Chapter 2: Acids, Bases and Salts

Important questions — the ones the board asked most (20)

1. Q1Case-based5 marksAsked: 2026

   Read the following passage and answer the questions that follow : In the given diagram, when electricity is passed through an aqueous solution of a common salt, a substance 'Z' is produced, along with the evolution of gases 'X' and 'Y'. When a burning matchstick is brought near the gas 'Y', it burns with a pop sound, whereas 'X' is used for disinfecting drinking water. When 'X' is passed through slaked lime, an insoluble substance 'A' is produced. (a) Write the names of the gases 'X' and 'Y'. (b) Write the balanced chemical equation for the formation of substance 'Z'. (c) (i) What happens : (I) If a drop of red litmus solution is added to the aqueous solution of substance 'Z' ? (II) If CO_2 gas is passed through ammoniacal solution of aqueous NaCl ? OR (c) (ii) Identify the substance 'A' and write the balanced chemical equation of the reaction which takes place in its preparation.

   

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   Model answer · chlor-alkali process, bleaching powder

   1. 1.(a) Gas X is chlorine (Cl₂), used to disinfect drinking water.1 mark
   2. 2.Gas Y is hydrogen (H₂), which burns with a pop sound.1 mark
   3. 3.(b) Z is sodium hydroxide: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g) (the chlor-alkali process).1 mark
   4. 4.(c)(i)(I) Sodium hydroxide is a base, so red litmus turns blue. OR (c)(ii) A is bleaching powder (calcium oxychloride, CaOCl₂).1 mark
   5. 5.(c)(i)(II) Sodium hydrogencarbonate is formed: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. OR (c)(ii) Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.1 mark

   Marks split from a structure derived from past papers (checked twice); not CBSE’s official scheme

2. Q2Case-based5 marksAsked: 2026

   Read the following passage and answer the questions that follow : In the given diagram, when electricity is passed through an aqueous solution of a common salt, a substance 'Z' is produced, along with the evolution of gases 'X' and 'Y'. When a burning matchstick is brought near the gas 'Y', it burns with a pop sound, whereas 'X' is used for disinfecting drinking water. When 'X' is passed through slaked lime, an insoluble substance 'A' is produced. (a) Write the names of the gases 'X' and 'Y'. (b) Write the balanced chemical equation for the formation of substance 'Z'. (c) (i) What happens : (I) If a drop of red litmus solution is added to the aqueous solution of substance 'Z' ? (II) If CO2 gas is passed through ammoniacal solution of aqueous NaCl ? OR (c) (ii) Identify the substance 'A' and write the balanced chemical equation of the reaction which takes place in its preparation.

   

   Medha Ma’am

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   Model answer · chlor-alkali process, bleaching powder

   1. 1.(a) Gas X is chlorine (Cl₂), used to disinfect drinking water.0.5 marks
   2. 2.Gas Y is hydrogen (H₂), which burns with a pop sound.0.5 marks
   3. 3.(b) Substance Z is sodium hydroxide (NaOH), formed in the chlor-alkali process.1 mark
   4. 4.2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)1 mark
   5. 5.(c)(i)(I) Sodium hydroxide is a base, so red litmus turns blue. OR (c)(ii) A is bleaching powder, CaOCl₂.1 mark
   6. 6.(c)(i)(II) Sodium hydrogencarbonate is formed: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. OR (c)(ii) Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.1 mark

   Marks split from a structure derived from past papers (checked twice); not CBSE’s official scheme

3. Q3Long answer5 marksAsked: 2024

   (a) (i) Five solutions A, B, C, D and E when tested with pH paper showed pH as 4, 1, 13, 7 and 10 respectively. Which solution is : (1) Strongly acidic (2) Strongly alkaline (3) Weakly acidic (4) Neutral and (5) Weakly alkaline ? Arrange the solutions in increasing order of H+ ion concentration. (ii) Write the name and formula of (1) an acidic salt and (2) a basic salt giving the name of the parent acid and parent base used to form the salt in each case. OR (b) Name and state in brief the process which is used to prepare sodium hydroxide from sodium chloride. In this process along with the main product two gases 'X' and 'Y' are also given off at the two electrodes. Name 'X' and 'Y' specifying the name of their respective electrode at which each gas is obtained. One of these gases when reacts with dry calcium hydroxide produces a compound 'Z' which is widely used in water treatment plants and textile industries. Name Z and write chemical equation for the reaction involved in its formation.

   

   Medha Ma’am

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   Watch it on the board · 140 s

   Model answer · chlor-alkali process, bleaching powder

   1. 1.(b) The process used to prepare sodium hydroxide from sodium chloride is the chlor-alkali process. It gets this name from its products: chlor for chlorine and alkali for sodium hydroxide.1 mark
   2. 2.In this process, electricity is passed through an aqueous solution of sodium chloride (called brine). The brine decomposes to form sodium hydroxide, and sodium hydroxide solution is formed near the cathode. 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)0.5 marks
   3. 3.Gas X is chlorine gas (Cl2). It is given off at the anode.1 mark
   4. 4.Gas Y is hydrogen gas (H2). It is given off at the cathode.1 mark
   5. 5.Compound Z is bleaching powder. It is formed when chlorine gas X reacts with dry slaked lime, which is calcium hydroxide, Ca(OH)2. Bleaching powder is represented as Ca(ClO)2, although its actual composition is quite complex. It is used to make drinking water free from germs and to bleach cotton and linen in the textile industry.0.5 marks
   6. 6.Chemical equation for the formation of bleaching powder by the action of chlorine on dry slaked lime: 2Ca(OH)2 + 2Cl2 → Ca(ClO)2 + CaCl2 + 2H2O1 mark

   • Write each gas together with its electrode: "chlorine gas (Cl2) ... at the anode" and "hydrogen gas (H2) ... at the cathode".
   • Write the balanced equation for the chlor-alkali process, with brine as the aqueous solution of sodium chloride.
   • Write that bleaching powder is formed from "dry slaked lime", Ca(OH)2, and give its balanced equation.

   Marks split as in the board’s published marking scheme

4. Q4Long answer5 marksAsked: 2024

   (a) A few crystals of ferrous sulphate were taken in a dry boiling tube and heated. Tiny water droplets were observed in the tube after some time. (i) From where did these water droplets appear ? Explain. (ii) What colour change will be observed during heating ? (iii) How many molecules of water are attached per molecule of FeSO4 crystal ? Write the molecular formula of crystalline forms of (I) Copper sulphate, and (II) Sodium carbonate. (iv) State how is Plaster of Paris obtained from gypsum. Write two uses of Plaster of Paris. OR (b) An acid 'X' present in tamarind when mixed with 'Y', produces a mixture 'Z'. 'Z' on addition to a dough when heated makes cakes soft and spongy. 'Y' is prepared from common salt and helps in faster cooking. (i) Write the common names of 'X', 'Y' and 'Z', and the chemical formula of 'Y'. (ii) How is 'Y' prepared and how does it help in making cakes soft and spongy ? Illustrate the reaction with suitable chemical equation. (iii) Write the name and chemical formula of a mild base other than 'Y' used as an antacid.

   How the model answer begins

   1. (a) (i) The water droplets come from the water of crystallisation in the ferrous sulphate crystals. Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. The crystals…

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5. Q5Case-based4 marksAsked: 2024

   Salts play a very important role in our daily life. Sodium chloride which is known as common salt is used almost in every kitchen. Baking soda is also a salt used in faster cooking as well as in baking industry. The family of salts is classified on the basis of cations and anions present in them. (a) Identify the acid and base from which Sodium chloride is formed. (b) Find the cation and the anion present in Calcium sulphate. (c) "Sodium chloride and washing soda both belong to the same family of salts." Justify this statement. OR (c) Define the term pH scale. Name the salt obtained by the reaction of Potassium hydroxide and Sulphuric acid and give the pH value of its aqueous solution.

   How the model answer begins

   1. (a) The acid is hydrochloric acid (HCl).

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6. Q6Short answer4 marksAsked: 2026

   The pH plays an important role in our daily life. Strength of acids and bases depends on the number of H+ ions and OH- ions produced by these respectively. pH scale helps in predicting the strength of acids and bases by measuring hydrogen ion concentration in a solution. Answer the following : (a) What type of indicator is used to predict how strong an acid or base is ? (b) Three solutions A, B and C showed pH as 3, 5 and 7. Which solution has the highest concentration of H+ ions ? (c) What are strong acids and weak acids ? OR (c) (i) Under what condition is rainwater called acid rain ? (ii) In the following acids, identify weak acids : Nitric acid, hydrochloric acid, acetic acid, sulphuric acid, formic acid.

   How the model answer begins

   1. (a) A universal indicator is used to judge how strong an acid or a base is. A universal indicator is a mixture of several indicators. It shows different colours at different concentrations of hydrogen ions in a…

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7. Q7Case-based4 marksAsked: 2025

   Common salt is a very important chemical compound for our daily life. It’s chemical name is sodium chloride and it is used as a raw material in the manufacture of caustic soda, washing soda, baking soda etc. It is also used in the preservation of pickles, butter, meat etc. (i) Name the acid and the base from which common salt can be obtained. (ii) State the nature (acidic/basic/neutral) of sodium chloride. Give reason for the justification for your answer. (iii) (A) What happens when electric current is passed through an aqueous solution of sodium chloride (called brine)? Name the products obtained along with the corresponding places in the electrolytic cell where each of these products is obtained. OR (iii) (B) How is washing soda obtained from sodium chloride? Give chemical equation of the reactions involved in the process.

8. Q8Case-based4 marksAsked: 2025

   [passage] (a) Write balanced chemical equations to show the products formed during electrolysis of brine. 1 (b) List two uses of any one product obtained during electrolysis of brine. 1 (c) (i) A mild non-corrosive basic salt 'A', used for faster cooking, is strongly heated to produce a compound 'B', that is used for removing permanent hardness of water. Identify A and B and also write the equation for the reaction that occurs when A is heated. 2 OR (c) (ii) Define water of crystallisation. Give two examples of salts that have water of crystallisation. 2

   How the model answer begins

   1. (a) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g): chlorine at the anode, hydrogen at the cathode and sodium hydroxide solution near the cathode.

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9. Q9Case-based4 marksAsked: 2025

   Seawater contains many salts dissolved in it. Common salt is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt and is mined like coal. The common salt is an important raw material for chemicals of daily use. (a) Write balanced chemical equations to show the products formed during electrolysis of brine. (b) List two uses of any one product obtained during electrolysis of brine. (c) (i) A mild non-corrosive basic salt 'A', used for faster cooking, is strongly heated to produce a compound 'B', that is used for removing permanent hardness of water. Identify A and B and also write the equation for the reaction that occurs when A is heated. OR (c) (ii) Define water of crystallisation. Give two examples of salts that have water of crystallisation.

   How the model answer begins

   1. (a) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g), with chlorine at the anode and hydrogen at the cathode.

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10. Q10Case-based4 marksAsked: 2025

    Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of 0 - 14. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator. Answer the following questions : (a) Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q? (b) A solution has a pH of 7. Name a compound you would use to (i) increase its pH, and (ii) decrease its pH. (c) (i) When the pH of a solution is decreased from 4 to 2, what effect does it produce on its hydronium ion concentration ? State the colour change shown by the pH paper. OR (c) (ii) A person is feeling pain and irritation in the stomach due to indigestion. What could be the pH of the fluid in the stomach ? Write the common name of the medicines people use for remedy. Give the chemical name of "milk of magnesia" often used for this purpose.

    How the model answer begins

    1. (a) Solution P is a strong acid, so it is placed at pH 0 to 4 on the pH scale. This is the very acidic end, below the neutral value of 7.

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11. Q11Case-based4 marksAsked: 2025

    37. Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of 0 – 14. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator. Answer the following questions : (a) Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q ? (b) A solution has a pH of 7. Name a compound you would use to (i) increase its pH, and (ii) decrease its pH. (c) (i) When the pH of a solution is decreased from 4 to 2, what effect does it produce on its hydronium ion concentration ? State the colour change shown by the pH paper. OR (c) (ii) A person is feeling pain and irritation in the stomach due to indigestion. What could be the pH of the fluid in the stomach ? Write the common name of the medicines people use for remedy. Give the chemical name of “milk of magnesia” often used for this purpose.

    How the model answer begins

    1. (a) Solution P is a strong acid, so it sits at the very acidic end of the pH scale, at pH 0 to 4. Values less than 7 on the pH scale represent an acidic solution, and a strong acid has a very low pH.

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12. Q12Case-based4 marksAsked: 2023

    While conducting practical work in the laboratory, the teacher divided the students into three groups and gave them different solutions and asked them to find the pH of the given solutions and classify them into acidic, basic and neutral solutions. Group A : Lemon juice, Vinegar, Colourless aerated drink Group B : Tomato juice, Coffee, Ginger juice Group C : Sodium hydroxide, Sodium chloride, Lime water (a) For the given solutions, in which group are the pH values likely to be (i) less than 7, and (ii) more than 7 ? (b) List two methods of determining the pH of solutions.

13. Q13Short answer3 marksAsked: 2024

    It is observed that Calcium on reaction with water floats on its surface. Explain why it happens. Also write a balanced chemical equation for the reaction that occurs. What happens when the aqueous solution of the product of this reaction reacts with Carbon dioxide gas ? Write a balanced chemical equation for the reaction.

14. Q14Short answer3 marksAsked: 2023

    (a) Suggest one remedial measure each to counteract the change in pH in human beings in following cases : (i) Production of too much acid in stomach during indigestion (ii) Stung by a honey bee / nettle leaves (b) Fresh milk has a pH of 6. When it changes into curd will its pH increase or decrease ? Why ?

    How the model answer begins

    1. Magnesium hydroxide (Milk of magnesia), a mild base, is often used to neutralise the excess acid produced in the stomach during indigestion.

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15. Q15Long answer3 marksAsked: 2026

    (a) Explain chlor-alkali process with chemical equation. Name the products formed at anode and cathode. OR (b) Write the preparation of following compounds with balanced chemical equation : (i) Baking soda (ii) Bleaching powder (iii) Plaster of Paris

16. Q16Short answer3 marksAsked: 2026

    Attempt either (a) or (b) : (a) Give reasons : (i) For dilution of an acid, acid is added into water and not water into acid. (ii) Dry HCl gas does not turn blue litmus red whereas dilute hydrochloric acid does. (iii) Blue colour of Copper(II) sulphate crystals turns white on heating. OR (b) Write the preparation of the following compounds with balanced chemical equation : (i) NaOH by chlor-alkali process (ii) Baking Soda (iii) Plaster of Paris

    How the model answer begins

    1. (i) Acid is added to water because mixing them gives out a lot of heat. When acid is added slowly to a large amount of water with stirring, the water takes up this heat. So there is no excessive local heating, and the…

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17. Q17Short answer3 marksAsked: 2025

    (a) State the chemical property in each case on which the following uses of baking soda are based upon : (i) as an anti-acids (ii) as a constituent in making baking powder (iii) in soda-acid fire-extinguishers OR (b) Write chemical equations to show what happens when an acid reacts with a (i) metal (ii) base and (iii) carbonate Write the name of the main product formed in each case.

18. Q18Short answer3 marksAsked: 2025

    (a) In an experiment a student dipped pH papers in four different solutions A, B, C and D and reported his observation as given below : (i) In which one of these solutions is the concentration of (1) hydrogen / hydronium ions; (2) hydroxyl ions maximum ? (ii) Give one example each of the two solutions identified in (i) above. (iii) What would be the pH of the resultant mixture obtained when these two solutions are mixed together in equal proportions ? Justify your answer. OR (b) (i) Name the gas liberated when an acid reacts with a metal. How is this gas tested ? (ii) Write the chemical equation for the reaction of zinc metal with (1) hydrochloric acid, and (2) sodium hydroxide. Name the compound of zinc obtained in each case.

    How the model answer begins

    1. (b)(i) When an acid reacts with a metal, the gas liberated is hydrogen gas (H₂). The metal displaces hydrogen from the acid and forms a salt. To test this gas, bring a burning candle or burning splinter near the gas, or…

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19. Q19Short answer3 marksAsked: 2024

    (i) The pH of a sample of tomato juice is 4.6. How is this juice likely to be in taste ? Give reason to justify your answer. (ii) How do we differentiate between a strong acid and a weak base in terms of ion-formation in aqueous solutions ? (iii) The acid rain can make the survival of aquatic animals difficult. How ?

20. Q20Short answer3 marksAsked: 2024

    27. Write the common name and the chemical name of the compound CaSO4 . 1/2 H2O. Write the method of its preparation. Give chemical equation for the reaction, when water reacts with CaSO4 . 1/2 H2O.

16 more questions of this chapter, each with its model answer, in the Ask Medha app.

Objective questions with the board’s key (10)

1. 1. CBSE 2026

   Which one of the following can be used as an acid-base indicator by a visually impaired (blind) student ?

   • (A) Turmeric
   • (B) Vanilla essence ✓
   • (C) Methyl orange
   • (D) Litmus

   Answer: (B)

2. 2. CBSE 2026

   18. The natural sources of oxalic acid, lactic acid and methanoic acid are respectively :

   • (A) Tomato, Curd, Ant sting ✓
   • (B) Tomato, Orange, Nettle sting
   • (C) Orange, Milk, Ant sting
   • (D) Orange, Sour milk, Nettle sting

   Answer: (A)

3. 3. CBSE 2026

   To a small amount of copper oxide in a beaker, when we add dilute hydrochloric acid slowly with stirring, the change in the colour of the solution is due to the formation of :

   • (A) Copper(II) oxide, which is black in colour.
   • (B) Copper(I) chloride, which is blue in colour.
   • (C) Copper(II) chloride, which is blue-green in colour. ✓
   • (D) Copper(I) oxide, which is black in colour.

   Answer: (C)

4. 4. CBSE 2026

   An aqueous solution turns red litmus blue. Excess addition of which of the following solution would reverse the change ?

   • (A) Sodium hydroxide
   • (B) Hydrochloric acid ✓
   • (C) Baking soda
   • (D) Slaked lime

   Answer: (B)

5. 5. CBSE 2026

   How will you protect yourself from the heat generated while diluting a concentrated acid ?

   • (A) By adding acid to water with constant stirring. ✓
   • (B) By adding water to acid with constant stirring.
   • (C) By adding water to acid followed by base.
   • (D) By adding base to acid with constant stirring.

   Answer: (A)

6. 6. CBSE 2026

   A metal carbonate reacts with a solution X which forms a salt, water and a gas Y. What are X and Y ?

   • (A) X - Sodium hydroxide, Y - Carbon dioxide
   • (B) X - Sodium hydroxide, Y - Hydrogen
   • (C) X - Hydrochloric acid, Y - Carbon dioxide ✓
   • (D) X - Hydrochloric acid, Y - Hydrogen

   Answer: (C)

7. 7. CBSE 2026

   Baking powder is a mixture of :

   • (A) Sodium carbonate + Acetic acid
   • (B) Sodium carbonate + Tartaric acid
   • (C) Sodium hydrogen carbonate + Tartaric acid ✓
   • (D) Sodium hydrogen carbonate + Acetic acid

   Answer: (C)

8. 8. CBSE 2026

   Tamarind is a natural source of which acid ?

   • (A) Acetic acid
   • (B) Tartaric acid ✓
   • (C) Oxalic acid
   • (D) Citric acid

   Answer: (B)

9. 9. CBSE 2026

   Rain water is called 'acid rain' when its :

   • (A) pH falls below 7.2.
   • (B) pH falls below 14.
   • (C) pH is above 7.
   • (D) pH falls below 5.6. ✓

   Answer: (D)

10. 10. CBSE 2026

    Study the following table and select the correct option : Salt | Acid used | Base used | Nature of Salt (A) NaCl | HCl | NaOH | Basic (B) Na2CO3 | H2CO3 | NaOH | Neutral (C) Na2SO4 | H2SO4 | NaOH | Acidic (D) CH3COONa | CH3COOH | NaOH | Basic

    • (A) NaCl, HCl, NaOH, Basic
    • (B) Na2CO3, H2CO3, NaOH, Neutral
    • (C) Na2SO4, H2SO4, NaOH, Acidic
    • (D) CH3COONa, CH3COOH, NaOH, Basic ✓

    Answer: (D)

Questions parents and students ask

How many marks does "Acids, Bases and Salts" carry in CBSE Class 10 Science?

In the CBSE papers of 2023–2026, this chapter carried about 7 marks and about 3 questions a year; by marks it ranks 5 in Science.

Which are the most important (VVI) questions of "Acids, Bases and Salts"?

The 20 questions above are from the board’s real papers, each with the years it was asked; the most-asked come first with their full model answers. All 36 written questions of this chapter, with answers, are in the Ask Medha app.

Do the answers follow the board’s marking scheme?

Yes. CBSE publishes its marking schemes; every model answer here is written point by point to the marks the board’s own scheme gives, and checked by a second, independent model.