Which gas is produced when zinc reacts with dilute hydrochloric acid? MCQ with Answer and Explanation

Explanation:
Permanent hardness is caused by dissolved chlorides and sulfates of calcium and magnesium. It cannot be removed by boiling. It is removed by adding washing soda (sodium carbonate, Na2CO3). The carbonate ions react with the soluble calcium and magnesium salts to form insoluble carbonates, which precipitate out.

Explanation:
Fermentation is an anaerobic biological process in which enzymes (zymase) convert glucose to ethanol and CO₂: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Oxidation would require oxygen. Esterification forms esters, saponification makes soap. Fermentation is used in brewing, winemaking, and baking (where CO₂ makes bread rise). The temperature is kept around 30-35°C.

Explanation:
AgNO₃ + NaCl → AgCl↓ + NaNO₃ is a double displacement (precipitation) reaction where ions exchange partners. Ag⁺ pairs with Cl⁻ to form a white precipitate of AgCl, and Na⁺ remains with NO₃⁻. Option A is combination, B is single displacement, D is decomposition. Double displacement generally forms a precipitate, gas, or water. Neutralization is also a double displacement: HCl + NaOH → NaCl + H₂O.