9th Class Chemistry Chapter # 7 Exercise Solutions – Punjab Board
Acid base Chemistry
Multiple Choice Questions (MCQs)
(i) Which acid is not used as a food or mixed with food?
Options:
(a) Tartaric acid
(b) Acetic acid
(c) Citric acid
(d) Formic acid
✅ Correct Answer: (d) Formic acid
📘 Explanation:
Formic acid is toxic and not used in food. The other acids are commonly found in foods or used as preservatives/flavouring agents.
(ii) While baking, which gas is responsible for raising the bread and making it soft?
Options:
(a) Oxygen
(b) Carbon dioxide
(c) Nitrogen
(d) Carbon monoxide
✅ Correct Answer: (b) Carbon dioxide
📘 Explanation:
Baking powder or baking soda reacts to produce CO₂ gas, which forms bubbles and causes the dough to rise and soften.
(iii) Predict the main characteristics of the reactions of metals with acids.
Options:
(a) Metals are dissolved
(b) Metals are converted into salts
(c) Hydrogen gas is evolved
(d) All the above mentioned characteristics are true
✅ Correct Answer: (d) All the above mentioned characteristics are true
📘 Explanation:
Acids react with metals to:
Dissolve the metal
Form a salt
Release hydrogen gas
(iv) How many hydroxide ions, calcium hydroxide will release in water?
Options:
(a) 1
(b) 2
(c) Zero
(d) 3
✅ Correct Answer: (b) 2
📘 Explanation:
Calcium hydroxide Ca(OH)2is a strong base and releases 2 OH⁻ ions per formula unit in aqueous solution.
(v) In a neutralization reaction between KOH and H₃PO₄, how many molecules of KOH will react with one molecule of H₃PO₄?
Options:
(a) 2
(b) 1
(c) 3
(d) 4
✅ Correct Answer: (c) 3
📘 Explanation:
Phosphoric acid H3PO4 is a triprotic acid (releases 3 H⁺). Potassium hydroxide KOHKOH provides 1 OH⁻ per molecule. So, 3 KOH molecules are required to neutralize 1 H₃PO₄.
(vi) Which acid is used in the preparation of soap?
Options:
(a) Tartaric acid
(b) Citric acid
(c) Acetic acid
(d) Oleic acid
✅ Correct Answer: (d) Oleic acid
📘 Explanation:
Soap is made by saponification of fats and oils, which often contain oleic acid, a fatty acid.
(vii) Which compound is formed when SO₂ is dissolved in water?
Options:
(a) SO₃
(b) H₂SO₃
(c) H₂SO₄
(d) H₂S₂O₃
✅ Correct Answer: (b) H₂SO₃
📘 Explanation:
Sulphur dioxide SO2 dissolves in water to form sulphurous acid:
SO2+H2O→H2SO3
(viii) Which of the following contains oxalic acid?
Options:
(a) Tomato
(b) Orange
(c) Tamarind
(d) Sour milk
✅ Correct Answer: (a) Tomato
📘 Explanation:
Tomatoes are rich in oxalic acid, which gives them a slightly sour taste.
(ix) Which compound in the following reaction is behaving as a conjugate base?
CH3COOH(aq)+H2O(l)⇌CH3COO−+H3O+
Options:
(a) CH₃COOH
(b) H₂O
(c) CH₃COO⁻
(d) H₃O⁺
✅ Correct Answer: (c) CH₃COO⁻
📘 Explanation:
CH₃COO⁻ is formed when CH₃COOH donates a proton (H⁺), so it acts as the conjugate base of the acid.
(x) When a chemical reaction is carried out with a substance Z, a gas is produced which turns red litmus paper blue. What is the reaction?
Options:
(a) Reaction of an acid with a metal carbonate
(b) Reaction of an acid with metal hydrogen carbonate
(c) Reaction of an alkali with an acid
(d) Reaction of an alkali with ammonium salt
✅ Correct Answer: (d) Reaction of an alkali with ammonium salt
📘 Explanation:
This reaction produces ammonia gas (NH₃), which is basic in nature and turns red litmus blue.
Questions for Short Answers
i. Choose Arrhenius Acids among the following compounds:
HF, NH₃, H₂SO₄, SO₃, H₂S, H₂O
Answer:
According to Arrhenius definition, an acid is a substance that increases the concentration of H⁺ (or H₃O⁺) ions in aqueous solution.
✅ Arrhenius acids among the given are:
HF (hydrofluoric acid)
H₂SO₄ (sulphuric acid)
H₂S (hydrosulphuric acid)
❌ Not acids (as per Arrhenius):
NH₃ – Base (gives OH⁻ in water indirectly)
SO₃ – Acidic oxide, not an acid itself until it reacts with water
H₂O – Neutral, acts as both acid or base depending on conditions
ii. How does calcium metal react with dilute H₂SO₄?
Answer:
Calcium metal reacts with dilute sulphuric acid (H₂SO₄) to form calcium sulphate (CaSO₄) and hydrogen gas (H₂).
Ca(s)+H2SO4(aq)→CaSO4(aq)+H2(g)
✅ Observation:
Effervescence of hydrogen gas is seen.
It’s an example of metal + acid → salt + hydrogen reaction.
iii. Which salt is formed when HCl reacts with BaCO₃?
Answer:
When hydrochloric acid (HCl) reacts with barium carbonate (BaCO₃), the products are:
Barium chloride (BaCl₂)
Water (H₂O)
Carbon dioxide (CO₂)
BaCO3+2HCl→BaCl2+H2O+CO2↑
✅ Salt formed: Barium chloride (BaCl₂)
iv. How will you justify that HSO₄⁻ is a Bronsted-Lowry acid?
Answer:
According to the Bronsted-Lowry definition, an acid is a substance that donates a proton (H⁺).
HSO₄⁻ can lose a proton to form SO₄²⁻:
HSO4−⇌H++SO42
Since it donates H⁺, it behaves as a Bronsted-Lowry acid.
✅ Conclusion: HSO₄⁻ is a weak acid that donates a proton, fitting Bronsted-Lowry definition.
v. Why is HCl not edible although it is present in the stomach and responsible for digestion of food?
Answer:
Hydrochloric acid (HCl) in the stomach:
Helps in digestion by activating enzymes and breaking down food.
It is present in very dilute form (around 0.5%–1%) in gastric juice.
❌ But concentrated HCl is:
Corrosive and highly acidic
Can burn tissues, damage mouth and esophagus, and is dangerous to health
✅ Conclusion: HCl is not edible in its concentrated form because it is harmful and corrosive, though a dilute amount naturally exists in the stomach for digestion.
Constructed Response Questions
i. What chemical name will you give to soap as a compound?
Answer:
Soap is chemically a salt of a fatty acid.
When a fat or oil reacts with sodium hydroxide (NaOH) during saponification, it produces sodium salt of long-chain fatty acid — commonly called soap.
✅ Chemical name of soap:
Sodium stearate (C₁₇H₃₅COONa) – if made from stearic acid.
🧼 Conclusion: Soap is generally a sodium or potassium salt of a fatty acid.
ii. In the presence of a drop of an acid, water is known to ionize as follows:
H2O⇌H++OH−
In your opinion, which name will be suitable for water: an acid, a base, or both?
Answer:
Water is a neutral molecule, but it can donate or accept a proton (H⁺) depending on what it reacts with.
In presence of a strong acid, water accepts H⁺ → acts as a base
In presence of a strong base, water donates H⁺ → acts as an acid
✅ Conclusion:
Water is amphoteric, i.e., it behaves as both acid and base, depending on the reacting substance.
iii. Why does Na₂CO₃ behave like a base in water?
Answer:
Sodium carbonate (Na₂CO₃) dissociates in water to give:
Na2CO3→2Na++CO32−
The carbonate ion (CO₃²⁻) is a weak base and reacts with water to form OH⁻ ions:
CO32−+H2O⇌HCO3−+OH−
This increase in OH⁻ ions makes the solution basic.
✅ Conclusion:
Na₂CO₃ behaves like a base because it produces hydroxide ions (OH⁻) in water due to hydrolysis.
iv. Is NaHCO₃ a base or an acid? Justify your answer.
Answer:
Sodium hydrogen carbonate (NaHCO₃), also called baking soda, is a basic salt.
It comes from a strong base (NaOH) and a weak acid (H₂CO₃).
In water:
NaHCO3⇌Na++HCO3−
Then:
HCO3−+H2O⇌H2CO3+OH−
This reaction releases OH⁻ ions, making the solution basic.
✅ Conclusion:
NaHCO₃ is a weak base, as it produces OH⁻ ions in water. However, it can also act as a weak acid in the right conditions, so it is amphiprotic but mostly basic in water.
v. What is the difference between a strong acid and a concentrated acid?
Answer:
| Strong Acid | Concentrated Acid |
|---|---|
| Completely ionizes in water | Has less water (more acid per unit volume) |
| Depends on chemical strength | Depends on amount of acid in solution |
| Example: HCl (strong even if dilute) | Example: Glacial acetic acid (concentrated but weak) |
| Strength ≠ concentration | Concentration ≠ strength |
✅ Conclusion:
A strong acid refers to the degree of ionization.
A concentrated acid refers to the amount of acid present in a given volume.
Descriptive Questions
i. Explain Arrhenius concept of acids and bases.
Answer:
Arrhenius Concept (1884) defines acids and bases based on ionization in water:
Arrhenius Acid: A substance that increases H⁺ ions (or H₃O⁺) in aqueous solution.
Example:HCl→H++Cl−
Arrhenius Base: A substance that increases OH⁻ ions in aqueous solution.
Example:NaOH→Na++OH−
✅ Limitation:
Only applies to aqueous solutions.
Cannot explain basic nature of substances like NH₃.
ii. Compare Arrhenius and Bronsted-Lowry concepts of acids and bases.
Answer:
| Property | Arrhenius Concept | Bronsted-Lowry Concept |
|---|---|---|
| Acid | Produces H⁺ in water | Donates a proton (H⁺) |
| Base | Produces OH⁻ in water | Accepts a proton (H⁺) |
| Medium | Only aqueous | Any solvent or gas phase |
| Example Acid | HCl → H⁺ + Cl⁻ | HCl donates H⁺ to H₂O |
| Example Base | NaOH → Na⁺ + OH⁻ | NH₃ + H₂O → NH₄⁺ + OH⁻ |
| Limitation | Only OH⁻ bases allowed | Covers more acid–base pairs |
✅ Conclusion:
Bronsted–Lowry theory is broader and explains acid–base behavior in non-aqueous media too.
v. State the reason of showing acidic character by both dry HCl gas and HCl solution in water.
Answer:
Dry HCl gas is not acidic — it does not ionize in the absence of water.
In water, HCl ionizes completely:
HCl+H2O→H3O++Cl−
– It shows acidic character (turns blue litmus red, reacts with metals, etc.)
✅ Conclusion:
Only aqueous HCl acts as an Arrhenius acid. Dry HCl gas doesn’t show acidic properties until it dissolves in water and releases H⁺ ions.
vi. Differentiate between an acid and its conjugate base.
Answer:
| Acid | Conjugate Base |
|---|---|
| Donates a proton (H⁺) | Formed after acid loses a proton |
| Strong acid → weak conjugate base | Weak acid → strong conjugate base |
| Example: HCl (acid) | Cl⁻ (conjugate base) |
| Example: CH₃COOH | CH₃COO⁻ |
✅ Conclusion:
When an acid loses a proton, it becomes its conjugate base, capable of accepting a proton in the reverse reaction.
Investigative Questions
i. Acids play significant roles within human body. Comment on this statement.
Answer:
Yes, acids play essential and diverse roles in the human body. Some key examples include:
✅ 1. Hydrochloric Acid (HCl) in the stomach:
Secreted by gastric glands.
Helps in digestion of food by:
Activating enzymes like pepsin.
Killing harmful microorganisms.
Maintaining an acidic pH (~1.5–3.5) for proper digestion.
✅ 2. Amino acids:
Building blocks of proteins, essential for growth and repair.
Each amino acid contains an acidic group (-COOH).
✅ 3. Fatty acids:
Components of lipids, important for energy storage and cell membrane structure.
✅ 4. Nucleic acids (DNA and RNA):
Carry genetic information.
Composed of phosphoric acid, sugars, and nitrogenous bases.
📘 Conclusion:
Acids are vital for digestion, metabolism, energy production, and genetic function in the human body.
ii. What is observed when CO₂ is passed through lime water:
(i) for a short duration
(ii) for a long duration
Answer:
✅ Chemical used:
Lime water is a dilute solution of calcium hydroxide: Ca(OH)₂
(i) For a short duration:
CO2+Ca(OH)2→CaCO3↓+H2O
Calcium carbonate (CaCO₃) forms as a white precipitate.
Observation: Lime water turns milky.
(ii) For a long duration:
CaCO3+CO2+H2O→Ca(HCO3)2
The white precipitate dissolves and solution becomes clear again.
This is due to formation of calcium hydrogen carbonate (soluble).
📘 Conclusion:
Short duration → Milky appearance (due to CaCO₃)
Long duration → Milkiness disappears (due to formation of soluble Ca(HCO₃)₂)