📖 Chapter CN05 · NDA Class 11–12 Level🎯 NDA Level : High Priority
Acids, Bases, and Important Compounds is one of the highest-scoring chapters in NDA Chemistry. Questions appear directly on the three acid-base theories, pH calculations, indicators, and properties/uses of named compounds. Important Compounds (baking soda, bleaching powder, plaster of Paris, KMnO₄ etc.) are factual and highly predictable — average students who memorise the formula, preparation, and 2–3 uses of each compound can score every mark here.
📌 What to expect in NDA (based on 2022–2025 pattern): (1) Three acid-base theories — Arrhenius, Brønsted-Lowry, Lewis — differences and limitations; (2) Conjugate acid-base pairs; amphoteric substances (water, HCO₃⁻); (3) pH scale — pH of common substances; relationship pH + pOH = 14; (4) Indicators — litmus, phenolphthalein, methyl orange — colour changes; (5) Baking soda, washing soda, bleaching powder, gypsum, plaster of Paris — formula, preparation, uses; (6) KMnO₄ — colour, oxidising agent, uses in disinfection and titrations.
Topics at a Glance
① Acid-Base Theories
Arrhenius, Brønsted-Lowry, Lewis
② pH Scale & Neutralisation
pH, pOH, indicators, salt hydrolysis
③ Sodium Compounds
Baking soda, washing soda, bleaching powder
④ Calcium & Other Compounds
Gypsum, Plaster of Paris, KMnO₄
1. Acid-Base Theories
1.1
Three Theories — Arrhenius, Brønsted-Lowry, Lewis
Each theory is broader than the previous — Lewis is the most general
Our understanding of acids and bases has evolved through three increasingly general theories. NDA tests their definitions, differences, and specific examples. The Lewis theory is the broadest, covering even reactions with no proton transfer.
ARRHENIUS (1884)
Ionisation in Water
Acid: produces H⁺ (or H₃O⁺) in water
Base: produces OH⁻ in water
HCl → H⁺ + Cl⁻ (acid ✓)
NaOH → Na⁺ + OH⁻ (base ✓)
Limitation: only works for aqueous solutions; NH₃ is a base but produces no OH⁻ directly
BRØNSTED-LOWRY (1923)
Proton Transfer
Acid: proton (H⁺) donor
Base: proton (H⁺) acceptor
Works in non-aqueous solvents too
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (NH₃ is base ✓)
Introduces conjugate acid-base pairs
Limitation: doesn't explain BF₃ + NH₃ (no proton transfer)
LEWIS (1923)
Electron Pair Transfer
Acid: electron pair acceptor
Base: electron pair donor
Most general — covers all acid-base reactions
BF₃ (Lewis acid) + NH₃ (Lewis base) → BF₃·NH₃
Explains reactions with no proton at all
No limitation — superset of both above
⚛ Conjugate Acid-Base Pairs — Brønsted-Lowry
When an acid donates H⁺, it becomes its conjugate base.
When a base accepts H⁺, it becomes its conjugate acid.
Example 1 — HCl in water:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻
Acid₁ Base₂ Conj.acid₂ Conj.base₁
Pair 1: HCl / Cl⁻ Pair 2: H₂O / H₃O⁺
Example 2 — NH₃ in water:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Base₁ Acid₂ Conj.acid₁ Conj.base₂
Pair 1: NH₄⁺ / NH₃ Pair 2: H₂O / OH⁻
Amphoteric substances (act as BOTH acid AND base):
Water (H₂O): acid towards NH₃; base towards HCl
HCO₃⁻ (bicarbonate): acid → CO₃²⁻; base → H₂CO₃
Strong acid → weak conjugate base (Cl⁻ is a very weak base). Weak acid → strong conjugate base (CH₃COO⁻ is a moderate base). This inverse relationship is key for NDA MCQs on conjugate pairs.
Q1. According to Lewis concept, a Lewis acid is a species that:
(a) Donates a proton (b) Accepts a proton (c) Accepts an electron pair (d) Donates an electron pair
Answer: (c) Accepts an electron pair
Lewis acid = electron pair acceptor; Lewis base = electron pair donor. BF₃ accepts the lone pair from NH₃ to form BF₃:NH₃ — no proton transfer occurs. This is why the Lewis concept is broader than Arrhenius and Brønsted-Lowry. Metal cations (Fe³⁺, Al³⁺) are also Lewis acids — they accept lone pairs from water molecules (ligands).
Q2. In the reaction: H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺, the conjugate base of H₂PO₄⁻ is:
(a) H₃PO₄ (b) HPO₄²⁻ (c) PO₄³⁻ (d) H₃O⁺
Answer: (b) HPO₄²⁻
H₂PO₄⁻ donates H⁺ → becomes HPO₄²⁻. The conjugate base is formed by the loss of one H⁺ from the acid. Rule: Conjugate base = acid − H⁺. So H₂PO₄⁻ (acid) − H⁺ = HPO₄²⁻ (conjugate base). The conjugate acid of water (H₂O + H⁺) is H₃O⁺.
Q3. Water is described as amphoteric. This means it:
(a) Always acts as an acid (b) Always acts as a base (c) Can act as both acid and base (d) Is neutral and does not react
Answer: (c) Can act as both acid and base
In H₂O + NH₃ → NH₄⁺ + OH⁻: water acts as an acid (donates H⁺).
In H₂O + HCl → H₃O⁺ + Cl⁻: water acts as a base (accepts H⁺).
HCO₃⁻ is another NDA-tested amphoteric species: with strong acid → acts as base (accepts H⁺ → H₂CO₃); with strong base → acts as acid (donates H⁺ → CO₃²⁻).
pH = −log₁₀[H⁺] (or −log₁₀[H₃O⁺])
pOH = −log₁₀[OH⁻]
At 25°C: pH + pOH = 14 (Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ mol²/L²)
Neutral: [H⁺] = [OH⁻] = 10⁻⁷ M → pH = 7
Acidic: [H⁺] > 10⁻⁷ M → pH < 7
Basic: [H⁺] < 10⁻⁷ M → pH > 7
Quick calculations:
[H⁺] = 10⁻³ M → pH = 3 (strong acid, e.g. HCl 0.001 M)
[H⁺] = 10⁻¹¹ M → pH = 11 → pOH = 14−11 = 3 → [OH⁻] = 10⁻³ M
Each unit change in pH = 10× change in [H⁺]
pH 3 is 10× more acidic than pH 4
pH 3 is 100× more acidic than pH 5
Kw (ionic product of water) = 1×10⁻¹⁴ at 25°C. Temperature affects Kw — as temperature rises, Kw increases slightly (water ionises more), so neutral pH falls below 7 at higher temperatures. Neutral pH = 7 applies only at 25°C.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
■ ACIDIC (pH < 7) — H⁺ > OH⁻
7
■ BASIC / ALKALINE (pH > 7) — OH⁻ > H⁺
Substance
pH (approx.)
Nature
Practical relevance
Gastric acid (HCl)
1–2
Strongly acidic
Digestive acid in stomach
Lemon juice
2–3
Acidic
Citric acid
Vinegar
2.5–3.5
Acidic
Acetic acid
Human blood
7.35–7.45
Slightly basic
Maintained by buffer system
Pure water
7
Neutral
Reference point
Baking soda solution
8–9
Weakly basic
NaHCO₃ in water
Seawater
7.5–8.4
Slightly basic
Dissolved salts
Milk of magnesia
10–11
Basic
Mg(OH)₂ antacid
NaOH solution
12–14
Strongly basic
Caustic soda
2.2
Indicators & Neutralisation
Indicators show whether a solution is acidic or basic by changing colour
Strong acid + strong base → neutral salt (pH = 7): NaCl, K₂SO₄
Strong acid + weak base → acidic salt (pH < 7): NH₄Cl, CuSO₄
Weak acid + strong base → basic salt (pH > 7): CH₃COONa, Na₂CO₃
Weak acid + weak base → depends on Ka/Kb: CH₃COONH₄
NDA asks: "NaCl solution is acidic/basic/neutral?" → neutral
📌 NDA Trap — Phenolphthalein is colourless in acid, pink in base: Students sometimes state "phenolphthalein is pink in acid" — this is wrong. It turns pink/magenta only when pH > 8.2 (basic). In acid and neutral solution it is completely colourless. Litmus is red in acid, blue in base — both colours are distinct and easy to remember. Methyl orange is red in acid, yellow in base — used for strong acid–weak base titrations.
📝 TOPIC-WISE PYQ
pH Scale & Indicators — NDA Pattern Questions
Q1. The pH of a solution is 3. What is the concentration of H⁺ ions?
(a) 10⁻³ M (b) 3 M (c) 10³ M (d) 0.3 M
Answer: (a) 10⁻³ M
pH = −log[H⁺] → 3 = −log[H⁺] → [H⁺] = 10⁻³ M = 0.001 M.
This means 1 mL of this solution contains 10⁻⁶ moles of H⁺. pH 3 is 10,000 times more acidic than neutral water (pH 7), since each pH unit represents a 10-fold change.
Q2. The colour of phenolphthalein indicator in a sodium hydroxide solution is:
(a) Red (b) Yellow (c) Colourless (d) Pink
Answer: (d) Pink
NaOH is a strong base (pH ~13). Phenolphthalein changes from colourless to pink/magenta in alkaline solutions (pH > 8.2). In acidic or neutral solutions, phenolphthalein remains colourless. This is commonly tested: "In HCl — colourless; in NaOH — pink."
Q3. A solution has pOH = 4 at 25°C. What is its pH?
(a) 4 (b) 10 (c) 7 (d) 14
Answer: (b) 10
At 25°C: pH + pOH = 14 → pH = 14 − 4 = 10. pH 10 is basic (pH > 7). The solution has [OH⁻] = 10⁻⁴ M, making it an alkaline solution. This relationship (pH + pOH = 14) is valid only at 25°C and is one of the most directly tested formula-recall questions in NDA Chemistry.
🧠 TRICKY QUESTIONS
Acid-Base Theory & pH — Conceptual Traps
Q. If the pH of a solution changes from 5 to 3, by how many times does the acidity increase?
Answer: 100 times (10²)
Each unit decrease in pH = 10× increase in [H⁺].
pH 5 → [H⁺] = 10⁻⁵ M; pH 3 → [H⁺] = 10⁻³ M
Ratio = 10⁻³/10⁻⁵ = 10² = 100 times more acidic.
The pH scale is logarithmic, not linear. A change of 2 units = 100-fold change in [H⁺]. This is heavily tested — students who don't know the log relationship answer "2 times."
Q. NH₄Cl is the salt of a strong acid (HCl) and a weak base (NH₃). Is its aqueous solution acidic, basic, or neutral?
Answer: Acidic (pH < 7)
In solution: NH₄Cl → NH₄⁺ + Cl⁻. Cl⁻ is the conjugate base of strong acid HCl — does not hydrolyse. NH₄⁺ is the conjugate acid of weak base NH₃ — does hydrolyse: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ → releases H⁺ → acidic solution.
Rule: Salt of strong acid + weak base → acidic. Examples: NH₄Cl, CuSO₄, FeCl₃ all give acidic solutions. Na₂CO₃ (weak acid + strong base) → basic. NaCl (strong acid + strong base) → neutral.
3. Important Chemical Compounds
📌 NDA Strategy for Important Compounds: Each compound is tested on (a) its correct formula/name, (b) one key preparation reaction, and (c) 2–3 uses. The table below and compound cards are structured exactly for this — memorise formula → preparation → uses for each compound. Questions from this section are the most predictable in NDA Chemistry.
3.0
Master Reference Table — All 6 Compounds
Formula, common name, IUPAC name, and key use at a glance
Common Name
Formula
IUPAC / Chemical Name
Key Use
Baking soda
NaHCO₃
Sodium hydrogen carbonate
Baking, antacid, fire extinguisher
Washing soda
Na₂CO₃·10H₂O
Sodium carbonate decahydrate
Laundry cleaning, water softening, glass manufacture
Answer: (b) CaSO₄·2H₂O (Gypsum)
Gypsum is heated at 120–180°C: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. PoP is the hemihydrate. Over-heating gives "dead burnt" CaSO₄ which does not set with water and is useless for casts. CaCO₃ (limestone) heated gives CaO (quicklime) — a different compound.
Q2. Bleaching powder is prepared by passing chlorine gas over:
Answer: (b) Slaked lime Ca(OH)₂
Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O. Slaked lime (not quicklime) is used — the reaction requires Ca(OH)₂. Bleaching powder is a mixed salt of calcium hypochlorite [Ca(OCl)₂] and calcium chloride (CaCl₂), often simplified as CaOCl₂. It releases nascent oxygen [O] in the presence of moisture/CO₂ — that [O] bleaches.
Q3. Baking soda is used as an antacid because it:
(a) Releases CO₂ which dilutes stomach acid (b) Reacts with stomach acid (HCl) and neutralises it (c) Absorbs water in the stomach (d) Is a strong acid itself
Answer: (b) Reacts with stomach acid and neutralises it
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑. The bicarbonate ion (HCO₃⁻) acts as a base, accepting H⁺ from excess gastric HCl. The CO₂ produced causes belching. It's mildly alkaline (pH ~8.3) so it doesn't over-neutralise and cause alkalosis — making it safer than NaOH. Taken in moderation; prolonged use can disturb acid-base balance.
Q4. Potassium permanganate solution is purple. After reacting with an acidic oxalic acid solution, the colour changes to:
(a) Green (b) Brown (c) Colourless (d) Orange
Answer: (c) Colourless
In acidic medium, MnO₄⁻ (purple, Mn = +7) is reduced to Mn²⁺ (colourless, pale pink). The reaction: 2KMnO₄ + 5H₂C₂O₄ + 3H₂SO₄ → 2MnSO₄ + K₂SO₄ + 10CO₂ + 8H₂O. This is the basis of permanganometry titration — the endpoint is marked by the first persistent pink/purple colour. In basic medium, KMnO₄ gives MnO₂ (brown precipitate) instead.
🧠 TRICKY QUESTIONS
Important Compounds — Most Commonly Confused
Q. What is the difference between baking soda (NaHCO₃) and washing soda (Na₂CO₃·10H₂O)? A student says both are sodium salts and do the same job.
Answer: They are very different in basicity, formula, and applications. Baking soda (NaHCO₃): mildly basic (pH ~8.3); used in baking (CO₂ leavening), antacid, fire extinguisher; edible. Washing soda (Na₂CO₃·10H₂O): strongly basic (pH ~11); not edible; used for laundry, water softening, glass. Na₂CO₃ is the fully deprotonated carbonate — much stronger base. Heating baking soda converts it to washing soda: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. You can distinguish them by: baking soda effervesces with acid (CO₂); washing soda gives the same reaction but is more corrosive/alkaline and is NOT used as an antacid.
Q. Gypsum (CaSO₄·2H₂O) and Plaster of Paris (CaSO₄·½H₂O) — what is the relationship? Why does PoP harden when mixed with water?
Answer: PoP is derived from gypsum by partial dehydration; it hardens by reverting to gypsum.
Gypsum →(heat 120°C) Plaster of Paris + water (partial loss of water of crystallisation)
Plaster of Paris + water →(room temp) Gypsum (setting reaction — hardens)
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (hard solid)
The setting is a hydration reaction — PoP absorbs water and recrystallises as gypsum, which is harder. The slight expansion during setting allows PoP to fill moulds perfectly. NDA trap: "PoP is used for casts but gypsum is not" — correct. Over-heated gypsum (dead burnt CaSO₄) cannot set at all as it has no affinity for water.
Q. What is the oxidation state of Mn in KMnO₄, and why is it such a powerful oxidising agent?
Answer: Mn is in +7 oxidation state — the highest possible for Mn.
K⁺ (+1) + Mn + 4O²⁻(−8) = 0 → Mn = +7.
Because Mn is at its maximum oxidation state (+7), it strongly tends to gain electrons and be reduced (to Mn²⁺ in acid, or Mn⁴⁺/MnO₂ in neutral/base). This electron affinity = oxidising power. In acid: MnO₄⁻ gains 5e⁻ per ion (from +7 to +2), making it a very powerful 5-electron oxidant per mole. This is why permanganometry can titrate reducing agents like Fe²⁺, C₂O₄²⁻, H₂O₂ accurately.
📄 CN05 Formula & Fact Sheet — Quick Reference
⚛ Acid-Base Theories
Arrhenius: acid → H⁺; base → OH⁻ (in water only)
Brønsted-Lowry: acid = H⁺ donor; base = H⁺ acceptor
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