Acids, Bases and Salts — Quick Revision Pack

Chapter 2 | NCERT Class 10 Science | 10–15 min revision

2-Minute Recap

Acids → sour, turn blue litmus red; bases → bitter/soapy, turn red litmus blue.
Acids in water give $H^{+} (a q)$ / $H_{3} O^{+}$ ; bases give $OH^{-} (a q)$ ; alkalis = water-soluble bases.
Acid + metal → salt + $H_{2}$ ; acid + carbonate/HCO₃⁻ → salt + $CO_{2}$ + $H_{2} O$ .
Neutralisation: acid + base → salt + water.
Metal oxides are basic; non-metal oxides are acidic.
Dry HCl does not show acidic behaviour on dry litmus — water is needed for $H_{3} O^{+}$ .
Dilution rule: always add acid to water (never water to concentrated acid).
pH scale 0–14: $< 7$ acidic, $= 7$ neutral, $> 7$ basic; lower pH → more $H^{+}$ .
Tooth enamel damaged below pH 5.5; acid rain often pH $< 5.6$ .
Chlor-alkali (brine electrolysis): $NaOH$ , $Cl_{2}$ , $H_{2}$ .
Bleaching powder $CaOCl_{2}$ / $Ca (ClO)_{2}$ ; baking soda $NaHCO_{3}$ ; washing soda $Na_{2} CO_{3} \cdot 10 H_{2} O$ .
Water of crystallisation; PoP = $CaSO_{4} \cdot \frac{1}{2} H_{2} O$ from gypsum at $373 K$ .

Memory Tricks (Mnemonics)

Litmus LRB: Acids turn blue litmus Red; bases turn red litmus Blue.
Phenolphthalein: Pink in Base — colourless in acid / neutral.
Dilution safety chant: “Acid into Water — Always Wise” (AW / never WA).
pH story strip: Body fluids ~7.0–7.8; acid rain $< 5.6$ ; tooth danger $< 5.5$ ; stomach acid ~1.2; bee sting acid → treat with mild base.
Brine products: NCH — NaOH, Chlorine, Hydrogen (chlor-alkali).
Baking vs washing: Baking = bicarbonate $NaHCO_{3}$ ; Washing = carbonate hydrate $Na_{2} CO_{3} \cdot 10 H_{2} O$ .

One-Liner Definitions (Board Gold)

Term	One-liner definition
Acid	Substance that produces $H^{+} (a q)$ / $H_{3} O^{+}$ ions in aqueous solution
Base	Substance that produces $OH^{-} (a q)$ ions in aqueous solution
Alkali	A base that is soluble in water
Indicator	Substance that shows different colours (or odours) in acidic and basic media
Neutralisation	Reaction of an acid with a base to form salt and water
Strong acid / base	Acid/base that ionises (almost) completely in water
Weak acid / base	Acid/base that ionises only partially in water
pH	Measure of the concentration of hydrogen ions in a solution (scale 0–14)
Salt	Ionic compound formed by neutralisation of an acid by a base
Water of crystallisation	Fixed number of water molecules present in one formula unit of a salt
Hydrated salt	Salt that contains water of crystallisation
Anhydrous salt	Salt from which water of crystallisation has been removed
Chlor-alkali process	Electrolysis of brine producing NaOH, Cl₂ and H₂
Plaster of Paris	$CaSO_{4} \cdot \frac{1}{2} H_{2} O$ obtained by heating gypsum at $373 K$

Must-Write Equations / Formulae

Acid + metal: $Zn (s) + H_{2} SO_{4} (a q) \to ZnSO_{4} (a q) + H_{2} (g)$ — pop test for $H_{2}$ .
Base + Zn: $2 NaOH (a q) + Zn (s) \to Na_{2} ZnO_{2} (s) + H_{2} (g)$ — sodium zincate.
Carbonate + acid: $Na_{2} CO_{3} + 2 HCl \to 2 NaCl + H_{2} O + CO_{2}$ .
Hydrogencarbonate + acid: $NaHCO_{3} + HCl \to NaCl + H_{2} O + CO_{2}$ .
Lime water milky: $Ca (OH)_{2} + CO_{2} \to CaCO_{3} + H_{2} O$ ; excess $CO_{2}$ → $Ca (HCO_{3})_{2}$ (clears).
Neutralisation: $NaOH + HCl \to NaCl + H_{2} O$ .
Metal oxide + acid: $CuO + 2 HCl \to CuCl_{2} + H_{2} O$ .
Bleaching powder (lab idea): $Ca (OH)_{2} + Cl_{2} \to CaOCl_{2} + H_{2} O$ .
Baking soda heat: $2 NaHCO_{3} heat Na_{2} CO_{3} + H_{2} O + CO_{2}$ .
Washing soda from baking soda: $Na_{2} CO_{3} + 10 H_{2} O \to Na_{2} CO_{3} \cdot 10 H_{2} O$ .
Gypsum → PoP: $CaSO_{4} \cdot 2 H_{2} O 373 K CaSO_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$ .
PoP sets: $CaSO_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to CaSO_{4} \cdot 2 H_{2} O$ .
Hydrate colour change: $CuSO_{4} \cdot 5 H_{2} O$ (blue) $heat$ anhydrous $CuSO_{4}$ (white).

Flowchart / Decision Map

(in words) 1. **Identify acid or base?** Taste/indicators/pH; or ask:

H^{+}

or

OH^{+}

in water? 2. **Reaction type with given reactant:** - Metal present → check acid (or special base + Zn) → salt +

H_{2}

- Carbonate / HCO₃⁻ → salt +

CO_{2}

+

H_{2} O

→ test with lime water - Acid + base → salt + water (neutralisation) - Metal oxide + acid / non-metal oxide + base → salt + water 3. **pH question?** Locate on 0–14; compare with 7; lower pH = more acidic = more

H^{+}

. 4. **Dilution / lab safety?** Always acid → water; say “exothermic”. 5. **Salt from common salt?** Brine electrolysis → NaOH, Cl₂, H₂ → bleaching powder / HCl / soaps, etc. 6. **Hydrate / PoP?** Mention water of crystallisation; PoP from gypsum at 373 K; store airtight.

Board Keywords to Underline

hydronium ion $H_{3} O^{+}$ / hydrogen ion $H^{+} (a q)$
hydroxide ion $OH^{-} (a q)$
indicator / olfactory indicator
neutralisation / salt and water
dilute / concentrate; strong / weak
pH scale / acidic / basic / neutral
chlor-alkali process
bleaching powder / baking soda / washing soda
water of crystallisation / hydrated / anhydrous
Plaster of Paris / gypsum / $373 K$

Common Mistakes (Don't Lose Marks)

Calling all H-containing compounds acids (glucose, alcohol) — Fix: acids must give $H^{+} (a q)$ in water.
Saying dry HCl turns dry litmus red — Fix: needs moisture/water to form $H_{3} O^{+}$ .
Adding water to concentrated acid — Fix: add acid to water.
Confusing strong with concentrated — Fix: strong = ionisation extent; concentrated = amount per volume.
Thinking pH 6 has fewer H⁺ than pH 8 — Fix: lower pH → more $H^{+}$ .
Swapping baking soda and washing soda — Fix: baking = $NaHCO_{3}$ ; washing = $Na_{2} CO_{3} \cdot 10 H_{2} O$ .
Forgetting CO₂ test details — Fix: lime water milky; excess CO₂ clears milkiness.
Writing PoP formula wrong — Fix: $CaSO_{4} \cdot \frac{1}{2} H_{2} O$ (or $2 CaSO_{4} \cdot H_{2} O$ ).

5 Rapid Fire (Answers at end)

Why does dry HCl gas not change the colour of dry litmus paper?
State the correct way to dilute concentrated sulphuric acid.
What are the three products of the chlor-alkali process?
Write the formula of Plaster of Paris and the temperature used to prepare it from gypsum.
Tooth enamel starts corroding below which pH?

Rapid Fire Answers

No water → no $H_{3} O^{+}$ / $H^{+} (a q)$ ions formed, so no acidic behaviour on dry litmus.
Add acid slowly to water with constant stirring (never water into acid).
Sodium hydroxide (NaOH), chlorine ( $Cl_{2}$ ), and hydrogen ( $H_{2}$ ).
$CaSO_{4} \cdot \frac{1}{2} H_{2} O$ ; prepared by heating gypsum at $373 K$ .
Below pH 5.5.

Night-Before Checklist

Indicators: litmus, phenolphthalein, methyl orange, olfactory
Acid + metal and carbonate/HCO₃⁻ equations (with CO₂ lime-water test)
Neutralisation definition + one equation
Metal oxides basic; non-metal oxides acidic
Dry HCl needs water — board favourite reason
Dilution: acid into water
pH: $< 7$ , $= 7$ , $> 7$ ; tooth 5.5; acid rain 5.6
Chlor-alkali products and one use of each
Bleaching powder, baking soda, washing soda formulae & uses
$CuSO_{4} \cdot 5 H_{2} O$ blue → white on heating
PoP formula, preparation, setting equation, airtight storage