Which gas is passed over dry slaked lime in the industrial preparation of bleaching powder?

Introduction / Context:
This question tests your knowledge of the industrial preparation of bleaching powder, an important chlorine based compound. Bleaching powder is used for disinfection and bleaching in households and industry. Knowing which gas is used in its production helps reinforce your understanding of the chemistry of chlorine and its compounds.

Given Data / Assumptions:

• Substance prepared: bleaching powder.
• The process involves passing a gas over dry slaked lime Ca(OH)2.
• Options include oxygen, hydrogen, nitrogen, chlorine and ammonia.
• Assume the standard process described in school chemistry textbooks.

Concept / Approach:
Bleaching powder is essentially calcium oxychloride, written as Ca(OCl)Cl or CaOCl2. It is produced industrially by passing chlorine gas over dry slaked lime. The chlorine reacts with calcium hydroxide to form bleaching powder and water. Therefore, the correct gas is chlorine, not oxygen or other gases.

Step-by-Step Solution:
Step 1: Recall the formula of slaked lime, which is calcium hydroxide, Ca(OH)2. Step 2: In the manufacturing process, dry Ca(OH)2 is spread on trays in a chamber. Step 3: Chlorine gas is passed over the dry slaked lime. Step 4: The simplified reaction is often written as: Ca(OH)2 + Cl2 → CaOCl2 + H2O. Step 5: CaOCl2 is bleaching powder, which contains both CaCl2 and Ca(OCl)2 like species. Step 6: Other gases such as oxygen, hydrogen, nitrogen and ammonia are not used in this preparation step. Step 7: Hence, the gas used is chlorine.

Verification / Alternative check:
Most introductory chemistry texts describe chlorine and its compounds in one chapter. In that chapter, the manufacture of bleaching powder is highlighted as a key industrial use of chlorine gas. The fact that bleaching powder is a chlorine containing bleaching agent should remind you of the direct role of chlorine gas in its production. Oxygen is associated more with combustion and oxidation processes, while ammonia is associated with fertilisers, not bleaching powder.

Why Other Options Are Wrong:
Oxygen does support combustion and is involved in many oxidation reactions but is not the gas used to make bleaching powder. Hydrogen and nitrogen do not form bleaching powder with slaked lime under these conditions. Ammonia reacts with certain substances to form fertilisers and other compounds, but not bleaching powder. Only chlorine reacts with Ca(OH)2 in the described industrial process to yield bleaching powder.

Common Pitfalls:
Because bleaching powder and bleaching agents in general are associated with oxidation, some students might guess oxygen. However, the key to avoiding this mistake is remembering that bleaching powder is a chlorine based compound. Another pitfall is to misremember the manufacturing process and confuse chlorine gas with other gases used in different industrial reactions. Keeping a separate mental list of key industrial processes like Haber process for ammonia, Contact process for sulphuric acid and bleaching powder preparation helps clarify such questions.

Final Answer:
In the industrial preparation of bleaching powder, the gas passed over dry slaked lime is chlorine.