KINETIC THEORY AND THE STATES OF MATTER 3

 QUESTIONS AND ANSWERS: 

Q25: How is hydrogen gas prepared in the laboratory, and how is it tested? 

A25:

• Preparation Equation:

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g) or

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g) or

Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g).

• Process: Reacting metals like zinc granules or magnesium ribbon with dilute hydrochloric acid or sulphuric acid in a conical flask, collecting the gas by displacement of water.
• Test: Put a burning/lighted splint in the gas; a “pop” sound indicates the presence of hydrogen gas.
• Drying: Pass the gas through anhydrous CaCl₂.

Q26: What are the physical and chemical properties of hydrogen gas, and what are its uses? 

A26:

• Physical properties: Lightest gas, lowest density, colourless, odourless, insoluble in water.
• Chemical properties: Neutral to litmus, unreactive under normal conditions, does not support combustion, burns in oxygen (air) with a pop sound to produce water (2H₂ (g) + O₂ (g) → 2H₂O (l)), reacts with halogens (e.g., H₂ (g) + F₂ (g) → 2HF (g)).
• Uses: Production of ammonia gas (Haber process), manufacture of margarine (hydrogenation), oxy-hydrogen flame for cutting and welding, fuel cells.

Q27: What safety precautions are important when preparing hydrogen gas? 

A27: Wear safety goggles and a lab coat, handle acids with care, and conduct the experiment in a well-ventilated area or under a fume hood.

Q28: How is carbon dioxide gas prepared in the laboratory, and how is it tested? 

A28:

• Preparation Equation: CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g).
• Process: Reacting marble chips (calcium carbonate) with dilute hydrochloric acid in a conical flask, collecting the gas by displacing water.
• Test: Pass the gas through limewater (saturated solution of calcium hydroxide), which turns milky.
• Drying: Pass the gas through concentrated sulphuric acid or anhydrous CaCl₂.

Q29: What are the physical and chemical properties of carbon dioxide gas, and what are its uses? 

A29:

• Physical properties: Colourless, odourless, denser than air, soluble in water, condenses into dry ice.
• Chemical properties: 
  Turns moist blue litmus red, turns limewater milky
  (CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l)), 
  excess CO₂ causes milkiness to disappear (CaCO₃ (s) + H₂O (l) + CO₂ (g) → Ca(HCO₃)₂ (aq)), reacts with water in photosynthesis (6CO₂ (g) + 6H₂O (l) → C₆H₁₂O₆ (s) + 6O₂ (g)),
  does not support combustion.
  
  
• Uses: Photosynthesis, fire extinguishers, fizzy drinks, refrigerants.

Q30: How is ammonia gas prepared in the laboratory, and how is it tested? 

A30:

• Preparation Equation: Ca(OH)₂ (aq) + 2NH₄Cl (aq) → CaCl₂ (aq) + 2H₂O (l) + 2NH₃ (g).
• Process: Mix ammonium chloride and calcium hydroxide (or sodium hydroxide pellets) in a test tube or flask, and gently heat the mixture. The gas is collected by upward displacement of air.
• Test:
• Moist red litmus paper turns blue.
• Forms dense white fumes with concentrated HCl vapour to form NH₄Cl
  (NH₃ + HCl → NH₄Cl).
• Drying: Pass the gas over solid calcium oxide (CaO).

Q31: What are the physical and chemical properties of ammonia gas, and what are its uses? 

A31:

• Physical properties: Colourless gas with a pungent, choking smell, less dense than air, soluble in water.
• Chemical properties: Turns moist red litmus blue, forms dense white fumes with HCl, burns in oxygen with a pale-yellow flame.
• Uses: Manufacture of fertilisers, explosives, nylon, plastics, pigment.

Q32: What safety precautions are important when preparing ammonia gas? 

A32: Wear safety goggles and a lab coat, handle chemicals (especially concentrated acids and bases) with care, and conduct the experiment in a well-ventilated area or under a fume hood as ammonia gas is pungent and irritating.