CA04 — Acids, Bases & Salts

✈ Chemistry – CA04 · AFCAT General Awareness AFCAT Level ★ High Priority

Acids, bases and salts are encountered in everyday life — from stomach acid to soaps. AFCAT tests pH values, indicator colours, and common salts directly. This chapter has a very high question frequency — purely factual, completely scoring.

📌 AFCAT Focus: pH = 7 (neutral), <7 (acid), >7 (base); litmus red in acid, blue in base; phenolphthalein colourless in acid, pink in base; baking soda = NaHCO₃ (not washing soda Na₂CO₃); neutralisation = acid + base → salt + water; plaster of Paris = CaSO₄·½H₂O.

1. pH Scale & Indicators

Fig. 1 — pH Scale 0–14 with Common Substances and Indicator Colour Changes

2. Important Salts & Their Uses

Common NameChemical Name / FormulaKey Use Baking SodaSodium hydrogen carbonate NaHCO₃Baking (releases CO₂); antacid; fire extinguisher Washing SodaSodium carbonate Na₂CO₃·10H₂OCleaning; water softening; glass & soap making Common SaltSodium chloride NaClFood preservation; raw material for NaOH and HCl Bleaching PowderCalcium hypochlorite Ca(OCl)ClDisinfecting water; bleaching cloth and paper Plaster of ParisCalcium sulphate hemihydrate CaSO₄·½H₂OFractured bone casts; building; sculptures AlumPotash alum K₂SO₄·Al₂(SO₄)₃·24H₂OWater purification; dyeing; leather tanning Blue VitriolCopper sulphate CuSO₄·5H₂OFungicide (Bordeaux mixture); electroplating

📝 AFCAT PYQs — Acids, Bases & Salts

Q1. The pH of pure water at 25°C is: AFCAT PYQ

(a) 0(b) 6(c) 7(d) 14

✔ Answer: (c) 7

Pure water is neutral — pH = 7. pH < 7 = acidic; pH > 7 = basic/alkaline. The pH scale runs 0 (most acidic) to 14 (most basic). This is the most fundamental chemistry fact on the AFCAT — students often confuse it with the number of carbon molecules or temperature. Just remember: pure water = neutral = pH 7.

Q2. Which indicator turns pink in a basic solution? AFCAT PYQ

(a) Litmus(b) Methyl orange(c) Phenolphthalein(d) Universal indicator

✔ Answer: (c) Phenolphthalein

Phenolphthalein is colourless in acid and neutral solutions, but turns pink/red in basic (alkaline) solutions. Litmus turns red in acid and blue in base. Methyl orange is red in acid and yellow in base. Phenolphthalein is the standard indicator for alkalinity. This is repeatedly asked directly in AFCAT.

Q3. Baking soda (used in cooking) is: AFCAT PYQ

(a) Na₂CO₃(b) NaHCO₃(c) NaCl(d) NaOH

✔ Answer: (b) NaHCO₃

Baking soda = NaHCO₃ (sodium hydrogen carbonate / sodium bicarbonate). When heated or mixed with an acid, it releases CO₂ gas which causes dough to rise. It is also an antacid that neutralises excess stomach acid. NaHCO₃ ≠ Na₂CO₃ (washing soda). This confusion between baking soda and washing soda is a direct AFCAT trap question.

Q4. Acid + Base → ? (Neutralisation reaction) AFCAT PYQ

(a) Salt only(b) Water only(c) Salt + Water(d) Acid + Water

✔ Answer: (c) 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 salt. The reaction is exothermic (releases heat). This is the definition directly tested in AFCAT — always "salt AND water", not just one of them.

🧠 Quick Memory Chart — CA04

🔥 pH Scale

pH < 7: acidic solution
pH = 7: neutral (pure water)
pH > 7: basic / alkaline
Stomach acid ≈ pH 1–2
NaOH solution ≈ pH 14

📌 Indicators

Litmus: red (acid), blue (base)
Phenolphthalein: colourless→PINK (base)
Methyl orange: red (acid)→yellow (base)
Neutralisation: acid+base→salt+water
Exothermic: heat is released

⚛ Key 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. Lemon juice has a pH of about 3. It is:

(a) Basic(b) Acidic(c) Neutral(d) Alkaline

E2. The formula of Plaster of Paris is:

(a) CaSO₄·2H₂O(b) CaSO₄(c) CaSO₄·½H₂O(d) CaCO₃

E3. Antacid tablets (milk of magnesia) have a pH of about 10. They are:

(a) Acidic (pH 10 is acidic)(b) Basic (pH 10 > 7)(c) Neutral(d) Very strongly acidic

Answers:
E1 → (b) Acidic [pH 3 < 7] | E2 → (c) CaSO₄·½H₂O [hemihydrate; gypsum = CaSO₄·2H₂O; different compound] | E3 → (b) Basic [pH 10 > 7; it is basic/alkaline; neutralises excess stomach acid (HCl) which is acidic]

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