Hard water contains dissolved salts of calcium and magnesium. Which substance causes temporary hardness?
A. Sodium bicarbonate
B. Calcium sulfate
C. Magnesium chloride
D. Calcium bicarbonate
Answer: Option D
Solution (By JKExamLibrary)
Temporary hardness is due to dissolved bicarbonates of Ca and Mg, which can be removed by boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. Permanent hardness is caused by chlorides and sulfates of Ca and Mg, not removed by boiling. Sodium bicarbonate does not cause hardness. Methods to remove permanent hardness include treatment with washing soda (Na₂CO₃), ion exchange, or calgon.
Explanation:
Washing soda (Na₂CO₃) reacts with Ca²⁺ and Mg²⁺ ions in hard water to form insoluble carbonates: Ca²⁺ + Na₂CO₃ → CaCO₃↓ + 2Na⁺. Similarly MgCO₃. These precipitates are removed, softening the water. Ion exchange uses zeolites or resins. Boiling removes only temporary hardness. Distillation is expensive.
Explanation:
Zn + 2HCl → ZnCl₂ + H₂↑. Zinc displaces hydrogen from acid because it is more reactive than hydrogen in the reactivity series. Hydrogen gas burns with a pop sound when a burning splint is brought near the mouth of the test tube. Metals above hydrogen in the series react with acids to evolve H₂; metals below (Cu, Ag) do not. Nitric acid is an oxidizer, so may not yield H₂.
Explanation:
Saponification is the alkaline hydrolysis of fats or oils (which are esters of fatty acids) using a strong base like NaOH or KOH. This reaction produces glycerol and the sodium or potassium salts of fatty acids, which are soaps. It is an irreversible reaction.
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