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Class 10 Science Chapter 2 Acids, Bases and Salts — Formulas & Key Points

Chapter 2 of NCERT Class 10 Science introduces acids, bases and salts through both qualitative tests and quantitative pH measurements. Mastery requires memorising indicator colour changes, recognising neutralisation patterns and writing balanced chemical equations for salt preparation. This formula sheet organises definitions, reaction templates and pH mathematics into exam-ready tables, ensuring every mark in the 2025 CBSE board paper is within reach.

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Key takeaways

✓Acids produce H⁺ (or H₃O⁺) in aqueous solution; bases produce OH⁻ ions.
✓pH = -log₁₀[H⁺]; values below 7 indicate acidic, above 7 alkaline, exactly 7 neutral.
✓Neutralisation: Acid + Base → Salt + Water is the cornerstone reaction for salt preparation.
✓Salts can be acidic, basic or neutral depending on parent acid and base strength.
✓Common laboratory indicators include litmus, methyl orange and phenolphthalein with distinct colour changes.
✓Water of crystallisation in salts like CuSO₄·5H₂O and washing soda Na₂CO₃·10H₂O must be counted in molar mass.
✓Chlor-alkali process and Solvay process are industrial methods named in NCERT for NaOH and Na₂CO₃ respectively.

Definitions and Fundamental Concepts

CBSE examiners award full marks only when definitions use NCERT language verbatim. Arrhenius definitions remain the baseline for Class 10, though NCERT briefly mentions Brønsted-Lowry. An acid releases hydrogen ions in water; a base releases hydroxide ions. The term 'alkali' is reserved for water-soluble bases. Strong acids and bases ionise completely in aqueous solution, while weak ones establish equilibrium. Students often lose marks by writing 'acid gives H⁺' without specifying 'in aqueous solution'. Understanding these distinctions prevents conceptual errors in pH and salt hydrolysis questions that appear regularly in Section-B of the board paper.

Acid (Arrhenius): Substance that releases H⁺ (or H₃O⁺) ions in aqueous solution.
Base (Arrhenius): Substance that releases OH⁻ ions in aqueous solution.
Alkali: A water-soluble base (e.g. NaOH, KOH).
Salt: Ionic compound formed by neutralisation, replacing H⁺ of acid with metal or ammonium ion.
Indicator: Substance showing different colours in acidic and basic media.
Neutralisation: Reaction between acid and base producing salt and water.

Core Chemical Reactions — Acids

NCERT lists five key reaction types for acids. Every board paper carries at least one 3-mark 'write balanced equation and identify type' question. Metal reactions yield hydrogen gas (pop test with burning splint). Metal carbonate and bicarbonate reactions release CO₂ (lime-water turns milky). Metal oxide and hydroxide reactions are neutralisations forming salt and water. Students must write state symbols (s, l, g, aq) for full credit in the CBSE marking scheme. Memorise the templates below, then substitute any acid (HCl, H₂SO₄, HNO₃) and corresponding salt name. Dil. H₂SO₄ is preferred with metals because conc. H₂SO₄ acts as oxidiser not typical acid.

Acid + Metal → Salt + Hydrogen gas; e.g. Zn + H₂SO₄ → ZnSO₄ + H₂↑
Acid + Metal carbonate → Salt + Water + Carbon dioxide; e.g. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
Acid + Metal hydrogen carbonate → Salt + Water + CO₂; e.g. NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
Acid + Metal oxide → Salt + Water; e.g. CuO + 2HCl → CuCl₂ + H₂O
Acid + Base (Metal hydroxide) → Salt + Water; e.g. NaOH + HCl → NaCl + H₂O

Core Chemical Reactions — Bases

Bases react with metals, non-metal oxides and acids. The base-plus-metal reaction is less common but NCERT highlights sodium and zinc reacting with NaOH to produce hydrogen. Non-metal oxides like CO₂ and SO₂ are acidic, so they neutralise bases without producing hydrogen. The neutralisation template is identical to the acid side but reversed. CBSE questions often ask students to identify the basic nature of metal oxides or to explain why plaster-of-Paris sets by reacting with water. Make sure to distinguish between metal oxides (basic) and non-metal oxides (acidic) when predicting reaction products and pH of resulting solutions.

Base + Metal → Salt + Hydrogen; e.g. 2NaOH + Zn → Na₂ZnO₂ + H₂↑ (sodium zincate)
Base + Non-metal oxide → Salt + Water; e.g. 2NaOH + CO₂ → Na₂CO₃ + H₂O
Base + Acid → Salt + Water (neutralisation); e.g. Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O
Metal oxide + Water → Metal hydroxide; e.g. CaO + H₂O → Ca(OH)₂ (slaking of lime)

pH Scale and Calculations

The pH scale runs from 0 to 14, with 7 being neutral at 25°C. pH is defined as the negative logarithm (base 10) of hydrogen ion concentration in mol/L. A one-unit pH change represents a tenfold change in [H⁺]. NCERT does not require log calculations beyond conceptual understanding for Class 10, but students should know pH < 7 is acidic, pH > 7 is basic. The 2024 CBSE board paper asked students to arrange solutions by increasing pH given their nature. Universal indicator shows red-orange (pH 1-4), yellow-green (pH 5-7), blue-violet (pH 8-14). Memorise that human blood pH is about 7.4, gastric juice around 1.5-2.0, pure water exactly 7.0 at 25°C.

pH = -log₁₀[H⁺] where [H⁺] is in mol/L
pH < 7: acidic; pH = 7: neutral; pH > 7: basic (alkaline)
[H⁺] × [OH⁻] = 10⁻¹⁴ at 25°C (ionic product of water)
pOH = -log₁₀[OH⁻] and pH + pOH = 14 at 25°C
Strong acid (HCl, H₂SO₄, HNO₃): pH 0-3; weak acid (CH₃COOH): pH 4-6
Strong base (NaOH, KOH): pH 12-14; weak base (NH₄OH): pH 8-11

Indicators and Colour Changes

NCERT specifies litmus (red/blue), methyl orange and phenolphthalein as the three main indicators. Turmeric, red cabbage and china-rose are natural indicators mentioned for activity demonstrations. CBSE board practical exams test litmus and phenolphthalein. Students must memorise exact colour changes because one-mark MCQs hinge on them. Litmus is neutral purple, turning red in acid and blue in base. Methyl orange is red below pH 3.1 and yellow above pH 4.4. Phenolphthalein is colourless in acid and pink in base. Olfactory indicators like onion and vanilla change smell, not colour. Universal indicator provides a pH range via a colour spectrum from red through green to violet.

Litmus: Red in acid, Blue in base, Purple in neutral
Methyl orange: Red (pH < 3.1), Orange (pH 3.1-4.4), Yellow (pH > 4.4)
Phenolphthalein: Colourless in acid/neutral (pH < 8.2), Pink in base (pH > 8.2)
Turmeric: Yellow in neutral/acid, Red-brown in base (contains curcumin)
Universal indicator: Red (pH 1-2), Orange (3-4), Yellow (5-6), Green (7), Blue (8-11), Violet (12-14)

Salts — Types, Hydrolysis and Preparation

Salts are classified by the strength of their parent acid and base. A salt from strong acid and strong base (NaCl from HCl + NaOH) is neutral pH 7. Strong acid + weak base (NH₄Cl from HCl + NH₄OH) gives acidic salt pH < 7. Weak acid + strong base (CH₃COONa from CH₃COOH + NaOH) gives basic salt pH > 7. Weak acid + weak base salts depend on relative strengths. NCERT lists four preparation methods: neutralisation, metal + acid, metal oxide/carbonate + acid, and double displacement. Water of crystallisation is fixed; heating removes it. Gypsum CaSO₄·2H₂O becomes plaster-of-Paris (CaSO₄·½H₂O) on heating at 373 K; rehydration hardens the plaster. The 2025 board carries 3-4 marks on salt preparation and properties.

Neutral salt: Strong acid + Strong base; pH = 7; e.g. NaCl, K₂SO₄
Acidic salt: Strong acid + Weak base; pH < 7; e.g. NH₄Cl, (NH₄)₂SO₄
Basic salt: Weak acid + Strong base; pH > 7; e.g. CH₃COONa, Na₂CO₃
Preparation by neutralisation: HCl + NaOH → NaCl + H₂O (evaporate water to crystallise)
Preparation from metal: Zn + H₂SO₄ → ZnSO₄ + H₂
Preparation from carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Water of crystallisation: Fixed H₂O molecules in crystal lattice, e.g. CuSO₄·5H₂O (blue vitriol)

Important Compounds and Their Uses

NCERT Chapter 2 highlights sodium hydroxide (caustic soda), sodium carbonate (washing soda), sodium hydrogen carbonate (baking soda), calcium oxide (quick lime), calcium hydroxide (slaked lime), plaster-of-Paris and bleaching powder. The chlor-alkali process electrolyses brine to produce NaOH, Cl₂ and H₂ at cathode and anode. The Solvay process manufactures Na₂CO₃ from NaCl and limestone. Baking soda NaHCO₃ decomposes on heating to release CO₂, used in fire extinguishers and baking. Washing soda Na₂CO₃·10H₂O softens hard water by precipitating Ca²⁺ and Mg²⁺. Bleaching powder CaOCl₂ releases Cl₂ for disinfection. Plaster-of-Paris sets within 10-15 minutes when mixed with water. CBSE board long-answer questions (5 marks) often ask for preparation, properties and uses of any two compounds; candidates must write chemical equations and at least three distinct uses.

NaOH (Sodium hydroxide): Made by chlor-alkali process; used in soap, paper, textiles; highly corrosive
Na₂CO₃·10H₂O (Washing soda): Prepared from Solvay process; removes permanent hardness of water; pH ~11
NaHCO₃ (Baking soda): Mild base pH ~8.5; used in baking (releases CO₂), antacid, fire extinguisher
CaO (Quick lime): Reacts vigorously with water releasing heat; used in cement, whitewashing
Ca(OH)₂ (Slaked lime): Used in whitewash, neutralising acidic soil, making bleaching powder
CaSO₄·½H₂O (Plaster-of-Paris): Sets hard in 10-15 min; used in casts, moulds, wall plaster
CaOCl₂ (Bleaching powder): Made by Cl₂ + dry Ca(OH)₂; disinfects water, bleaches cotton

Common Notation and Unit Mistakes

CBSE marking schemes penalise missing state symbols, incorrect ion charges and wrong formulae. Always write aqueous (aq) for solutions, solid (s) for precipitates, gas (g) for evolved gases and liquid (l) for water. The hydrogen ion in water is H₃O⁺ (hydronium) not free H⁺, though NCERT accepts both. pH has no unit because it is a logarithmic scale; writing 'pH = 7 units' costs marks. Salt formulae must balance charges: calcium chloride is CaCl₂ not CaCl. Students often write HCO₃ instead of HCO₃⁻ or omit the dot in hydrated salts like CuSO₄·5H₂O. In stoichiometry, molar mass must include water of crystallisation. Double-check oxidation states when writing salt formulae from metal and acid radical combinations to avoid losing easy marks in Section-A objective questions.

State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous — compulsory in equations
pH is dimensionless; never write pH = 5 mol/L
Hydronium ion H₃O⁺ is preferred over H⁺ in solution, though both accepted in Class 10
Charge balance: CaCl₂, Al₂(SO₄)₃, NH₄Cl — check oxidation numbers
Water of crystallisation: write the dot, e.g. Na₂CO₃·10H₂O not Na₂CO₃ 10H₂O
Ion formulae: HCO₃⁻, SO₄²⁻, PO₄³⁻ — include charge superscript

Memory Tricks and Mnemonics

Mnemonics save precious revision time the night before the board exam. Remember indicator colours with 'Little Red Base Blue' for litmus. For methyl orange think 'MO Reds in Acid'. Phenolphthalein: 'Phenol Pinks in Base'. The five uses of baking soda spell BAFFS: Baking, Antacid, Fire extinguisher, Food preservative, Soda-acid fire extinguisher. For strong acids remember HNO₃ (Nitric), H₂SO₄ (Sulphuric), HCl (Hydrochloric) as 'N S C' (No Stomach Can handle them at full strength). Water of crystallisation: 'FIVE Blue Copper' for CuSO₄·5H₂O and 'TEN Washing' for Na₂CO₃·10H₂O. Chlor-alkali gives three products: NaOH, Cl₂, H₂ — remember 'N C H' in alphabetical order at cathode (N, H) and anode (C for Cl). CBSETUTOR.ai offers over 500 chapter-specific flashcards and quizzes with instant photo-upload doubt solving at just ₹999 per month flat for all classes, helping students cement these mnemonics through daily practice and personalised AI feedback on mock tests aligned with the 2025 CBSE pattern.

Litmus: 'Little Red (acid) Base Blue'
Methyl Orange: 'MO Reds in Acid, Yellows in Base'
Phenolphthalein: 'Phenol Pinks in Base, Colourless in Acid'
Strong acids: HNO₃, H₂SO₄, HCl — 'N S C No Stomach Can'
Baking soda uses: BAFFS (Baking, Antacid, Fire, Food, Soda-acid)
Water of crystallisation: FIVE Blue Copper, TEN Washing soda
Chlor-alkali products: Cathode (NaOH, H₂), Anode (Cl₂)

Three Solved Mini-Examples

Worked examples consolidate formula application and reveal common mistakes. Example 1 practices stoichiometry with water of crystallisation. Example 2 applies the salt classification rule. Example 3 combines pH concept with neutralisation. CBSE awards full marks only when students show complete working, balanced equations and correct units where applicable. Writing just the final answer forfeits method marks. Board examiners also check for logical flow: state the formula or principle first, substitute values, simplify step-by-step and box the final answer. These three examples mirror the 2-3 mark numericals and reasoning questions seen in CBSE papers from 2022 to 2024, covering the most frequently tested sub-topics of Chapter 2.

Last-Minute Revision Box

Use this one-glance checklist to verify every formula and key fact within five minutes before entering the exam hall. Tick off each item mentally or on scrap paper. Ensure you can write the definition of acid, base and salt in one sentence each. Recall the five acid reactions and three base reactions by heart. Know the exact pH values: neutral 7, gastric juice 2, blood 7.4. State the colour of litmus, methyl orange and phenolphthalein in acid and base. List at least three uses each for NaOH, Na₂CO₃, NaHCO₃ and plaster-of-Paris. Write the formulae with water of crystallisation for washing soda and blue vitriol. Remember that bleaching powder is CaOCl₂ and releases Cl₂. Finally, revise the chlor-alkali and Solvay process in two lines. This box is designed for the frantic final hour when panic sets in and structured notes feel overwhelming; treat it as your emergency lifeline to secure Section-A MCQs and ensure no silly mistake costs a rank in the CBSE merit list.

Definitions: Acid (H⁺ in aq), Base (OH⁻ in aq), Salt (ionic product of neutralisation)
Five acid reactions: +Metal → Salt+H₂; +Carbonate → Salt+H₂O+CO₂; +Base → Salt+H₂O; +Metal oxide → Salt+H₂O; +Bicarbonate → Salt+H₂O+CO₂
Three base reactions: +Acid → Salt+H₂O; +Non-metal oxide → Salt+H₂O; +Metal → Salt+H₂
pH: <7 acid, =7 neutral, >7 base; pH+pOH=14; [H⁺][OH⁻]=10⁻¹⁴
Indicators: Litmus R/B, MO Red<3.1 Yellow>4.4, Phenol Colourless<8.2 Pink>8.2
Salts: Neutral (NaCl), Acidic (NH₄Cl pH<7), Basic (Na₂CO₃ pH>7)
Water of crystallisation: CuSO₄·5H₂O (blue, 250 g/mol), Na₂CO₃·10H₂O (286 g/mol), CaSO₄·2H₂O (gypsum), CaSO₄·½H₂O (plaster)
Chlor-alkali: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂; Solvay: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ → Na₂CO₃
Common uses: NaOH (soap, paper); Na₂CO₃ (glass, water softening); NaHCO₃ (baking, antacid, fire); Plaster (casts, moulds); Bleaching powder (disinfect, bleach)

Frequently asked questions

What is the difference between an acid and a base according to NCERT Class 10 Science Chapter 2?+

Acids release hydrogen ions (H⁺ or H₃O⁺) when dissolved in water, while bases release hydroxide ions (OH⁻). Acids turn blue litmus red and have pH less than 7; bases turn red litmus blue and have pH greater than 7. NCERT emphasises Arrhenius definitions for Class 10.

How is pH calculated and what does a pH of 3 mean?+

pH equals the negative logarithm of hydrogen ion concentration: pH = -log₁₀[H⁺]. A pH of 3 means [H⁺] = 10⁻³ mol/L, indicating a strongly acidic solution. Each unit decrease in pH represents a tenfold increase in acidity. pH has no units because it is a logarithmic scale.

Why does phenolphthalein turn pink in sodium hydroxide solution but remain colourless in hydrochloric acid?+

Phenolphthalein is a pH-sensitive indicator that is colourless in acidic and neutral solutions (pH < 8.2) and turns pink in basic solutions (pH > 8.2). Sodium hydroxide is a strong base with pH around 13-14, causing the colour change. This property is used in acid-base titrations to detect the endpoint.

What are the five important chemical reactions of acids listed in NCERT?+

Acids react with (1) metals to produce salt and hydrogen gas, (2) metal carbonates to produce salt, water and carbon dioxide, (3) metal hydrogen carbonates to produce salt, water and CO₂, (4) metal oxides to produce salt and water (neutralisation), and (5) bases to produce salt and water (neutralisation).

How do you prepare sodium chloride salt in the laboratory using neutralisation?+

Mix dilute hydrochloric acid with sodium hydroxide solution in a beaker until the mixture becomes neutral (test with litmus). The reaction is HCl + NaOH → NaCl + H₂O. Heat the solution gently to evaporate water, then allow it to cool for crystallisation of sodium chloride. Filter and dry the crystals.

What is water of crystallisation and which salts contain it?+

Water of crystallisation is a fixed number of water molecules chemically bound in the crystal structure of certain salts. Examples include copper sulphate pentahydrate (CuSO₄·5H₂O, blue crystals), washing soda (Na₂CO₃·10H₂O) and gypsum (CaSO₄·2H₂O). Heating removes this water, often changing the salt's colour and molar mass.

Why is ammonium chloride solution acidic even though it is a salt?+

Ammonium chloride is formed from a strong acid (HCl) and a weak base (NH₄OH). In water, NH₄⁺ ions undergo hydrolysis releasing H⁺ ions: NH₄⁺ + H₂O ⇌ NH₄OH + H⁺. The excess H⁺ makes the solution acidic with pH less than 7. Salts from strong acid and weak base always produce acidic solutions.

What is the chlor-alkali process and why is it important?+

The chlor-alkali process electrolyses brine (concentrated NaCl solution) to produce sodium hydroxide at the cathode, chlorine gas at the anode and hydrogen gas at the cathode. Equation: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. It is the industrial method for manufacturing caustic soda, widely used in soap, paper and textiles.

How does baking soda work in a fire extinguisher?+

Baking soda (sodium hydrogen carbonate, NaHCO₃) decomposes on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The carbon dioxide released does not support combustion and forms a blanket over the fire, cutting off oxygen supply. This principle is used in soda-acid fire extinguishers where acid reacts with baking soda to generate CO₂ instantly.

What are the main uses of washing soda and how is it prepared?+

Washing soda (Na₂CO₃·10H₂O) is used to remove permanent hardness of water by precipitating calcium and magnesium ions, in glass and detergent manufacture, and to neutralise acidic effluents. It is prepared industrially by the Solvay process from sodium chloride and limestone, then recrystallised with ten molecules of water to form the decahydrate.

Why does copper sulphate crystal turn white on heating and blue again when water is added?+

Blue copper sulphate crystals are CuSO₄·5H₂O. On heating above 373 K, water of crystallisation is lost: CuSO₄·5H₂O → CuSO₄ + 5H₂O, leaving anhydrous white CuSO₄. Adding water reverses the process, regenerating the blue pentahydrate. This colour change is used to test for the presence of water.

How can CBSETUTOR.ai help with mastering CBSE Class 10 Science Chapter 2 Acids, Bases and Salts?+

CBSETUTOR.ai provides 24×7 AI-powered tutoring with instant photo-upload doubt solving, chapter-wise formula flashcards and mock tests aligned with the latest CBSE pattern. Students get step-by-step solutions for every NCERT exercise question and personalised feedback on weak areas. At a flat ₹999 per month for all subjects and classes 6-12, families save on expensive coaching. A 3-day free trial lets students experience one-on-one AI guidance before committing.

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