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Science practice

Chemistry - Acids, Bases & Salts Questions with Answers

Acids, bases and salts questions cover indicators, the pH scale, neutralisation, the reactions of acids with metals, bases and carbonates, and the chemical names and uses of common salts, at NCERT class 10 level. RRB Group D, ALP, NTPC and SSC papers usually ask for the chemical formula or use of a compound such as baking soda, a pH value, or the colour of an indicator. Tip: learn the formula and the common name together, because the two are often swapped in the options.

Formulas and key facts

  • pH below 7 is acidic, pH = 7 is neutral, pH above 7 is basic
  • pH = -log₁₀[H⁺]; a fall of one pH unit means ten times more H⁺ ions
  • Acid + metal → salt + hydrogen gas
  • Acid + base → salt + water (neutralisation)
  • Acid + metal carbonate or hydrogencarbonate → salt + water + carbon dioxide
  • Litmus: acids turn blue litmus red, bases turn red litmus blue
  • Phenolphthalein is colourless in acid and pink in base; methyl orange is red in acid and yellow in base
  • Baking soda: NaHCO₃ (sodium hydrogencarbonate); washing soda: Na₂CO₃·10H₂O
  • Bleaching powder: CaOCl₂; plaster of Paris: CaSO₄·½H₂O; gypsum: CaSO₄·2H₂O
  • Plaster of Paris is made by heating gypsum at 373 K
  • Common salt NaCl is the raw material for sodium hydroxide, baking soda, washing soda and bleaching powder
  • Antacids such as milk of magnesia are mild bases that neutralise excess stomach acid

Worked examples

Q1. Find the pH of 0.001 M hydrochloric acid, assuming it is fully ionised.

  1. HCl is a strong acid, so [H⁺] = 0.001 M = 10⁻³ M.
  2. pH = -log₁₀(10⁻³) = 3.
  3. pH 3 is below 7, so the solution is acidic.

Answer: pH = 3

Q2. The pH of a solution changes from 6 to 3. By what factor does the hydrogen ion concentration change?

  1. Each pH unit is a factor of 10 in [H⁺].
  2. pH falls by 3 units, so [H⁺] changes by 10³.
  3. [H⁺] increases, because a lower pH means a stronger acid.

Answer: [H⁺] increases 1000 times

Q3. Sodium carbonate reacts with excess hydrochloric acid: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. What mass of CO₂ is released from 10.6 g of Na₂CO₃? (Atomic masses: Na = 23, C = 12, O = 16.)

  1. Molar mass of Na₂CO₃ = 2 × 23 + 12 + 3 × 16 = 106 g/mol.
  2. Moles of Na₂CO₃ = 10.6 / 106 = 0.1 mol.
  3. The equation shows 1 mol Na₂CO₃ gives 1 mol CO₂, so 0.1 mol CO₂ is formed.
  4. Molar mass of CO₂ = 12 + 2 × 16 = 44 g/mol; mass = 0.1 × 44 = 4.4 g.

Answer: 4.4 g of CO₂

Shortcuts that save time

  • Lower pH means stronger acid, higher pH means stronger base. Place the options on a 0 to 14 line before choosing, which avoids the common reversal error that costs marks.
  • Learn the compound names as pairs with their one-line use: baking soda for antacid and baking, washing soda for cleaning and hard water, bleaching powder for disinfecting water, plaster of Paris for setting fractures.
  • When an acid reacts with a carbonate, the gas is carbon dioxide and it turns lime water milky; with a metal, the gas is hydrogen and it burns with a pop sound. Do not mix up the two.

Where this appears

RRB Group DGeneral Science section
RRB ALPGeneral Science section
RRB NTPCGeneral Awareness section
SSC CGLGeneral Awareness section

Chemistry - Acids, Bases & Salts

Practice Set
Attempted: 0Correct: 0Incorrect: 0
1 of 1 (adapting live)Medium
Which of the following is NOT a use of plaster of Paris? (i) Manufacture of glass, soap and paper (ii) used in plaster for supporting fractured bones. (iii) making toys, materials for decoration and for making surfaces smooth. (iv) Used in chemistry laboratories for sealing air gaps in apparatus where air tight arrangement is required.

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Chemistry - Acids, Bases & Salts questions with answers

10 sample questions from this topic with the correct option and the solution.

  1. Q1. The Contact Process is used for the manufacturing of which of the following chemicals?

    1. Ammonia
    2. Nitric acid
    3. Hydrochloric acid
    4. Sulphuric acid

    Answer: (D) Sulphuric acid

    Solution
    The Contact Process is the industrial method for manufacturing sulphuric acid. Sulphur dioxide is oxidised to sulphur trioxide using a vanadium pentoxide catalyst. The sulphur trioxide is then absorbed in concentrated sulphuric acid to form oleum, which is diluted to give sulphuric acid. Ammonia is made by the Haber process and nitric acid by the Ostwald process.

  2. Q2. Which of the following is a use of plaster of Paris? (i) Manufacture of glass, soap and paper (ii) As a cleaning agent for domestic purposes (iii) Making toys, materials for decoration and for making surfaces smooth (iv) For baking

    1. only i
    2. only ii
    3. only iii
    4. only iv

    Answer: (C) only iii

    Solution
    Plaster of Paris is used for making toys, decorative materials and for making surfaces smooth. It is also used for setting fractured bones. Manufacture of glass, soap and paper is a use of washing soda, and baking is a use of baking soda, not plaster of Paris. So only statement iii is correct.

  3. Q3. The correct way of making a solution of acid in water is to ________.

    1. Add acid to water
    2. Mix acid and water simultaneously
    3. Add water to acid
    4. Add water to acid in a shallow container

    Answer: (A) Add acid to water

    Solution
    Dissolving an acid in water is a highly exothermic process. If water is added to concentrated acid, the heat released can make the mixture splash out and cause burns. So the acid must always be added slowly to water with constant stirring, so the heat is absorbed by the large amount of water.

  4. Q4. Which of the following are neutralisation reactions? (i) NaOH (aq) + HCl (aq) → NaCl (aq) + H 2 O (l) (II) HNO 3 (aq) + KOH (aq) → KNO 3 (aq) + H 2 O (I) (III) HCl (aq) + NaOH (aq) → NaCl (aq) + H 2 O (l))

    1. i, ii and iii
    2. i and iii
    3. i and ii
    4. ii and iii

    Answer: (A) i, ii and iii

    Solution
    A neutralisation reaction is one in which an acid reacts with a base to form salt and water. All three reactions involve a strong acid (HCl or HNO3) reacting with a strong base (NaOH or KOH) to give a salt and water. Hence i, ii and iii are all neutralisation reactions.

  5. Q5. The foul smell of rancid butter is due to -

    1. Butyric acid
    2. Palmitic acid
    3. Stearic acid
    4. Caproic acid

    Answer: (A) Butyric acid

    Solution
    When butter turns rancid, its fats are hydrolysed and oxidised, releasing free butyric acid. Butyric acid (butanoic acid) has a strong, unpleasant odour, which gives rancid butter its foul smell. Palmitic and stearic acids are long-chain fatty acids that are odourless solids.

  6. Q6. The general reaction between a metal oxide and an acid can be written as:

    1. Metal oxide + acid ⇨ salt + water
    2. Metal oxide + acid ⇨ salt + precipitate
    3. Metal oxide + acid ⇨ water + gas
    4. Metal oxide + acid ⇨ salt + gas

    Answer: (A) Metal oxide + acid ⇨ salt + water

    Solution
    Metal oxides are basic in nature, so they react with acids in a neutralisation reaction. This reaction gives a salt and water, for example CuO + 2HCl gives CuCl2 + H2O. No gas or precipitate is formed in the general reaction.

  7. Q7. Substances whose odour changes in acidic or basic media are called-

    1. Both acidic and basic indicators
    2. Acidic indicators
    3. Olfactory indicators
    4. Basic indicators

    Answer: (C) Olfactory indicators

    Solution
    Olfactory indicators are substances whose smell changes in acidic or basic media. Examples are onion, vanilla and clove oil, which lose their odour in a base but keep it in an acid (or vice versa). Acidic and basic indicators usually change colour, not odour, so olfactory indicators is the correct term.

  8. Q8. The addition of which of the following adds fizz to soft drinks?

    1. Citric acid
    2. Carbonic acid
    3. Acetic acid
    4. Sulphuric acid

    Answer: (B) Carbonic acid

    Solution
    Soft drinks are made by dissolving carbon dioxide gas in water under pressure. The dissolved CO2 forms carbonic acid, and when the bottle is opened the gas escapes as bubbles, giving the fizz. Citric acid, acetic acid and sulphuric acid do not produce this effect.

  9. Q9. The acid and base which form sodium hydrogen carbonate are:

    1. HCl and NaCl
    2. HCl and NaOH
    3. H 2 CO 3 and Na 2 CO 3
    4. H 2 CO 3 and NaOH

    Answer: (D) H 2 CO 3 and NaOH

    Solution
    Sodium hydrogen carbonate (NaHCO3) is an acid salt formed by partial neutralisation of a weak dibasic acid. Carbonic acid (H2CO3) reacts with sodium hydroxide (NaOH) in a 1:1 ratio: H2CO3 + NaOH -> NaHCO3 + H2O. Hence the acid and base are H2CO3 and NaOH.

  10. Q10. The action of chlorine on dry Ca(OH) 2, produces which of the following?

    1. Hydrochloric acid
    2. Calcium chloride
    3. Calcium oxide
    4. Bleaching powder

    Answer: (D) Bleaching powder

    Solution
    When chlorine gas is passed over dry slaked lime, Ca(OH)2, calcium oxychloride is formed: Ca(OH)2 + Cl2 -> CaOCl2 + H2O. This compound is commonly called bleaching powder. It is used for disinfecting water and bleaching.