Acids, Bases, Salts and the pH Scale

Attempt the quiz above first, then review every question with its correct answer and a short explanation below.

1. Q1.The pH of pure water at 25 °C is:

   • A.0
   • B.10
   • C.14
   • D.7✓ Correct Answer

   Explanation: Pure water is neutral, with pH 7. Values below 7 are acidic and above 7 are basic.

2. Q2.The sour taste of lemon is due to the presence of:

   • A.acetic acid
   • B.lactic acid
   • C.citric acid✓ Correct Answer
   • D.oxalic acid

   Explanation: Citrus fruits such as lemon and orange contain citric acid.

3. Q3.Vinegar is a dilute solution of:

   • A.formic acid
   • B.citric acid
   • C.acetic acid✓ Correct Answer
   • D.tartaric acid

   Explanation: Vinegar contains about 5 to 8 per cent acetic acid in water.

4. Q4.Blue litmus paper turns red in a solution that is:

   • A.basic
   • B.acidic✓ Correct Answer
   • C.neutral
   • D.colloidal

   Explanation: Acids turn blue litmus red; bases turn red litmus blue.

5. Q5.Sodium hydroxide (NaOH) is commonly known as:

   • A.baking soda
   • B.caustic soda✓ Correct Answer
   • C.washing soda
   • D.bleaching powder

   Explanation: NaOH is a strong base called caustic soda; it is used in making soap.

6. Q6.The chemical name of baking soda is:

   • A.sodium carbonate
   • B.sodium hydrogencarbonate✓ Correct Answer
   • C.sodium chloride
   • D.calcium carbonate

   Explanation: Baking soda is NaHCO3. Washing soda is Na2CO3·10H2O.

7. Q7.Milk of magnesia, used as an antacid, is chemically:

   • A.magnesium chloride
   • B.sodium hydroxide
   • C.magnesium hydroxide✓ Correct Answer
   • D.aluminium oxide

   Explanation: Mg(OH)2 is a mild base that neutralises excess stomach acid.

8. Q8.Bleaching powder is produced by passing chlorine gas over:

   • A.quicklime
   • B.limestone
   • C.caustic soda
   • D.dry slaked lime✓ Correct Answer

   Explanation: Ca(OH)2 + Cl2 gives CaOCl2, which is bleaching powder.

9. Q9.Washing soda crystals contain how many molecules of water of crystallisation?

   • A.10✓ Correct Answer
   • B.5
   • C.7
   • D.2

   Explanation: Washing soda is Na2CO3·10H2O.

10. Q10.Plaster of Paris is prepared by heating:

    • A.gypsum✓ Correct Answer
    • B.limestone
    • C.quicklime
    • D.chalk

    Explanation: Heating gypsum (CaSO4·2H2O) at about 373 K gives Plaster of Paris (CaSO4·½H2O).

11. Q11.The pH of normal human blood is about:

    • A.6.4
    • B.8.4
    • C.5.4
    • D.7.4✓ Correct Answer

    Explanation: Blood is slightly basic, with a pH of about 7.35 to 7.45.

12. Q12.Rain water is called acid rain when its pH falls below about:

    • A.7.0
    • B.5.6✓ Correct Answer
    • C.6.5
    • D.4.0

    Explanation: Clean rain is slightly acidic (pH about 5.6) because of dissolved carbon dioxide; below that it is acid rain.

13. Q13.Which of the following is a strong acid?

    • A.acetic acid
    • B.carbonic acid
    • C.citric acid
    • D.hydrochloric acid✓ Correct Answer

    Explanation: HCl ionises almost completely in water, so it is a strong acid. The others are weak acids.

14. Q14.The reaction of an acid with a base to form a salt and water is called:

    • A.hydrolysis
    • B.esterification
    • C.neutralisation✓ Correct Answer
    • D.oxidation

    Explanation: Acid + base gives salt + water, for example HCl + NaOH gives NaCl + H2O.

15. Q15.The chlor-alkali process uses which solution as its raw material?

    • A.lime water
    • B.dilute sulphuric acid
    • C.brine (sodium chloride solution)✓ Correct Answer
    • D.bleaching powder solution

    Explanation: Electrolysis of brine gives sodium hydroxide, chlorine and hydrogen.

16. Q16.The pH of gastric juice in the human stomach is about:

    • A.5.0 to 6.0
    • B.7.0 to 8.0
    • C.9.0 to 10.0
    • D.1.5 to 2.0✓ Correct Answer

    Explanation: Stomach acid, mainly hydrochloric acid, is strongly acidic with a pH of about 1.5 to 2.

17. Q17.Aqua regia, which can dissolve gold and platinum, is a mixture of concentrated HCl and HNO3 in the ratio:

    • A.1 : 3
    • B.3 : 1✓ Correct Answer
    • C.1 : 1
    • D.2 : 1

    Explanation: Aqua regia is 3 parts concentrated hydrochloric acid to 1 part concentrated nitric acid.

18. Q18.The pH of a 0.001 M solution of hydrochloric acid is:

    • A.3✓ Correct Answer
    • B.1
    • C.4
    • D.11

    Explanation: HCl is fully ionised, so [H+] = 10⁻³ M and pH = -log(10⁻³) = 3.

19. Q19.Which scientists independently proposed in 1923 that an acid is a proton donor and a base is a proton acceptor?

    • A.Brønsted and Lowry✓ Correct Answer
    • B.Arrhenius and Faraday
    • C.Lewis and Dalton
    • D.Bohr and Rutherford

    Explanation: The Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors. Arrhenius defined them by ions in water; Lewis by electron pairs.

20. Q20.Which compound, known as "hypo", is used as a fixer in photography?

    • A.sodium thiosulphate✓ Correct Answer
    • B.sodium chloride
    • C.potassium nitrate
    • D.copper sulphate

    Explanation: Sodium thiosulphate (hypo) dissolves unexposed silver halide from film and paper, fixing the image.