AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Washing soda (sodium carbonate, Na₂CO₃) is used to remove permanent hardness of water, as it reacts with dissolved calcium and magnesium salts to form insoluble carbonates.
The question is from the topic of salts and their uses (Chapter 2). Examiners expect the chemical name sodium carbonate and its common name washing soda. Permanent hardness is caused by soluble chlorides and sulphates of Ca²⁺/Mg²⁺, which washing soda removes by precipitation. Note: this topic is referenced under uses of washing soda in the CBSE syllabus, so knowing both the name and the reason is ideal even for a 1-mark answer.