Which substance is considered useful for reducing permanent hardness of water?
Answer and explanation
Correct answer: Washing soda
Permanent hardness is caused mainly by dissolved calcium and magnesium sulphates or chlorides, which do not disappear on boiling. Washing soda, Na₂CO₃, supplies carbonate ions that precipitate these ions as insoluble calcium or magnesium carbonates. Removing them softens the water. Vinegar, lemon juice, and hydrochloric acid add acidity rather than removing hardness, so option A is correct.
Frequently asked questions
What is the correct answer to this question?
Washing soda
Why is this the correct answer?
Permanent hardness is caused mainly by dissolved calcium and magnesium sulphates or chlorides, which do not disappear on boiling. Washing soda, Na₂CO₃, supplies carbonate ions that precipitate these ions as insoluble calcium or magnesium carbonates. Removing them softens the water. Vinegar, lemon juice, and hydrochloric acid add acidity rather than removing hardness, so option A is correct.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
