Gyanyaan हिंदी

Chapter 2: Acids, Bases and Salts

Class 10 Science · MCQs with answers

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  1. What colour does an acid turn blue litmus paper?

    1. (A) Green
    2. (B) Yellow
    3. (C) Red
    4. (D) No change
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    Correct answer: (C) Red

    Explanation: Acids turn blue litmus red, while bases turn red litmus blue.

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  2. What colour does phenolphthalein show when added to sodium hydroxide solution?

    1. (A) Colourless
    2. (B) Pink
    3. (C) Blue
    4. (D) Yellow
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    Correct answer: (B) Pink

    Explanation: Phenolphthalein turns pink in a basic solution and stays colourless in an acidic one. NaOH is a base.

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  3. Which of the following is an olfactory indicator?

    1. (A) Litmus
    2. (B) Methyl orange
    3. (C) Phenolphthalein
    4. (D) Clove oil
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    Correct answer: (D) Clove oil

    Explanation: An olfactory indicator's smell changes in an acidic or basic medium. Onion, vanilla and clove oil are olfactory indicators.

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  4. Which gas is given off when dilute sulphuric acid reacts with zinc granules?

    1. (A) Oxygen
    2. (B) Carbon dioxide
    3. (C) Hydrogen
    4. (D) Chlorine
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    Correct answer: (C) Hydrogen

    Explanation: Zn + H₂SO₄ → ZnSO₄ + H₂. A metal reacting with an acid gives hydrogen gas, which burns with a "pop" sound when a lit matchstick is brought near it.

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  5. When dilute HCl is added to sodium carbonate, the gas given off turns lime water milky. Which gas is it?

    1. (A) Carbon dioxide
    2. (B) Hydrogen
    3. (C) Oxygen
    4. (D) Chlorine
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    Correct answer: (A) Carbon dioxide

    Explanation: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. CO₂ reacts with lime water, Ca(OH)₂, to form insoluble CaCO₃, which makes it milky.

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  6. What forms when an acid reacts with a base?

    1. (A) Salt and hydrogen
    2. (B) Water only
    3. (C) Oxide and hydrogen
    4. (D) Salt and water
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    Correct answer: (D) Salt and water

    Explanation: Acid + base → salt + water. For example, NaOH + HCl → NaCl + H₂O. This is called a neutralisation reaction.

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  7. What is the pH of pure water?

    1. (A) 14
    2. (B) 7
    3. (C) 5
    4. (D) 0
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    Correct answer: (B) 7

    Explanation: Pure water is neutral, with a pH of 7. A pH below 7 means an acidic solution, and above 7 a basic one.

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  8. Four solutions have pH values 2, 5, 9 and 12. Which has the highest concentration of hydrogen ions (H⁺)?

    1. (A) The one with pH 2
    2. (B) The one with pH 5
    3. (C) The one with pH 9
    4. (D) The one with pH 12
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    Correct answer: (A) The one with pH 2

    Explanation: The lower the pH, the higher the H⁺ ion concentration and the more strongly acidic the solution. So the pH 2 solution has the most H⁺.

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  9. Below what pH in the mouth does tooth decay start?

    1. (A) 7.5
    2. (B) 6.5
    3. (C) 5.5
    4. (D) 3.5
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    Correct answer: (C) 5.5

    Explanation: Tooth enamel (calcium phosphate) starts to corrode when the pH falls below 5.5. Bacteria make acid from leftover food, which is why basic toothpaste helps.

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  10. Which antacid is taken when the stomach makes too much acid?

    1. (A) Vinegar
    2. (B) Milk of magnesia
    3. (C) Lemon juice
    4. (D) Dilute HCl
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    Correct answer: (B) Milk of magnesia

    Explanation: Milk of magnesia (magnesium hydroxide) is a mild base that neutralises the extra acid in the stomach.

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  11. Which acid does an ant sting inject into the skin?

    1. (A) Citric acid
    2. (B) Lactic acid
    3. (C) Tartaric acid
    4. (D) Methanoic acid
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    Correct answer: (D) Methanoic acid

    Explanation: An ant sting contains methanoic (formic) acid. Applying a mild base such as baking soda eases the pain.

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  12. Which acid is found in tomatoes?

    1. (A) Acetic acid
    2. (B) Tartaric acid
    3. (C) Oxalic acid
    4. (D) Lactic acid
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    Correct answer: (C) Oxalic acid

    Explanation: Tomatoes contain oxalic acid. Vinegar has acetic acid, tamarind has tartaric acid, and curd has lactic acid.

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  13. What is the chemical formula of bleaching powder?

    1. (A) CaOCl₂
    2. (B) CaCO₃
    3. (C) Ca(OH)₂
    4. (D) CaSO₄
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    Correct answer: (A) CaOCl₂

    Explanation: Bleaching powder is CaOCl₂. It is made by passing chlorine gas over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.

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  14. What is the chemical name of baking soda?

    1. (A) Sodium carbonate
    2. (B) Sodium chloride
    3. (C) Sodium hydroxide
    4. (D) Sodium hydrogencarbonate
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    Correct answer: (D) Sodium hydrogencarbonate

    Explanation: Baking soda is sodium hydrogencarbonate (NaHCO₃). On heating it releases CO₂, which makes cakes and pakoras rise.

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  15. What is the formula of washing soda?

    1. (A) NaHCO₃
    2. (B) Na₂CO₃·10H₂O
    3. (C) CaSO₄·2H₂O
    4. (D) NaCl
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    Correct answer: (B) Na₂CO₃·10H₂O

    Explanation: Washing soda is the crystalline form of sodium carbonate, with 10 molecules of water of crystallisation: Na₂CO₃·10H₂O.

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  16. What is the chemical formula of Plaster of Paris?

    1. (A) CaSO₄·½H₂O
    2. (B) CaSO₄·2H₂O
    3. (C) CaSO₄
    4. (D) CaCO₃
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    Correct answer: (A) CaSO₄·½H₂O

    Explanation: Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O). It is made by heating gypsum (CaSO₄·2H₂O) to 373 K.

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  17. How many molecules of water of crystallisation does one formula unit of copper sulphate crystal contain?

    1. (A) 10
    2. (B) 7
    3. (C) 5
    4. (D) 2
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    Correct answer: (C) 5

    Explanation: Blue copper sulphate crystals are CuSO₄·5H₂O. On heating they lose this water and turn white.

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  18. In the chlor-alkali process, which products form from the electrolysis of aqueous sodium chloride?

    1. (A) Na, Cl₂ and O₂
    2. (B) NaOH, Cl₂ and H₂
    3. (C) NaOH, O₂ and H₂
    4. (D) Na₂CO₃, Cl₂ and H₂
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    Correct answer: (B) NaOH, Cl₂ and H₂

    Explanation: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chlorine forms at the anode, hydrogen at the cathode, and sodium hydroxide near the cathode. The name "chlor-alkali" comes from these products.

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  19. What is the safe way to dilute a concentrated acid?

    1. (A) Slowly add water to the acid
    2. (B) Mix both quickly at once
    3. (C) Heat the acid first
    4. (D) Slowly add the acid to water while stirring
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    Correct answer: (D) Slowly add the acid to water while stirring

    Explanation: Dissolving acid in water is highly exothermic. Adding water to acid can release so much heat that the mixture splashes and burns, so always add acid slowly to water.

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  20. What are bases that dissolve in water called?

    1. (A) Salts
    2. (B) Indicators
    3. (C) Alkalis
    4. (D) Oxides
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    Correct answer: (C) Alkalis

    Explanation: Bases that dissolve in water are called alkalis, such as NaOH and KOH. All alkalis are bases, but not all bases are alkalis.

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  21. Which ions do bases produce when dissolved in water?

    1. (A) OH⁻ ions
    2. (B) H⁺ ions
    3. (C) Cl⁻ ions
    4. (D) Na⁺ ions
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    Correct answer: (A) OH⁻ ions

    Explanation: Bases produce hydroxide (OH⁻) ions in water, while acids produce hydrogen (H⁺) ions.

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  22. Rainwater is called acid rain when its pH falls below what value?

    1. (A) 3.6
    2. (B) 5.6
    3. (C) 6.6
    4. (D) 7.6
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    Correct answer: (B) 5.6

    Explanation: When the pH of rainwater falls below 5.6, it is called acid rain. It harms life in rivers and lakes.

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  23. Assertion (A): Dry HCl gas does not change the colour of dry blue litmus paper. Reason (R): HCl produces hydrogen ions only in the presence of water.

    1. (A) Both A and R are true, and R is the correct explanation of A.
    2. (B) Both A and R are true, but R is not the correct explanation of A.
    3. (C) A is true, but R is false.
    4. (D) A is false, but R is true.
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    Correct answer: (A) Both A and R are true, and R is the correct explanation of A.

    Explanation: Acidic behaviour comes from H⁺ ions, which do not form without water. So dry HCl does not turn litmus red, and R correctly explains A.

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  24. Assertion (A): Plaster of Paris should be stored in a moisture-proof container. Reason (R): Plaster of Paris turns into hard gypsum when it absorbs water.

    1. (A) Both A and R are true, and R is the correct explanation of A.
    2. (B) Both A and R are true, but R is not the correct explanation of A.
    3. (C) A is true, but R is false.
    4. (D) A is false, but R is true.
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    Correct answer: (A) Both A and R are true, and R is the correct explanation of A.

    Explanation: When it gets moisture, Plaster of Paris sets into gypsum and becomes useless. That is why it must be kept dry, so R correctly explains A.

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  25. Assertion (A): Sodium hydroxide is an alkali. Reason (R): All bases are soluble in water.

    1. (A) Both A and R are true, and R is the correct explanation of A.
    2. (B) Both A and R are true, but R is not the correct explanation of A.
    3. (C) A is true, but R is false.
    4. (D) A is false, but R is true.
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    Correct answer: (C) A is true, but R is false.

    Explanation: NaOH dissolves in water, so it is an alkali (A is true). But not all bases dissolve in water (copper oxide does not, for example), so R is false.

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