| Publisher | : Oriental Book Company Private Limited |
| Material | : Madhyamik Physical Science Solution |
| Subject | : Physical Science |
| Class | : 9 (Madhyamik) |
| Chapter Name | : Acids, Bases and Salts |
In-Text Questions
Question 4.4.1
Give examples of two substances belongs to class acid.
Answer
- Hydrochloric acid (HCl)
- Sulphuric acid (H₂SO₄)
Question 4.4.2
What happens when a dry blue litmus paper is introduced into a jar containing dry HCl gas ?
Answer
Dry HCl gas does not show acidic properties as it does not ionize in the absence of water. Therefore, the dry blue litmus paper remains unchanged.
Question 4.4.3
What the products obtained by reaction of acids and bases ?
Answer
The reaction between an acid and a base produces salt and water. This reaction is called a neutralisation reaction.
Example:
HCl + NaOH → NaCl + H₂O
Question 4.4.4
Give example of an indicator and its colour in acid and alkaline medium.
Answer
Indicator: Phenolphthalein
- In acidic medium: Colourless
- In alkaline medium: Pink
Question 4.4.5
What is the molar concentration of H⁺ and OH⁻ ions is pure water ?
Answer
In pure water at 25°C,
- [H⁺] = [OH⁻] = 10⁻⁷ M
Question 4.4.6
Examine the Table 4.4.2 above and answer the following questions.
(i) Which solution is the strongest-acid ?
(ii) Which one is most basic ?
(iii) Blood is acidic or basic ?
Answer
(i) Green mango juice (pH = 2.0)
(ii) Toothpaste (pH = 9.0)
(iii) Blood is slightly basic (pH = 7.4).
Question 4.4.7
What is muriatic acid, why this name was given ? (Ans. HCl, It was first prepared from sea water i.e. marine salt, so it is called muriatic acid).
Answer
Muriatic acid is Hydrochloric acid (HCl). It was first prepared from sea water (marine salt), hence the name muriatic acid.
Question 4.4.8
Why copper does not react with dil HCl?
Answer
Copper (Cu) does not react with dilute HCl because it is less electropositive than hydrogen and does not displace hydrogen from HCl.
Question 4.4.9
Why nitric acid does not liberate hydrogen in reaction with Zinc ?
Answer
Nitric acid (HNO₃) is a strong oxidising agent and reacts with zinc (Zn) to form zinc nitrate (Zn(NO₃)₂) and oxides of nitrogen (NO, NO₂) instead of hydrogen gas.
Reaction:
Zn + 4HNO₃ → Zn(NO₃)₂ + 2NO₂ + 2H₂O
Question 4.4.10
What is nascent hydrogen ?
Answer
Nascent hydrogen refers to hydrogen atoms produced at the moment of formation from a reaction. It is considered more reactive than molecular hydrogen (H₂).
Example:
Zn + H₂SO₄ → ZnSO₄ + 2[H] (nascent hydrogen)
Question 4.4.11
How could you identify a colourless solution is of sulphuric acid ?
Answer
A colourless solution can be identified as sulphuric acid (H2SO4) by performing the BaCl₂ test. Addition of barium chloride (BaCl2) to H₂SO₄ forms a white precipitate of BaSO4, confirming its presence.
Reaction:
BaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl
Question 4.4.12
How would you get hydrogen from a base ?
Answer
Hydrogen gas can be obtained by reacting an alkali (base) like NaOH with a reactive metal like zinc (Zn).
Reaction:
Zn + 2NaOH → Na2ZnO2 + H2↑
Question 4.4.13
Between NaOH burn and HNO₃ burn which one is more serious.
Answer
NaOH burns are more serious because alkali burns penetrate tissues more rapidly, causing deep and severe burns, while HNO₃ burns cause surface burns with a yellow stain.
Question 4.4.14
At what pH tooth decay takes place ?
Answer
Tooth decay occurs when the pH of the mouth falls below 5.5.
Question 4.4.15
Why ZnO is an amphoteric oxide ?
Answer
Zinc oxide (ZnO) is amphoteric because it reacts with both acids and bases to form salts and water.
As a base:
ZnO + 2HCl → ZnCl2 + H₂O
As an acid:
ZnO + 2NaOH → Na2ZnO2 + H₂O
Question 4.4.16
Mention the oxides which are responsible for acid rain ?
Answer
The oxides responsible for acid rain are:
- Sulphur dioxide (SO2)
- Sulphur trioxide (SO3)
- Nitrogen dioxide (NO2)
- Carbon dioxide (CO2)
Question 4.4.17
What do you mean by end point of neutralisation reaction.
Answer
The end point of a neutralisation reaction is the stage at which an acid and a base completely react to form salt and water, with the indicator showing a colour change.
Example using Phenolphthalein:
- Before neutralisation: Pink (alkali)
- At neutralisation: Colourless (neutral)
Question 4.4.18
Mention two compounds which are used in antacids.
Answer
- Magnesium hydroxide (Mg(OH)2) – Milk of magnesia
- Aluminium hydroxide (Al(OH)3)
These compounds neutralize excess stomach acid and provide relief from acidity and heartburn.
Very Short Answer type Questions
Multiple Choice questions :
Question 1 (i)
An aqueous solution turns blue litmus solution red. Excess addition of which of the following solutions to the above solution reverse the change—
(a) HCl solution
(b) Vinegar solution
(c) H2SO4 solution
(d) Caustic soda solution.
Answer
(d) Caustic soda solution
Explanation
The given solution turns blue litmus red, meaning it is acidic. To reverse this, we need a base, and caustic soda (NaOH) is a strong base.
Question 1 (ii)
Which of the following substance change the colour of pH paper to greenish-blue ?
(a) Common salt
(b) Vinegar
(c) An antacid
(d) Orange juice.
Answer
(c) An antacid
Explanation
pH paper turns greenish-blue in a slightly basic medium. Antacids, such as Mg(OH)₂, are mild bases that neutralize stomach acid.
Question 1 (iii)
To protect tooth decay you are advised to brush your teeth regularly, the nature of commonly used paste is—
(a) Neutral
(b) Acidic
(c) Basic
(d) Corrosive
Answer
(c) Basic
Explanation
Toothpaste is slightly basic to neutralize acids formed by bacteria in the mouth and prevent tooth decay.
Question 1 (iv)
If you soak a pH paper into gastric juices, it will indicate a pH—
(a) more than 7
(b) less than 7
(c) equal to 7
(d) less than 1
Answer
(b) less than 7
Explanation
Gastric juice contains hydrochloric acid (HCl), which makes its pH below 7, indicating acidity.
Question 1 (v)
Which of the following gases is responsible for acid rain ?
(a) N₂
(b) CH₄
(c) SO₂
(d) H₂.
Answer
(c) SO₂
Explanation
Sulfur dioxide (SO₂) is a major cause of acid rain as it dissolves in water to form sulfuric acid.
Fill in the blanks (with words given in the bracket) :
Question 2 (i)
An acid salt can be obtained from a ___ acid. (monobasic/polybasic)
Answer
polybasic
Explanation
Acid salts form from polybasic acids (acids that can donate more than one proton, e.g., H₂SO₄).
Question 2 (ii)
All alkalis are ___ (bases/not bases)
Answer
bases
Explanation
All alkalis (soluble bases) are bases, but not all bases dissolve in water.
Question 2 (iii)
An alkali reacts with ___ to give ___ and water. (acid/base/salt)
Answer
acid, salt
Explanation
A base reacts with an acid in a neutralization reaction, forming a salt and water.
Question 2 (iv)
Tooth enamel is made of ___ (Ca3(PO4)2/CaCO3)
Answer
Ca3(PO4)2
Explanation
Tooth enamel is primarily made of calcium phosphate (Ca₃(PO₄)₂), which is resistant to acids.
Complete the following reactions :
Question III (i)
2HCl + ___ → CaCl₂ + 2H₂O
Answer
Ca(OH)2
Question III (ii)
HNO₃ + NH₄OH → ___ + H₂O
Answer
NH4NO3
Question III (iii)
H₂SO₄ + ___ → CuSO₄ + SO₂↑ + H₂O
Answer
Cu
Question III (iv)
___ + NaOH → NaAlO₂ + H₂
Answer
Al2O3
Indicate which of the following statements are true and which are false :
Question IV (i)
A substance which produce H⁺ ion in water is an acid.
Answer
True
Explanation
Acids produce H⁺ ions in solution, which is the Arrhenius definition of acids.
Question IV (ii)
The hydrogen of an acid can be replaced by negative radical.
Answer
True
Explanation
Acids release H⁺ ions, which can be replaced by metallic or positive radicals to form salts.
Question IV (iii)
All alkalis are bases but all bases are not alkalis.
Answer
True
Explanation
All alkalis are bases, but some bases (like Cu(OH)2) are insoluble in water, so they are not alkalis.
Question IV (iv)
Both acid and normal salt can be obtained from H₂SO₄.
Answer
True.
Explanation
Sulfuric acid (H₂SO4) can form both normal salts (Na2SO4) and acid salts (NaHSO4).
Question IV (v)
A pH paper gives different colours with acids and bases.
Answer
True
Explanation
pH paper shows different colors in acids and bases, indicating pH variations.
Short answer type questions
Question 1
Define an acid.
Answer
An acid is a substance that produces H⁺ ions in aqueous solution and turns blue litmus red.
Question 2
Which of the following acids give both normal and acid salts and which will give only normal salt ?
(a) H2SO4
(b) HNO3
(c) HCl
Answer
(a) H2SO4 → Both normal and acid salts
(b) HNO3 → Only normal salt
(c) HCl → Only normal salt
Question 3
Identify the acid salts and normal salts among the following ?
(a) KHSO4
(b) CaSO4
(c) Na2CO3
(d) Ca(HCO3)2
Answer
(a) Acid salt
(b) Normal salt
(c) Normal salt
(d) Acid salt
Question 4
Define a base.
Answer
A base is a substance that produces OH⁻ ions in aqueous solution and turns red litmus blue.
Question 5
What do you mean by neutralisation ?
Answer
Neutralisation is a reaction between an acid and a base to form salt and water, resulting in the loss of their acidic and basic properties.
Example:
HCl + NaOH → NaCl + H2O
Long answer type questions
Question 1
Give Arrhenous definition of acids and bases.
Answer
- Acids are substances that produce H⁺ ions in aqueous solution.
- Bases are substances that produce OH⁻ ions in aqueous solution.
Question 2
(a) Show the ionisation of HCl in water.
(b) Why HCl does not show acid character in anhydrous condition.
Answer
(a) HCl + H₂O → H₃O⁺ + Cl⁻
(b) HCl does not ionise in the absence of water, so H⁺ ions are not formed, and it does not show acidic properties.
Question 3
(a) Give examples of three types of salts.
(b) What is an acid-base indicator ?
Answer
(a) Examples of three types of salts
- Normal salt – NaCl
- Acid salt – NaHSO₄
- Basic salt – Zn(OH)Cl
(b) A chemical substance that changes colour in acidic and basic medium to indicate pH.
Example: Litmus paper – Red in acid, Blue in base.
Question 4
How pH of a solution is measured ?
Answer
pH of a solution is measured using a pH paper or universal indicator that shows different colours at different pH levels.
Question 5
Mention one industrial use of each of the following :
(a) HCl
(b) H₂SO₄
(c) HNO₃
(d) NaOH
Answer
(a) HCl – Used in pickling of steel
(b) H₂SO₄ – Used in manufacturing fertilizers
(c) HNO₃ – Used in manufacturing explosives
(d) NaOH – Used in soap and detergent production
Question 6
(a) What is nascent hydrogen ? Show a reaction to explain that the idea of nascent hydrogen is baseless.
(b) How will you prove that nitric acid contains hydrogen and nitrogen ?
Answer
(a) Nascent hydrogen is hydrogen at the moment of formation. However, it is unnecessary because actual reducing agents are stronger.
Example:
Zn + 2HCl → ZnCl₂ + H₂↑
When nitric acid reacts with zinc, it forms zinc nitrate and releases oxides of nitrogen.
Question 7
How will you identify a colourless liquid is H₂SO₄ ?
Answer
Add BaCl₂ to the liquid. If a white precipitate of BaSO4 forms, it confirms the presence of H2SO4.
Question 8
What precautionary measure will you take for safe handling of concentrated acids and bases ?
Answer
Precautionary measure are:-
- Wear gloves and goggles
- Store in tightly closed bottles
- Always add acid to water (not water to acid)
- Work in a well-ventilated area
