence of carbonate.

Precipitation Reaction

The concept of precipitation is crucial in identifying certain chemical compounds. In these reactions, solutes react to form an insoluble product, known as the precipitate.

When the solid \( Y_1 \), which is calcium oxide \( CaO \), dissolves in water, it forms calcium hydroxide \( Ca(OH)_2 \). Upon reacting with an excess of barium chloride \( BaCl_2 \), a white precipitate of barium carbonate \( BaCO_3 \) is formed, illustrating a classic precipitation reaction:

\[ Ca(OH)_2 + BaCl_2 \rightarrow BaCO_3 + 2HCl \]

Precipitation reactions such as this are important because they enable chemists to isolate and identify substances based on their solubility and reaction behavior. In this context, the effervescence of the white precipitate when treated with acid, releasing \( CO_2 \), helps further confirm its identity as barium carbonate.