Problem 115 - A Complete Resource Book in Chemistry for JEE Main

Question

Some large white transparent crystals are left out in a bowl for several days. They are then observed to have changed their form into white powder. The crystals may have been of (a) calcium oxide (b) sodium carbonate (c) ammonium chloride (d) sodium chloride

Step-by-Step Solution

Answer

The crystals were likely sodium carbonate.

Step 1: Understand the Type of Change

The problem describes a physical transformation where crystalline solid changes into a powder form when left out, indicating a process like efflorescence or loss of water of crystallization.

Step 2: Analyze Potential Candidates

Evaluate each compound: (a) Calcium oxide does not form crystals or powder under typical conditions. (b) Sodium carbonate (specifically sodium carbonate decahydrate) can lose water to form a white powder. (c) Ammonium chloride does not typically leave any residue when left out. (d) Sodium chloride does not change its form significantly upon exposure to air.

Step 3: Conclusion

Sodium carbonate is a common crystalline substance that loses water molecules when exposed to air, turning into a powder. This process, known as efflorescence, aligns with the observation made in the exercise.

Key Concepts

Crystallization

Crystallization is the process through which a solid forms, where the atoms or molecules are structured in a specific, repeated pattern extending in all directions. This orderly pattern is what gives crystals their unique shapes and sizes. It is a fundamental method used to purify solid substances. In the context of sodium carbonate, crystallization involves the organized arrangement of sodium and carbonate ions along with water molecules. These water molecules form part of the crystal structure, contributing to the solid's overall stability.

Crystals can vary in transparency and size, sometimes appearing as large, sparkling formations. When exposed to air, some crystalline substances undergo transformation due to interactions with the environment, such as humidity changes, leading to loss of water molecules and changes in appearance.

Sodium Carbonate

Sodium carbonate, also known as washing soda or soda ash, is a versatile chemical compound with the formula Na extsubscript{2}CO extsubscript{3}. It often appears in a crystalline form when hydrated as sodium carbonate decahydrate, which is noted for its large transparent crystals.

It is commonly used in household cleaning products and as a water softener. When sodium carbonate crystals are left in an open environment, they tend to lose their water of crystallization over time. This process results in the conversion of these crystals into a fine white powder, which is predominantly anhydrous sodium carbonate. This characteristic behavior is crucial in understanding other sodium carbonate applications and why it must be stored in airtight containers to maintain its crystalline state.

Physical Transformation

Physical transformation refers to changes that occur in a substance's physical state or appearance without altering its chemical composition. These changes are usually reversible, such as dissolving, melting, evaporation, and crystallization.

In the case of sodium carbonate, the transformation from crystalline form to powder due to efflorescence is a typical physical change. Efflorescence is the process where water molecules evaporate from a hydrated crystal, leading to a structural transition. As the water leaves, it causes the breakdown of the orderly arrangement of ions, resulting in a powdery form.

Understanding this transformation aids in recognizing how and why certain materials should be handled and stored properly to preserve their physical properties. This knowledge can also extend to lessons in environmental conditions and their impacts on material stability.