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CC05 — Acids, Bases & Salts

📖 CC05  ·  CDS General Science — Chemistry ★ High Priority

Acids, bases and salts are encountered in everyday life — from stomach acid to soaps and baking soda. CDS tests pH values, indicator colours, definitions of acids and bases by three theories, and the properties and uses of common salts.

📌 CDS Focus: Arrhenius definition (most basic); pH scale (7 = neutral, <7 = acid, >7 = base); litmus paper (red = acid, blue = base); indicators (phenolphthalein colourless in acid, pink in base); neutralisation reactions; baking soda (NaHCO₃) vs washing soda (Na₂CO₃·10H₂O); plaster of Paris.

1. Definitions of Acids and Bases

Fig. 1 — Three Theories of Acids and Bases: Arrhenius, Brønsted-Lowry, Lewis
THREE THEORIES — Acids and Bases Theory Acid is... Base is... Example ARRHENIUS (1884, simplest) In aqueous solution Gives H⁺ ions in water Gives OH⁻ ions in water HCl→H⁺+Cl⁻ NaOH→Na⁺+OH⁻ Fails for NH₃ BRØNSTED -LOWRY (1923, broader) Proton donor (H⁺) Proton acceptor HCl + H₂O → H₃O⁺ + Cl⁻ NH₃ is a base here LEWIS (1923, broadest) No H needed Electron pair acceptor Electron pair donor BF₃ (Lewis acid) NH₃ (Lewis base) Broadest; no water needed

2. The pH Scale

Fig. 2 — pH Scale 0–14 with Common Substances and Indicator Colours
pH SCALE — Acidic (0–7) | Neutral (7) | Basic / Alkaline (7–14) ← ACIDIC NEUTRAL BASIC / ALKALINE → 0 1 2 3 4 7 9 11 13 14 Battery acid (0) Gastric acid (1–2) Lemon juice (3) Pure water pH = 7 Baking soda pH ≈ 8–9 Milk of magnesia NaOH pH = 14 INDICATORS — Colour in Acid, Neutral and Base Indicator In Acid (pH < 7) Neutral In Base (pH > 7) Litmus RED Purple BLUE Phenolphthalein Colourless Colourless PINK / RED Methyl orange RED Orange YELLOW

3. Important Salts & Their Uses

Salt (Common Name)Chemical Name / FormulaKey Use Common SaltSodium chloride (NaCl)Food preservation, seasoning; raw material for NaOH and HCl Baking SodaSodium hydrogen carbonate (NaHCO₃)Baking (releases CO₂); antacid; fire extinguishers Washing SodaSodium carbonate (Na₂CO₃·10H₂O)Cleaning agent; water softening; glass & soap making Bleaching PowderCalcium hypochlorite Ca(OCl)ClDisinfecting water; bleaching cloth and paper Plaster of ParisCalcium sulphate hemihydrate CaSO₄·½H₂OPlaster casts (fractures); building; sculptures AlumPotassium aluminium sulphate K₂SO₄·Al₂(SO₄)₃·24H₂OWater purification; dyeing; leather tanning Blue VitriolCopper sulphate (CuSO₄·5H₂O)Fungicide (Bordeaux mixture); electroplating

📝 CDS PYQs — Acids, Bases & Salts

Q1. The pH of pure water at 25°C is: CDS PYQ
(a) 0(b) 6(c) 7(d) 14
✔ Answer: (c) 7
Pure water has pH = 7 — it is neutral. pH < 7 = acidic; pH > 7 = basic/alkaline. pH = −log[H⁺]. In pure water at 25°C, [H⁺] = [OH⁻] = 10⁻⁷ mol/L. This is a foundational CDS chemistry fact — the pH scale ranges from 0 (most acidic) to 14 (most basic).
Q2. Which indicator turns pink/red in a basic solution? CDS PYQ
(a) Litmus(b) Methyl orange(c) Phenolphthalein(d) Universal indicator
✔ Answer: (c) Phenolphthalein
Phenolphthalein is colourless in acidic and neutral solutions but turns pink/red in basic solutions. Litmus turns red in acid and blue in base. Methyl orange turns red in acid, orange at neutral, and yellow in base. Phenolphthalein is commonly used to identify basic solutions and as an endpoint indicator in acid-base titrations.
Q3. Baking soda used in cooking is: CDS PYQ
(a) Na₂CO₃(b) NaHCO₃(c) NaCl(d) NaOH
✔ Answer: (b) NaHCO₃
Baking soda = sodium hydrogen carbonate (NaHCO₃). When heated or mixed with acid (like lemon juice or vinegar in recipes), it releases CO₂ gas which causes the dough to rise (leavening effect). NaHCO₃ is also used as an antacid to neutralise excess stomach acid. Na₂CO₃ is washing soda (different compound).
Q4. Neutralisation reaction always produces: CDS PYQ
(a) Acid + Base(b) Salt + Water(c) Gas + Precipitate(d) Only water
✔ Answer: (b) Salt + Water
Neutralisation: Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O. The acid's H⁺ and the base's OH⁻ combine to form water; remaining ions form a salt. The reaction is exothermic (releases heat). This is the definition that CDS tests directly — salt + water is always the product of neutralisation.

🧠 Quick Memory Chart — CC05

🔥 pH Scale
  • pH < 7: acidic
  • pH = 7: neutral (pure water)
  • pH > 7: basic / alkaline
  • Stomach acid ≈ pH 1–2
  • NaOH solution ≈ pH 14
📌 Indicators
  • Litmus: red in acid, blue in base
  • Phenolphthalein: colourless → pink (base)
  • Methyl orange: red (acid) → yellow (base)
  • Neutralisation: acid + base → salt + water
⚛ Common Salts
  • Baking soda: NaHCO₃ (CO₂ releaser)
  • Washing soda: Na₂CO₃·10H₂O
  • Plaster of Paris: CaSO₄·½H₂O
  • Bleaching powder: Ca(OCl)Cl
  • Alum: water purification

📝 Practice Exercise

E1. Which of the following is a strong acid?
(a) Acetic acid(b) Citric acid(c) Hydrochloric acid(d) Carbonic acid
E2. The formula of plaster of Paris is:
(a) CaSO₄·2H₂O(b) CaSO₄·½H₂O(c) CaCO₃(d) Ca(OH)₂
E3. Milk of magnesia (antacid) has pH:
(a) 3–4(b) 6–7(c) 7(d) 10–11
Answers: E1 → (c) Hydrochloric acid (HCl) [fully dissociates in water; acetic, citric, carbonic are weak acids]  |  E2 → (b) CaSO₄·½H₂O (hemihydrate; gypsum = CaSO₄·2H₂O)  |  E3 → (d) 10–11 [it is a base — Mg(OH)₂; used to neutralise excess stomach acid]
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