Chapter 2: Acids, Bases and Salts

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

1. Q1Short answer2 marksAsked: 2024, 2025

   Why is an aqueous solution of sodium carbonate alkaline ?

   

   Medha Ma’am

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   Model answer · pH of salts, sodium carbonate

   1. 1.Sodium carbonate is a salt of a strong base and a weak acid, as it is formed from the reaction of a strong base, sodium hydroxide, and a weak acid, carbonic acid.1 mark
   2. 2.When sodium carbonate dissolves in water, it undergoes hydrolysis, which produces hydroxyl ions in the solution. Since salts of a strong base and weak acid are basic in nature with pH value more than 7, the aqueous solution of sodium carbonate turns out to be alkaline.1 mark

   • Mention clearly that sodium carbonate is a salt of a strong base and a weak acid.
   • Use the word hydrolysis while explaining why hydroxyl ions are produced.
   • State that the solution is basic with pH more than 7 due to hydroxyl ions.

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

2. Q2Short answer2 marksAsked: 2021, 2023

   What is the role of acid in our stomach ?

   

   Medha Ma’am

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   Hear it from Medha Ma’am · 25 s

   Model answer · role of acid in stomach

   1. 1.Hydrochloric acid is secreted by gastric glands present in the stomach. It helps in the digestion of food without harming the stomach by making the food medium acidic, and this acidic medium helps in the action of the pepsin enzyme.1 mark
   2. 2.This hydrochloric acid also destroys the harmful microorganisms or bacteria present in the food that we eat, thus protecting the body from infection.1 mark

   • Name the acid as 'hydrochloric acid' secreted by gastric glands, not just 'acid'.
   • Mention the enzyme 'pepsin' by name for the first point.
   • Write 'microorganisms or bacteria' clearly for the second point.

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

3. Q3Long answer5 marksAsked: 2025

   Explain the importance of pH in daily life.

   

   Medha Ma’am

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   Hear it from Medha Ma’am · 105 s

   Model answer · importance of pH in daily life

   1. 1.Living organisms can survive only in a narrow range of pH change, as living cells of plants and animals are highly sensitive to acids and bases. When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water, and the survival of aquatic life in such rivers becomes difficult.1 mark
   2. 2.Plants require a specific pH range for their healthy growth. To find out the pH required for the healthy growth of a plant, soil is tested, and if the soil is too acidic or too basic, it is treated with suitable substances such as quick lime to correct the soil pH so that plants can grow well.1 mark
   3. 3.Our body works within the pH range of 7.0 to 7.8, and this body pH must remain nearly constant for the body to work properly. Our stomach produces hydrochloric acid, which helps in the digestion of food without harming the stomach.1 mark
   4. 4.Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium phosphate, is the hardest substance in the body and does not dissolve in water, but is corroded when the pH in the mouth is below 5.5. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.1 mark
   5. 5.Bee-sting leaves an acid, which is methanoic acid, and this causes pain and irritation when a honeybee stings. Use of a mild base like baking soda on the stung area gives relief from this pain and irritation.1 mark

   • In point 1, write both 'sensitive' (of living cells to acids and bases) and 'aquatic life' (whose survival becomes difficult).
   • In point 3, state the exact range 'pH range of 7.0 to 7.8' and mention hydrochloric acid helping in 'digestion' of food.
   • In point 4, give the exact value 'pH of the mouth is lower than 5.5' and name 'basic' toothpastes used to neutralise the acid.

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

4. Q4Long answer5 marksAsked: 2026

   Explain the Arrhenius theory of acid and base.

   How the model answer begins

   1. According to the Arrhenius theory, acids are substances that dissociate in aqueous solution to give hydrogen ions (H+). These hydrogen ions cannot exist alone, but they exist after combining with water molecules, so…

   The full answer from Medha Ma’am →
5. Q5Long answer5 marksAsked: 2026

   What is pH ? State the importance of pH in daily life.

   How the model answer begins

   1. pH is a scale used to measure the hydrogen ion concentration in a solution. On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline). Higher the hydronium ion concentration, lower is the pH…

   The full answer from Medha Ma’am →
6. Q6Long answer5 marksAsked: 2022

   Mention any five importances of pH in our daily life.

   How the model answer begins

   1. Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change. If this range changes, it becomes difficult for the survival of plants and animals.

   The full answer from Medha Ma’am →
7. Q7Long answer5 marksAsked: 2022

   Mention five importance of pH in our daily life.

   How the model answer begins

   1. Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change, so this pH value of the body must be maintained for the survival of plants and animals.

   The full answer from Medha Ma’am →
8. Q8Short answer2 marksAsked: 2021

   What happens when CO2 (gas) is passed through lime water, (i) in small amount (ii) in excess amount?

   How the model answer begins

   1. When carbon dioxide gas is passed through lime water in small amounts, the lime water turns milky. This happens due to the formation of calcium carbonate. The reaction is: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

   The full answer from Medha Ma’am →
9. Q9Short answer2 marksAsked: 2021

   What is a neutralization reaction ? Give one example.

   How the model answer begins

   1. The reaction in which an acid and a base react with each other to form salt and water is called a neutralisation reaction. It can be written in general as Base + Acid → Salt + Water.

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10. Q10Short answer2 marksAsked: 2021

    What is baking powder?

    How the model answer begins

    1. Baking powder is a mixture of baking soda (sodium hydrogencarbonate, NaHCO3) and a mild edible acid such as tartaric acid.

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11. Q11Short answer2 marksAsked: 2024

    Name the sodium compound which is used for softening hard water.

    How the model answer begins

    1. Sodium carbonate is the sodium compound used for removing permanent hardness of water. It is also called washing soda and its chemical formula is Na2CO3.

    The full answer from Medha Ma’am →
12. Q12Short answer2 marksAsked: 2025

    What is an indicator ? Write the name of an indicator.

    How the model answer begins

    1. Indicators are substances that indicate the presence of acid or base in a solution by a change in colour or odour.

    The full answer from Medha Ma’am →
13. Q13Short answer2 marksAsked: 2025

    Why is it necessary to keep the plaster of Paris away from moisture? Explain.

    How the model answer begins

    1. Plaster of Paris is calcium sulphate hemihydrate, CaSO4.½H2O, a white powder. If it is kept near moisture, it reacts with water present in the moisture and changes into gypsum, which has the formula CaSO4.2H2O.

    The full answer from Medha Ma’am →
14. Q14Short answer2 marksAsked: 2025

    What are acids? Give any two examples. Explain the effect of an acid on an indicator.

    How the model answer begins

    1. Acidic nature of a substance is due to the formation of H+(aq) ions in solution. These substances are sour in taste. Hydrochloric acid (HCl) is one such substance which gives H+ ions in aqueous solution.

    The full answer from Medha Ma’am →
15. Q15Short answer2 marksAsked: 2026

    What effect does the concentration of H+ ions have on the nature of the solution ?

    How the model answer begins

    1. The acidic nature of the solution increases as the concentration of H+ ions increases. Higher the hydronium ion concentration, lower is the pH value, so a lower pH means the solution is more strongly acidic.

    The full answer from Medha Ma’am →
16. Q16Short answer2 marksAsked: 2026

    What do you understand by electrolysis ?

    How the model answer begins

    1. Electrolysis is a process in which chemical decomposition occurs when an electric current is passed through an aqueous solution or molten state of an electrolyte. For example, when electricity is passed through an…

    The full answer from Medha Ma’am →
17. Q17Short answer2 marksAsked: 2026

    What is a base ? What is the difference between a base and an alkali ?

    How the model answer begins

    1. Bases are substances that are bitter in taste and turn red litmus paper blue.

    The full answer from Medha Ma’am →
18. Q18Short answer2 marksAsked: 2023

    Why does an acid not show acidic behaviour in the absence of water ?

    How the model answer begins

    1. The separation of H+ ion from an acid molecule cannot occur in the absence of water, so acids are unable to produce hydrogen ions (H+) without water. For example, HCl + H2O → H3O+ + Cl–. Hydrogen ions cannot exist…

    The full answer from Medha Ma’am →
19. Q19Short answer2 marksAsked: 2023

    Why do compounds like alcohol and glucose not show acidic character in aqueous solution?

    How the model answer begins

    1. Molecules of alcohol and glucose contain hydrogen atoms, but they do not produce H+ ions in aqueous solution.

    The full answer from Medha Ma’am →
20. Q20Short answer2 marksAsked: 2023

    While diluting an acid, acid should be added to water and not water to acid. Why?

    How the model answer begins

    1. The process of dissolving an acid in water is a highly exothermic one, which means a large amount of heat is released when an acid is mixed with water.

    The full answer from Medha Ma’am →

6 more questions of this chapter, each with its model answer, in the AskMedha app.

Objective questions with the board’s key (10)

1. 1. Bihar Board 2026

   Dental caries is caused by

   • (A) Eating sweet things
   • (B) Not cleaning the teeth
   • (C) Acid formation by bacteria
   • (D) All of these ✓

   Answer: (D)

2. 2. Bihar Board 2026

   What is the chemical formula of chalk ?

   • (A) MgCO_3
   • (B) CaO
   • (C) CaCO_3 ✓
   • (D) Ca(HCO_3)_2

   Answer: (C)

3. 3. Bihar Board 2026

   The chemical formula of baking soda is

   • (A) NaHCO3 ✓
   • (B) NaOH
   • (C) Na2CO3
   • (D) KOH

   Answer: (A)

4. 4. Bihar Board 2026

   Which of the following is a salt ?

   • (A) HCl
   • (B) NaOH
   • (C) K2SO4 ✓
   • (D) NH4OH

   Answer: (C)

5. 5. Bihar Board 2026

   Which of the following causes acid rain ?

   • (A) CO2
   • (B) SO2 ✓
   • (C) CO
   • (D) None of these

   Answer: (B)

6. 6. Bihar Board 2026

   Which of the following compounds is a weak acid ?

   • (A) HCl
   • (B) HNO3
   • (C) CH3COOH ✓
   • (D) H2SO4

   Answer: (C)

7. 7. Bihar Board 2026

   Chemical formula of marble is

   • (A) MgCO3
   • (B) CaO
   • (C) CaCO3 ✓
   • (D) CuSO4

   Answer: (C)

8. 8. Bihar Board 2026

   Which of the following is a strong acid ?

   • (A) H2SO4
   • (B) HCl
   • (C) HNO3
   • (D) All of these ✓

   Answer: (D)

9. 9. Bihar Board 2026

   A solution turns blue litmus paper, red. Its pH will be probably

   • (A) 5 ✓
   • (B) 7
   • (C) 8
   • (D) 11

   Answer: (A)

10. 10. Bihar Board 2026

    Which of the following is a natural source of oxalic acid ?

    • (A) Orange
    • (B) Tomato ✓
    • (C) Vinegar
    • (D) Tamarind

    Answer: (B)

Questions parents and students ask

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

In the Bihar Board papers of 2021–2026, this chapter carried about 7 marks and about 9 questions a year; by marks it ranks 4 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 26 written questions of this chapter, with answers, are in the AskMedha app.

Do the answers follow the board’s marking scheme?

BSEB does not publish a marking scheme. Each answer is written point by point on a structure derived from past papers and checked by a second, independent model; it is never called the board’s official scheme.