Chapter 7: Acid Base ChemistryShort Questions with Answers
47 short questions of 9th Class Chemistry Chapter 7, Acid Base Chemistry, with exam-ready answers in the book's words: the textbook exercise questions plus SLO-based questions asked in board papers. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
Set A: Textbook exercise short questions 5 questions
The short questions at the end of the chapter in your book.
Short Q 7.1
Choose Arrhenius acids among the following compounds: HF, NH₄⁺, H₂SO₃, SO₃, H₂S, H₂O.
Answer
An Arrhenius acid is a substance which dissociates in water to give protons (H⁺) or hydroxonium ions (H₃O⁺).
Species
Arrhenius acid?
Reason
HF
Yes
HF ⇌ H⁺ + F⁻ in water
NH₄⁺
No
an ion; the book: "NH₄⁺ is not Arrhenius acid" (it is a Bronsted-Lowry acid)
H₂SO₃
Yes
H₂SO₃ ⇌ H⁺ + HSO₃⁻
SO₃
No
has no hydrogen; it is an acidic oxide that forms H₂SO₄ with water
H₂S
Yes
H₂S ⇌ H⁺ + HS⁻
H₂O
No
neutral; gives equal H⁺ and OH⁻
Arrhenius acids: HF, H₂SO₃ and H₂S.
Short Q 7.2
How does calcium metal react with dilute H₂SO₄?
Answer
Calcium is a reactive metal, so it dissolves in the acid, forms a salt and evolves hydrogen gas:
Ca(s) + H₂SO₄(aq) → CaSO₄(s) + H₂(g)
Calcium sulphate is only sparingly soluble, so it forms a coating on the calcium and the reaction soon slows down.
Short Q 7.3
Which salt is formed when HCl reacts with BaCO₃?
Answer
Barium chloride (BaCl₂). Acids decompose metal carbonates evolving carbon dioxide:
BaCO₃(s) + 2HCl(aq) → BaCl₂(aq) + H₂O(l) + CO₂(g)
Short Q 7.4
How will you justify that HSO₄⁻ is a Bronsted-Lowry acid?
Answer
A Bronsted-Lowry acid is a substance that donates a proton. HSO₄⁻ still has one hydrogen it can give away:
HSO₄⁻(aq) + H₂O(l) ⇌ SO₄²⁻(aq) + H₃O⁺(aq)
It donates a proton to water, so it is an acid; SO₄²⁻ is its conjugate base. (HSO₄⁻ can also accept a proton to become H₂SO₄, so it is amphoteric, but in water it acts mainly as an acid.)
Short Q 7.5
Why is HCl not edible although it is present in the stomach and responsible for digestion of food?
Answer
Hydrochloric acid is a strong mineral acid that ionises completely and is highly corrosive. Concentrated or even moderately dilute HCl would burn the mouth, throat and food pipe.
The stomach makes only a very dilute solution of HCl (about 0.1 M) for digestion, and its lining is protected by a layer of mucus that the acid cannot attack. The mouth and throat have no such protection. When the stomach makes too much acid, even this causes acidity and heartburn.
Set B: More short questions 42 questions
The book's in-text exercises first, then short questions on every topic of the chapter (SLO-based), as asked in board papers.
Practice exercise questions (in text)
SQ 7.1
Name some fruits which contain citric acid.
Answer
Lemon, orange, lime (kinu, mosambi), grapefruit and strawberries.
SQ 7.2
Activity 7.1: show with blue and red litmus whether tap water, battery water, rain water, soap solution, toothpaste, shampoo and bleach are acidic or basic.
Answer
Substance
Nature
Tap water
about neutral
Battery water
acidic
Rain water
slightly acidic
Soap solution
basic
Toothpaste
basic
Shampoo
about neutral / slightly acidic
Bleach
basic
Acids turn blue litmus red; bases turn red litmus blue.
SQ 7.3
In what ways are mineral acids useful for us?
Answer
Sulphuric acid: fertilizers (ammonium sulphate, superphosphate), car batteries, detergents, paints.
Hydrochloric acid: cleaning metals before galvanising, making PVC and medicines; in our stomach it digests food.
SQ 7.4
How do chloride ions exist in water?
Answer
As hydrated chloride ions, Cl⁻(aq). Water molecules surround each Cl⁻ with their partially positive hydrogen ends pointing towards the ion, held by ion-dipole attraction.
Hydrated Na⁺ and Cl⁻ (from Chapter 3).
SQ 7.5
Why does ammonium hydroxide only partially ionise in water?
Answer
NH₄OH is really ammonia dissolved in water. Ammonia is a weak base: only a small fraction of NH₃ molecules accept a proton from water, and the NH₄⁺ and OH⁻ formed readily recombine. So an equilibrium is set up with most of the ammonia un-ionised: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
SQ 7.6
Give two examples of Bronsted-Lowry bases which are not bases by Arrhenius definition.
Answer
NH₃ (accepts H⁺ to form NH₄⁺) and CO₃²⁻ (accepts H⁺ to form HCO₃⁻). CN⁻ and Cl⁻ are other examples: none of these contains OH⁻.
SQ 7.7
Activity 7.2: what is observed when zinc granules are warmed with dilute H₂SO₄ and a burning match is brought near?
Answer
Zinc dissolves with fizzing and a colourless gas is given off. The gas burns with a pop sound at the match flame: it is hydrogen.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
SQ 7.8
Activity 7.3: how do sodium bicarbonate and vinegar clean a blocked drain?
Answer
Vinegar (acetic acid) reacts with sodium bicarbonate, releasing bubbles of carbon dioxide that loosen the blockage; boiling water then flushes it out. NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. Caustic cleaners (NaOH) dissolve grease by turning it into soap.
Acids, bases and alkalis
SQ 7.9
What are the properties of acids?
Answer
Sour taste.
Turn blue litmus red.
Give H₃O⁺ ions in water.
React with metals (H₂), bases (salt + water) and carbonates (CO₂).
SQ 7.10
What are the properties of bases?
Answer
Bitter taste and slippery touch.
Turn red litmus blue.
Give OH⁻ ions in water (alkalis).
Neutralise acids; liberate ammonia from ammonium salts.
SQ 7.11
Differentiate between natural and mineral acids.
Answer
Natural (organic) acids
Mineral acids
Obtained from natural sources.
Prepared from minerals like NaCl or NaNO₃.
Mostly weak.
Generally very strong.
Citric, lactic, acetic, tartaric acid.
HCl, H₂SO₄, HNO₃.
SQ 7.12
Differentiate between a base and an alkali.
Answer
A base is an oxide or hydroxide of a metal that neutralises an acid (CuO, Cu(OH)₂, MgO). An alkali is a base that dissolves in water (NaOH, KOH). All alkalis are bases, but all bases are not alkalis.
SQ 7.13
Why is Na₂O a basic oxide?
Answer
Na₂O contains the oxide ion O²⁻, which is a very strong base with a strong tendency to react with water to produce hydroxide ions: Na₂O + H₂O → 2NaOH. It also reacts with acids to form salt and water.
SQ 7.14
What is a neutralisation reaction?
Answer
Acids and bases cancel the properties of each other when mixed in equal amounts. The reaction is called a neutralisation reaction; a salt and water are formed. HCl + NaOH → NaCl + H₂O.
SQ 7.15
Name the alkalis called caustic soda, caustic potash and lime water.
Acid: a substance which dissociates in water to give H⁺ or H₃O⁺ (HCl, HNO₃, H₂SO₄, HCN). Base: a substance which dissociates in water to give OH⁻ (NaOH, KOH, Ba(OH)₂).
SQ 7.17
What is the hydroxonium ion? How is it formed?
Answer
H₃O⁺. A proton is very small with a very high charge density, so it forms a strong bond with a lone pair of a water molecule: H⁺ + H₂O → H₃O⁺. Up to four water molecules may surround it.
H₃O⁺ and OH⁻ surrounded by water.
SQ 7.18
How did Arrhenius explain neutralisation?
Answer
A strong acid and a strong base dissolved in water completely dissociate into ions. The hydroxonium and hydroxyl ions then react to form water with the evolution of heat: H₃O⁺ + OH⁻ → 2H₂O. The salt stays as hydrated spectator ions.
SQ 7.19
Give two limitations of the Arrhenius concept.
Answer
Na₂CO₃, K₂CO₃ and NH₃ have no OH group, yet they behave as bases in water.
Water must be present; it cannot explain acid-base reactions in the gas phase (HCl + NH₃ → NH₄Cl) or in other solvents.
Strong and weak acids and bases
SQ 7.20
On what does the strength of an acid depend?
Answer
Upon the extent to which it is ionised in water. A strong acid ionises completely (HCl); a weak acid ionises partially (CH₃COOH).
SQ 7.21
Why are organic acids weak acids?
Answer
Organic acids ionise up to a very limited extent. Acetic acid is only 0.132 % ionised: out of 1000 molecules only about 1.32 dissociate.
SQ 7.22
Write the ionisation of sulphuric acid in two steps.
Antacids are weak bases (magnesium hydroxide, aluminium hydroxide) that neutralise the extra stomach acid; their concentration of hydroxyl ions is too low to harm the throat or stomach. Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
SQ 7.25
What causes stomach acidity?
Answer
Fatty and spicy foods cause the stomach lining to produce more hydrochloric acid than is needed. The person feels a burning sensation below the breastbone, a sour taste in the mouth and heartburn.
Bronsted-Lowry concept
SQ 7.26
Define Bronsted-Lowry acid and base.
Answer
An acid is a substance that donates a proton. A base is a substance that accepts a proton. Water need not be present.
SQ 7.27
Why are all Arrhenius acids Bronsted-Lowry acids?
Answer
Every Arrhenius acid gives H⁺ in water, which means it donates a proton to water: HCl + H₂O → H₃O⁺ + Cl⁻. That is exactly the Bronsted-Lowry definition.
SQ 7.28
Show that water is amphoteric.
Answer
HCl + H₂O → H₃O⁺ + Cl⁻
water accepts H⁺: base
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
water donates H⁺: acid
SQ 7.29
What are conjugate acid and conjugate base?
Answer
When an acid donates a proton, the species left is its conjugate base; when a base accepts a proton, it becomes its conjugate acid. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, OH⁻ is the conjugate base (of water) and NH₄⁺ the conjugate acid (of ammonia).
Conjugate pairs.
SQ 7.30
Identify the conjugate pairs in HCl + H₂O → H₃O⁺ + Cl⁻.
Answer
HCl / Cl⁻ (acid and its conjugate base) and H₂O / H₃O⁺ (base and its conjugate acid).
SQ 7.31
How does HCl gas show acidic character without water?
Answer
It donates a proton to ammonia in the gas phase: HCl(g) + NH₃(g) → NH₄Cl(s), white fumes. By Bronsted-Lowry, a proton donor is an acid, whether water is present or not.
Reactions of acids and bases
SQ 7.32
Write the reaction of an acid with a metal oxide.
Answer
2HCl(aq) + MgO(s) → MgCl₂(aq) + H₂O(l)
Acids with alkalis or metal oxides form salts and water.
SQ 7.33
Which metals do not evolve hydrogen with acids? Why?
Answer
Cu, Ag, Au and Pt. They are less reactive than hydrogen, so they cannot displace it from acids.
SQ 7.34
Write the reactions of acids with a carbonate and a hydrogen carbonate.
Answer
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
NaHCO₃ + HCl → NaCl + H₂O + CO₂
SQ 7.35
How do alkalis react with ammonium salts?
Answer
Alkalis react with ammonium salts and liberate ammonia gas, which has a sharp smell and turns red litmus blue.
NaOH + NH₄Cl → NaCl + H₂O + NH₃
SQ 7.36
How is ammonia gas tested?
Answer
It has a sharp, pungent smell; it turns moist red litmus blue; and it gives white fumes of NH₄Cl with a glass rod dipped in concentrated HCl.
SQ 7.37
What is soap chemically?
Answer
The sodium or potassium salt of a long-chain fatty acid, e.g. sodium stearate C₁₇H₃₅COONa, made by heating fats or oils with NaOH (saponification).
Acid rain
SQ 7.38
Define acid rain. How is it formed?
Answer
When rain water has pH between 4.2 and 4.4 it is called acid rain. Burning fossil fuels releases SO₂ and NO₂; with moisture in the air they form acid droplets (H₂SO₄, HNO₃) that fall as acid rain.
Formation of acid rain.
SQ 7.39
Write the equations for the formation of sulphuric and nitric acids in air.
Answer
2SO₂ + O₂ → 2SO₃; SO₃ + H₂O → H₂SO₄
2NO₂ + H₂O → HNO₂ + HNO₃
SQ 7.40
What are the effects of acid rain on soil and aquatic life?
Answer
It makes soil more acidic and washes away nutrients plants need; many plants cannot grow. It makes lakes, rivers and ponds too acidic for aquatic animals, so many no longer have fish.
SQ 7.41
Why does acid rain damage marble buildings and statues?
Answer
Marble and limestone are calcium carbonate. Acids decompose carbonates: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. The stone slowly dissolves and crumbles, as at the Taj Mahal. Acid rain also corrodes iron and steel.
SQ 7.42
How can acid rain be reduced?
Answer
Burn less fossil fuel; remove sulphur from fuels; fit scrubbers in power stations (lime absorbs SO₂) and catalytic converters in vehicles (remove NOₓ); use clean energy such as solar and wind. Lime is added to acidic lakes and soils to neutralise them.
