Chemical Reactions and Equations — Important Questions

Calcium hydroxide

Step 1: The reaction is CaO + H₂O → Ca(OH)₂. Step 2: Calcium oxide (quicklime) combines with water to form calcium hydroxide (slaked lime). Step 3: This is a vigorous reaction that releases lot of heat, making it exothermic. Step 4: Slaked lime solution is used for whitewashing walls. Step 5: Calcium carbonate forms later when slaked lime reacts with CO₂ from air.

Hydrogen

Step 1: The reaction is Zn + H₂SO₄ → ZnSO₄ + H₂ (or with HCl: Zn + 2HCl → ZnCl₂ + H₂). Step 2: Active metals like zinc displace hydrogen from acids. Step 3: Hydrogen gas bubbles are evolved, which can be tested with a burning splint. Step 4: The gas burns with a 'pop' sound, confirming it's hydrogen. Step 5: This is a displacement reaction where zinc displaces hydrogen from the acid.

Oxidation

Step 1: Rusting involves iron reacting with oxygen and moisture from air. Step 2: The reaction is 4Fe + 3O₂ + 6H₂O → 2Fe₂O₃.6H₂O (rust). Step 3: Iron gains oxygen during this process, which is definition of oxidation. Step 4: The reddish-brown coating (rust) weakens iron objects over time. Step 5: This is a slow oxidation process that causes corrosion, different from fast oxidation like burning.

Silver and chlorine

Step 1: The reaction is 2AgCl → 2Ag + Cl₂ (in presence of sunlight). Step 2: White silver chloride decomposes into grey metallic silver and chlorine gas. Step 3: This is photolytic decomposition - decomposition caused by light energy. Step 4: This principle is used in black and white photography. Step 5: Silver bromide behaves similarly, also used in photographic films.