Question

The compound \(\Lambda\) on heating gives a colourless gas and residuc which dissolves in water to give B. When cxcess of \(\mathrm{CO}_{2}\) is passed into \(\mathrm{B}\) and gently heated, \(\Lambda\) is formed. The compound \(\Lambda\) is (1) \(\mathrm{CaSO}_{4} 2 \mathrm{II}_{2} \mathrm{O}\) (2) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (3) \(\mathrm{CaCO}_{3}\) (4) \(\mathrm{K}_{2} \mathrm{CO}_{3}\)

Short answer

\( \text{CaCO}_3 \)

Step-by-step solution

- Identify the properties of \(\text{compound} \Lambda\)

The compound \(\text{compound} \Lambda\) on heating gives a colourless gas and a residue. This indicates that \(\text{compound} \Lambda\) is a carbonate, as heating a carbonate typically releases \(\text{CO}_2\) gas. Among the given options, \( \text{CaCO}_3, \text{Na}_2 \text{CO}_3, \text{K}_2 \text{CO}_3 \) are carbonates.

- Determine the reaction product B

When \(\text{CaCO}_3\) is heated, it decomposes to form \(\text{CaO}\) and releases \(\text{CO}_2\). The residue that dissolves in water to give \(\text{B}\) is \(\text{CaO}\) (lime), which dissolves to form \(\text{Ca(OH)}_2\) (B). None of the other options decompose in this manner.

- Forming \(\text{compound} \Lambda\) again

When excess \(\text{CO}_2\) is passed through \(\text{Ca(OH)}_2\) solution and heated gently, \(\text{Ca(OH)}_2\) converts back into \(\text{CaCO}_3\), which proves that \(\text{compound} \Lambda\) is \(\text{CaCO}_3\).

Key concepts

carbonate decomposition

Carbonate decomposition refers to the breaking down of carbonate compounds when they are heated. This results in the formation of metal oxides and the release of carbon dioxide gas (\text{CO}\text{2}). For instance, calcium carbonate (\text{CaCO}_3) decomposes into calcium oxide (\text{CaO}) and carbon dioxide when heated: $$ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 $$ This type of reaction is common among carbonate compounds and is an example of a thermal decomposition reaction. When carbonate compounds decompose, they typically leave behind a residue that can dissolve in water to form a different type of compound. The released carbon dioxide often appears as a colorless gas, which can help in identifying the reaction taking place. Understanding carbonate decomposition is essential because it plays a significant role in various industrial processes, such as the manufacture of cement and lime. By studying these reactions, students gain insight into the broader category of chemical reactions and learn how to predict the products of these reactions based on the starting materials.

chemical reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds, resulting in new products. In the case of the decomposition of calcium carbonate (\text{CaCO}_3), the reaction can be represented as: $$ \text{CaCO}_3 (s) \rightarrow \text{CaO} (s) + \text{CO}_2 (g) $$ Here, \text{CaCO}_3 (solid) undergoes thermal decomposition to form \text{CaO} (solid) and \text{CO}_2 (gas). This specific type of reaction falls under the category of decomposition reactions, where a single compound breaks down into two or more simpler substances. When \text{CaO} (calcium oxide) dissolves in water, it forms \text{Ca(OH)}_2 (calcium hydroxide), a solution which can react further with carbon dioxide: $$ \text{CaO} (s) + \text{H}_2\text{O} (l) \rightarrow \text{Ca(OH)}_2 (aq) $$ $$ \text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2\text{O} (l) $$ Ultimately, understanding chemical reactions and their representation through balanced chemical equations allows students to predict the outcomes of various reactions, balance chemical equations, and gain deeper insight into the nature of reactants and products involved in these processes.

thermal decomposition

Thermal decomposition is a process in which a chemical compound breaks down into two or more simpler substances when heated. This process is critical in many industrial and scientific applications. For example, when calcium carbonate (\text{CaCO}_3) is subjected to heat, it undergoes thermal decomposition to form calcium oxide (\text{CaO}) and carbon dioxide (\text{CO}_2} gas: $$ \text{CaCO}_3 (s) \rightarrow \text{CaO} (s) + \text{CO}_2 (g) $$ The remaining solid residue, \text{CaO}, can dissolve in water to form \text{Ca(OH)}_2 (calcium hydroxide). The thermal decomposition of carbonates, such as \text{CaCO}_3, is a common example of this reaction type. Thermal decomposition reactions are endothermic, meaning they absorb heat from the surroundings. This is observed in many carbonate compounds, such as \text{Na}_2\text{CO}_3 and \text{K}_2\text{CO}_3, though each compound has a different stability and decomposition temperature. Understanding thermal decomposition is crucial for fields such as material science, geology, and environmental science, where the stability and breakdown of materials under heat is of primary concern.