Which salt causes permanent hardness? MCQ with Answer and Explanation

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Which salt causes permanent hardness?

A. Mg(HCO₃)₂

B. Na₂CO₃

C. CaSO₄

D. Ca(HCO₃)₂

Answer: Option C

Solution (By JKExamLibrary)

Calcium sulfate and magnesium sulfate/chloride cause permanent hardness. Bicarbonates cause temporary hardness removable by boiling.

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Question #1

Plaster of Paris should be stored in a moisture-proof container because:

A. It dissolves in water

B. It sublimes

C. It evaporates

D. It reacts with moisture to form a hard solid (gypsum)

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Correct Answer: Option D

Explanation:
CaSO₄·½H₂O readily absorbs moisture to set to CaSO₄·2H₂O (gypsum), becoming hard and useless for molding. So it must be kept dry. Reaction: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.

This question belongs to: Science chemistry

Question #2

Sodium hydroxide (caustic soda) is manufactured by:

A. Ostwald process

B. Chlor-alkali process

C. Haber process

D. Contact process

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Correct Answer: Option B

Explanation:
The chlor-alkali process electrolyzes concentrated NaCl solution (brine) to produce NaOH, Cl₂, and H₂. 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. The mercury, diaphragm, or membrane cell can be used. Contact process is for H₂SO₄, Haber for NH₃, Ostwald for HNO₃.

This question belongs to: Science chemistry

Question #3

Synthetic detergents have advantage over soaps because:

A. They work well in hard water

B. They are biodegradable

C. They are cheaper

D. They are natural products

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Correct Answer: Option A

Explanation:
Detergents do not form scum with Ca²⁺/Mg²⁺, so they clean effectively in hard water. Soaps form insoluble precipitates. Biodegradability varies; some detergents are non-biodegradable.

This question belongs to: Science chemistry

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