Acids, Bases and Salts – important questions for Boards 2027
CBSE Class 10 Science
When sodium hydrogencarbonate reacts with dilute hydrochloric acid, which gas is produced and what happens when it is passed through lime water? (A) Hydrogen; lime water turns milky (B) Carbon dioxide; lime water turns milky (C) Carbon dioxide; a pop sound is heard (D) Oxygen; lime water remains unchanged
Answer
(B) Carbon dioxide; lime water turns milky — all metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.
How do indicators show whether a substance is acidic or basic?
Answer
Indicators show whether a substance is acidic or basic through a change in colour. Red and blue litmus, phenolphthalein and methyl orange are colour indicators; olfactory indicators such as onion change odour in acidic or basic media.
What is water of crystallisation? State how many water molecules are present in one formula unit of hydrated copper sulphate and write its formula.
Answer
1. Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. 2. Five water molecules are present in one formula unit of hydrated copper sulphate. 3. Its chemical formula is CuSO₄·5H₂O, showing that the five water molecules are part of the salt’s crystal composition, rather than moisture simply clinging to its surface.
Describe what is observed when blue copper sulphate crystals are heated and then moistened with a few drops of water. Explain the colour changes and the appearance of droplets in the boiling tube.
Answer
Blue copper sulphate crystals are hydrated crystals that change on heating and regain their colour when water is added. 1. The blue crystals contain water of crystallisation, the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in each formula unit of copper sulphate. 2. On heating, the water of crystallisation is removed, and the blue crystals turn white as copper sulphate without water of crystallisation forms. The released water appears as droplets on the cooler inner surface of the boiling tube. 3. The droplets therefore come from the heated crystals, not from outside the tube. The change is represented by CuSO₄·5H₂O → (heat) CuSO₄ + 5H₂O. 4. When two or three drops of water are added to the white sample, its blue colour reappears as hydrated copper sulphate forms again. Thus, the colour change shows that copper sulphate crystals contain water of crystallisation.
What is formed when excess carbon dioxide is passed through calcium carbonate in water? (A) Insoluble calcium oxide (B) Soluble calcium hydrogencarbonate (C) Calcium chloride and water (D) Sodium hydrogencarbonate and oxygen
Answer
(B) Soluble calcium hydrogencarbonate — on passing excess carbon dioxide, calcium carbonate forms calcium hydrogencarbonate, which is soluble in water.
What are olfactory indicators?
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Common salt is used to prepare sodium hydroxide and sodium hydrogencarbonate. Briefly state how each is prepared from common salt and write a balanced chemical equation for each process.
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Gypsum also contains water of crystallisation. State how many water molecules are present in one formula unit of gypsum and write its chemical formula. Relate this to the idea of a fixed number of water molecules in a salt formula unit.
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Why are non-metallic oxides considered acidic in nature? (A) They react with bases to produce salt and water (B) They react with metals to produce hydrogen (C) They dissolve only in acids (D) They produce hydroxide ions in water
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What happens when zinc granules are added to dilute sulphuric acid? State what happens to the hydrogen atoms of the acid.
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69 more probable questions in this chapter, every one with a full answer.
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