Science (086) — CBSE Class 10 board question

(i) When concentrated acid is mixed with water, a large amount of heat is released. If water is added to acid, the heat generated can cause the mixture to splash and cause burns. Adding acid to water ensures the heat is absorbed by the large volume of water gradually, making it safe.

(ii) Acids produce H⁺ (or H₃O⁺) ions only in the presence of water. Dry HCl gas has no water, so it cannot produce H⁺ ions and does not show acidic properties. Dilute HCl is dissolved in water, producing H⁺ ions, which turn blue litmus red.

(iii) Copper sulphate crystals (CuSO₄·5H₂O) contain water of crystallisation. On heating, this water is lost, converting the blue hydrated salt into white anhydrous copper sulphate (CuSO₄).

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• These are standard reasons from the Acids, Bases and Salts chapter (Chapter 2, Science Class 10).
• For (i), the key word is safety and heat absorption by water.
• For (ii), the key concept is ionisation requires water — no water = no H⁺ ions = no acidic effect.
• For (iii), the key term is water of crystallisation — its removal causes the colour change. Always name both forms of the compound.
• Note: The source passages provided were unrelated to this question; the answers are grounded in standard NCERT Class 10 Chemistry content.