Acids, Bases and Salts – Class 10 Science Chapter 2 Notes & NCERT PDF

Class 10ScienceChapter 2NCERT book: Science - Class 10

Notes and a simple summary of Chapter 2 of the NCERT Class 10 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 2: Acids, Bases and Salts

Introduction

Sour taste means acid; bitter taste and soapy touch mean base. Earlier you used litmus, turmeric and china rose to test them. In this chapter you study how acids and bases react, why they behave as they do in water, how their strength is measured on the pH scale, and the uses of important salts made from common salt.

Key Concepts

1. Indicators

Litmus (from lichen): acids turn blue litmus red; bases turn red litmus blue.

Synthetic indicators: methyl orange and phenolphthalein (phenolphthalein turns pink in bases and stays colourless in acids).

Olfactory indicators change smell in acid or base: onion, vanilla essence, clove oil (their smell is not detected in basic solution).

2. Reactions of acids and bases

Acid + metal → salt + hydrogen gas: Zn + H₂SO₄ → ZnSO₄ + H₂. The gas burns with a pop sound.

Base + metal: zinc with NaOH gives sodium zincate and hydrogen: 2NaOH + Zn → Na₂ZnO₂ + H₂ (not all metals react this way).

Acid + metal carbonate/hydrogencarbonate → salt + CO₂ + water: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The CO₂ turns lime water milky (CaCO₃ forms); excess CO₂ makes it clear again as soluble Ca(HCO₃)₂. Limestone, chalk and marble are forms of CaCO₃.

Acid + base → salt + water (neutralisation): NaOH + HCl → NaCl + H₂O. With phenolphthalein, pink NaOH turns colourless when acid is added.

Metal oxides are basic: CuO + 2HCl → CuCl₂ + H₂O (the solution turns blue-green).

Non-metal oxides are acidic: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

3. What do acids and bases have in common?

All acids give H⁺ ions in water — shown by glucose/alcohol solutions not conducting electricity while HCl, H₂SO₄ solutions do. H⁺ cannot exist alone; it combines with water to form the hydronium ion (H₃O⁺).

Dry HCl gas does not change dry blue litmus; acidic behaviour appears only in water. (Moist litmus is used; the guard tube with calcium chloride keeps the gas dry.)

Bases that dissolve in water are alkalis (NaOH, KOH, Mg(OH)₂, NH₄OH); they give OH⁻ ions.

Dissolving acid or base in water is highly exothermic: always add acid to water slowly with stirring, never water to acid, or the mixture may splash and the container may break. Mixing lowers ion concentration — dilution.

4. Strength — the pH scale

pH (p = potenz, German for power) runs from 0 (very acidic) to 14 (very alkaline); 7 is neutral. More H⁺ → lower pH.

Universal indicator shows different colours at different pH.

Strong acids give many H⁺ ions (HCl); weak acids give fewer (acetic acid).

5. Importance of pH in everyday life

Our body works within pH 7.0 to 7.8.

Acid rain: rain with pH below 5.6; it harms aquatic life in rivers.

Soil: plants need a particular pH range.

Digestion: the stomach makes hydrochloric acid; excess causes indigestion, treated with antacids like milk of magnesia (Mg(OH)₂).

Tooth decay starts when mouth pH falls below 5.5; tooth enamel (calcium hydroxyapatite) corrodes. Basic toothpastes neutralise the acid.

Self-defence: a bee sting leaves an acid — rub baking soda. Nettle leaves inject methanoic acid; rubbing the dock plant leaf gives relief. Natural acids: vinegar (acetic), orange (citric), tamarind (tartaric), tomato (oxalic), sour milk (lactic), ant and nettle sting (methanoic).

6. Salts

Salts with the same positive or negative radical form families (e.g. sodium salts, chloride salts).

pH of salts: strong acid + strong base → neutral salt (NaCl, pH 7); strong acid + weak base → acidic salt (NH₄Cl); weak acid + strong base → basic salt (Na₂CO₃, CH₃COONa).

Common salt (NaCl): from sea water and rock salt deposits; raw material for many chemicals.

Chlor-alkali process: electricity through brine gives NaOH (at the cathode), Cl₂ (anode) and H₂ (cathode): 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.

Bleaching powder (CaOCl₂): chlorine + dry slaked lime. Uses: bleaching cotton, linen and wood pulp; bleaching clothes in laundry; oxidising agent; disinfecting drinking water.

Baking soda (NaHCO₃): from NaCl, water, CO₂ and NH₃. Mild, non-corrosive base. On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Baking powder = baking soda + mild edible acid (tartaric acid); the CO₂ makes bread and cake soft and spongy. Also used in antacids and soda-acid fire extinguishers.

Washing soda (Na₂CO₃·10H₂O): recrystallising sodium carbonate. Used in glass, soap and paper industries, making borax, cleaning, and removing permanent hardness of water.

Water of crystallisation: fixed number of water molecules in one formula unit of a salt. Blue CuSO₄·5H₂O turns white on heating and blue again when water is added. Na₂CO₃·10H₂O has 10.

Plaster of Paris (CaSO₄·½H₂O): made by heating gypsum (CaSO₄·2H₂O) at 373 K. With water it sets back into gypsum as a hard mass — used by doctors for supporting fractured bones, making toys, decoration and smooth surfaces.

Important Reactions (balanced)

Zn + H₂SO₄ → ZnSO₄ + H₂

2NaOH + Zn → Na₂ZnO₂ + H₂

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

NaHCO₃ + HCl → NaCl + H₂O + CO₂

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂

NaOH + HCl → NaCl + H₂O

CuO + 2HCl → CuCl₂ + H₂O

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (heat)

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

Activities in the Chapter

Activity 2.1/2.2 – Test acids and bases with litmus, phenolphthalein, methyl orange; onion, vanilla and clove as olfactory indicators.

Activity 2.3 – Zinc + dilute H₂SO₄: bubbles of hydrogen, which burns with a pop.

Activity 2.5 – Sodium carbonate / hydrogencarbonate + HCl: gas turns lime water milky (CO₂).

Activity 2.6 – NaOH + phenolphthalein turns pink; adding HCl makes it colourless — neutralisation.

Activity 2.7 – Copper oxide + HCl: solution turns blue-green.

Activity 2.8 – Glucose and alcohol solutions do not light the bulb; HCl and H₂SO₄ do.

Activity 2.11 – pH of various solutions with universal indicator.

Activity 2.15 – Heat copper sulphate crystals: water droplets appear, colour turns white.

Board Exam Focus

Name the gas and write the reaction when acid meets a metal or a carbonate; test for H₂ (pop) and CO₂ (lime water).

pH questions: which solution is more acidic, and everyday examples (tooth decay 5.5, acid rain 5.6).

Chlor-alkali products and uses; preparation and uses of bleaching powder, baking soda, washing soda, Plaster of Paris.

Why dry HCl does not turn dry litmus red; why acid is added to water.

Key Terms

Indicator (सूचक)

Olfactory indicator (गंधीय सूचक)

Neutralisation (उदासीनीकरण)

Alkali – water-soluble base (क्षार)

Hydronium ion H₃O⁺ (हाइड्रोनियम आयन)

pH scale (pH पैमाना)

Universal indicator (सार्वत्रिक सूचक)

Salt (लवण)

Brine – concentrated NaCl solution (लवण जल)

Chlor-alkali process (क्लोर-क्षार प्रक्रिया)

Water of crystallisation (क्रिस्टलन जल)

Plaster of Paris (प्लास्टर ऑफ़ पेरिस)

Antacid (प्रतिअम्ल)

Dilution (तनुकरण)

Common Mistakes

Adding water to concentrated acid — always add acid to water.

Thinking a higher pH means a stronger acid — a lower pH is more acidic.

Confusing baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O).

Writing Plaster of Paris as CaSO₄·2H₂O — that is gypsum; POP is CaSO₄·½H₂O.

Believing all salts are neutral — their pH depends on the strength of the acid and base.

💡 Key Learning Points

✓Indicators (litmus, phenolphthalein, methyl orange, olfactory indicators) identify acids and bases.
✓Acids react with metals to give hydrogen and with carbonates to give CO₂; acids and bases neutralise each other to form salt and water.
✓Acids give H⁺ (H₃O⁺) ions and bases give OH⁻ ions only in water; dilution must be done by adding acid to water.
✓The pH scale (0–14) measures strength; pH matters for the body, soil, digestion, teeth and acid rain.
✓Common salt is the raw material for NaOH, bleaching powder, baking soda and washing soda; water of crystallisation explains hydrated salts like gypsum and POP.

👨‍🏫 Teaching Tips

→Let students smell onion strips dipped in HCl and in NaOH to discover olfactory indicators themselves.
→Demonstrate the pop test for hydrogen and the lime-water test for CO₂ side by side.
→Show the conductivity setup with glucose vs HCl to explain why only acids give H⁺ ions in water.
→Use universal indicator on household liquids (lemon juice, soap, milk of magnesia, tap water) and build a class pH chart.
→Draw a flowchart from 'common salt' to NaOH, Cl₂, H₂, bleaching powder, baking soda and washing soda.
→Heat blue copper sulphate crystals and re-add water to show water of crystallisation reversibly.
→Link each salt to its daily use (cake, laundry, bone plaster, water disinfection) with real samples.

📋 Assessment Questions

Write the reaction when zinc reacts with dilute H₂SO₄ and describe the test for the gas.
Solution A has pH 4 and B has pH 9 — which is acidic and which has more H⁺ ions?
Why should water not be added to concentrated acid?
Name the products of the chlor-alkali process and one use of each.
How is Plaster of Paris prepared and why must it be stored in a moisture-proof container?

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