Class 10 Science Chapter 2 Notes | Acids, Bases and Salts | CBSE Board Exam 2026–27
Prepare for your Class 10 Science and Chemistry examinations with these comprehensive, NCERT-based notes on Acids, Bases and Salts. This chapter explains the chemical properties of acids and bases, indicators, pH scale, neutralisation reactions, important chemical equations, common salts and their uses.
These exam-oriented notes cover the major concepts, definitions, observations, reaction equations and applications that students should revise for CBSE board examinations. Use the memory tips, comparison tables and quick revision section to strengthen your understanding and improve answer-writing accuracy.
Chapter 2: Acids, Bases and Salts – Class 10 Science Notes
Subject: Science – Chemistry
Class: 10
Chapter: 2 – Acids, Bases and Salts
Study focus: NCERT concepts, chemical reactions, balanced equations, reasoning questions and applications in daily life.
1. Acids and Bases: Basic Concepts
Acids and bases are common substances found in food, medicines, cleaning products and laboratory chemicals. Their characteristic properties help us identify them using indicators and chemical tests.
Acids
- Generally sour in taste.
- Turn blue litmus red.
- Produce hydrogen ions in aqueous solution.
- Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃) and acetic acid (CH₃COOH).
Bases
- Many are bitter and feel soapy to touch.
- Turn red litmus blue.
- Produce hydroxide ions in aqueous solution.
- Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide [Ca(OH)₂].
2. Natural Sources of Acids
| Natural source | Acid present |
|---|---|
| Vinegar | Acetic acid |
| Curd / sour milk | Lactic acid |
| Lemon | Citric acid |
| Orange | Citric acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Ant sting | Methanoic acid |
| Nettle sting | Methanoic acid |
3. Indicators and Their Colour Changes
Indicators are substances that show a change in colour or smell in acidic and basic media. They help identify the nature of a solution.
| Indicator | In acidic solution | In basic solution |
|---|---|---|
| Blue litmus | Turns red | Remains blue |
| Red litmus | Remains red | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Remains yellow | Turns reddish-brown |
Types of indicators
- Natural indicators: Litmus, turmeric and coloured petals of certain flowers.
- Synthetic indicators: Phenolphthalein and methyl orange.
- Olfactory indicators: Substances whose smell changes in acidic or basic media, such as onion, vanilla essence and clove oil.
- Universal indicator: A mixture of indicators used to estimate the pH of a solution.
4. Chemical Properties of Acids and Bases
4.1 Reaction of acids with metals
Many metals react with dilute acids to produce a salt and hydrogen gas.
Example: Zinc with dilute sulphuric acid
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Example: Magnesium with dilute hydrochloric acid
Mg + 2HCl → MgCl₂ + H₂↑
Test for hydrogen gas: When a burning splint is brought near hydrogen, it burns with a characteristic pop sound.
4.2 Reaction of bases with certain metals
Some metals, such as zinc, react with strong bases to release hydrogen gas.
2NaOH + Zn → Na₂ZnO₂ + H₂↑
Products: Sodium zincate and hydrogen gas.
4.3 Reaction of metal carbonates and hydrogencarbonates with acids
Sodium carbonate with hydrochloric acid:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
Sodium hydrogencarbonate with hydrochloric acid:
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
Test for carbon dioxide: Pass the gas through lime water. It turns milky due to the formation of insoluble calcium carbonate.
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
When excess carbon dioxide is passed through lime water, the milkiness disappears because soluble calcium hydrogencarbonate forms.
CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
4.4 Neutralisation reaction
When an acid reacts with a base, their effects are neutralised and salt and water are formed.
HCl + NaOH → NaCl + H₂O
The ionic reaction responsible for neutralisation is:
H⁺(aq) + OH⁻(aq) → H₂O(l)
4.5 Reaction of metallic oxides with acids
Metallic oxides generally behave as basic oxides and react with acids to form salt and water.
CuO + 2HCl → CuCl₂ + H₂O
Copper oxide dissolves in hydrochloric acid, producing a blue-green solution of copper(II) chloride.
4.6 Reaction of non-metallic oxides with bases
Non-metallic oxides such as carbon dioxide react with bases to form salt and water.
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
This behaviour is similar to the reaction of acids with bases, so many non-metallic oxides are acidic in nature.
5. What Do All Acids and Bases Have in Common?
Acids in aqueous solution
Acids produce hydrogen ions in water. These ions associate with water molecules to form hydronium ions.
HCl + H₂O → H₃O⁺ + Cl⁻
The presence of H⁺(aq), represented more accurately as H₃O⁺ in water, is responsible for acidic properties.
Bases in aqueous solution
Bases produce hydroxide ions in water.
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
Mg(OH)₂ → Mg²⁺ + 2OH⁻
Bases that dissolve in water are called alkalis. Therefore, every alkali is a base, but every base is not necessarily an alkali.
Why do acids and bases conduct electricity in aqueous solution?
They produce ions in water, and these mobile ions carry electric current through the solution.
Why do glucose and alcohol solutions not show acidic properties?
Although they contain hydrogen, they do not ionise in water to produce H⁺ ions in the manner that acids do.
Why does dry HCl gas not change dry blue litmus paper?
In the absence of water, HCl does not produce the hydrogen ions responsible for its acidic behaviour. Moist blue litmus turns red because water allows these ions to form.
6. Dilution of Acids and Bases
Dilution means adding water to an acid or base solution to decrease the concentration of ions per unit volume.
- Dilution of an acid decreases the concentration of hydronium ions per unit volume, so its pH generally increases towards 7.
- Dilution of a base decreases the concentration of hydroxide ions per unit volume, so its pH generally decreases towards 7.
- Dissolving concentrated acids or bases in water releases heat and is an exothermic process.
7. The pH Scale
The pH scale measures how acidic or basic a solution is. The chapter describes a general scale from 0 to 14.
| pH value | Nature of solution | Meaning |
|---|---|---|
| Less than 7 | Acidic | Higher hydrogen-ion concentration relative to a neutral solution. |
| Exactly 7 | Neutral | Neutral solution, such as pure water under standard conditions. |
| Greater than 7 | Basic / alkaline | Greater hydroxide-ion concentration relative to a neutral solution. |
The pH scale is logarithmic: a change of one pH unit corresponds to a tenfold change in hydrogen-ion concentration in dilute aqueous solutions.
8. Importance of pH in Everyday Life
8.1 pH in the human body
Our body functions within a narrow pH range. The textbook states that the body works within approximately pH 7.0–7.8, and living organisms are sensitive to significant changes in pH.
8.2 Acid rain
Rainwater with a pH below 5.6 is called acid rain. It can lower the pH of river and lake water, making survival difficult for aquatic organisms.
8.3 pH in digestion
The stomach produces hydrochloric acid to help digest food. Excess acid can cause indigestion and irritation. Antacids, such as magnesium hydroxide (milk of magnesia), neutralise excess stomach acid.
8.4 Tooth decay
Tooth decay starts when the pH of the mouth falls below 5.5. Bacteria act on food particles and sugars to produce acids that can corrode tooth enamel. Cleaning teeth and using generally basic toothpaste help neutralise excess acid.
8.5 Soil pH
Plants need a suitable soil pH for healthy growth. Farmers may use quicklime (CaO), slaked lime [Ca(OH)₂] or chalk (CaCO₃) to treat acidic soil.
8.6 Stings from animals and plants
- A bee sting is described in the textbook as leaving an acid that causes pain and irritation. A mild base such as baking soda is traditionally used to help neutralise the acid.
- Nettle leaves inject methanoic acid, which causes a burning sensation.
9. Salts and Their Families
Salts are formed in reactions such as neutralisation. Salts having the same positive ion or the same negative ion may belong to the same family.
- Sodium salts: NaCl and Na₂SO₄.
- Chloride salts: NaCl and KCl.
pH of salts
| Acid and base used | Usual nature of salt solution |
|---|---|
| Strong acid + strong base | Neutral; pH about 7 |
| Strong acid + weak base | Acidic; pH below 7 |
| Strong base + weak acid | Basic; pH above 7 |
10. Chemicals Obtained from Common Salt
Common salt (sodium chloride, NaCl) is an important raw material used to manufacture sodium hydroxide, bleaching powder, baking soda and washing soda.
10.1 Sodium hydroxide – NaOH
Sodium hydroxide is prepared by electrolysis of an aqueous solution of sodium chloride called brine. This is the chlor-alkali process.
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
Products to remember:
- Chlorine gas is released at the anode.
- Hydrogen gas is released at the cathode.
- Sodium hydroxide solution is formed in the process.
10.2 Bleaching powder
Formula used in the textbook: Ca(ClO)₂. The textbook notes that its actual composition is more complex.
Preparation: Chlorine reacts with dry slaked lime.
2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O
Uses:
- Bleaching cotton, linen, wood pulp and washed clothes.
- Acting as an oxidising agent in chemical industries.
- Helping make drinking water free from germs.
10.3 Baking soda – NaHCO₃
Chemical name: Sodium hydrogencarbonate.
Preparation reaction:
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Reaction on heating:
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Uses:
- Used in baking powder, which helps bread and cakes rise and become soft and spongy.
- Used in antacids to neutralise excess stomach acid.
- Used in soda-acid fire extinguishers.
10.4 Washing soda – Na₂CO₃·10H₂O
Chemical name: Sodium carbonate decahydrate.
Washing soda is obtained by recrystallisation of sodium carbonate.
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
Uses:
- Used in glass, soap and paper industries.
- Used in the manufacture of sodium compounds such as borax.
- Used as a domestic cleaning agent.
- Used to remove permanent hardness of water.
11. Water of Crystallisation
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.
Copper sulphate crystals
- Hydrated copper sulphate is blue and has the formula CuSO₄·5H₂O.
- On heating, it loses water of crystallisation and becomes white anhydrous copper sulphate.
- Adding water to the anhydrous salt restores the blue colour.
CuSO₄·5H₂O → CuSO₄ + 5H₂O
Gypsum and Plaster of Paris
Gypsum: CaSO₄·2H₂O
When gypsum is heated to about 373 K, it loses part of its water of crystallisation and forms Plaster of Paris.
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
Plaster of Paris: Calcium sulphate hemihydrate, CaSO₄·½H₂O.
When mixed with water, it changes back into gypsum and forms a hard solid mass.
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Uses of Plaster of Paris:
- Supporting fractured bones in the correct position.
- Making toys and decorative materials.
- Making surfaces smooth.
12. Important Chemical Equations for Board Exams
Practise writing these equations with correct formulas, products and balancing.
| No. | Reaction | Balanced equation |
|---|---|---|
| 1 | Zinc + sulphuric acid | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| 2 | Magnesium + hydrochloric acid | Mg + 2HCl → MgCl₂ + H₂ |
| 3 | Sodium carbonate + hydrochloric acid | Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ |
| 4 | Sodium hydrogencarbonate + hydrochloric acid | NaHCO₃ + HCl → NaCl + H₂O + CO₂ |
| 5 | Carbon dioxide + lime water | Ca(OH)₂ + CO₂ → CaCO₃ + H₂O |
| 6 | Hydrochloric acid + sodium hydroxide | HCl + NaOH → NaCl + H₂O |
| 7 | Copper oxide + hydrochloric acid | CuO + 2HCl → CuCl₂ + H₂O |
| 8 | Electrolysis of brine | 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ |
| 9 | Heating baking soda | 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ |
| 10 | Gypsum to Plaster of Paris | CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O |
13. Common Board Exam Mistakes to Avoid
- Do not confuse baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O).
- Do not write that all hydrogen-containing compounds are acids. Acids produce H⁺ ions in aqueous solution.
- Remember that an alkali is a base that dissolves in water.
- Do not confuse the gas evolved when an acid reacts with a metal (usually hydrogen) with the gas evolved when an acid reacts with a carbonate (carbon dioxide).
- Do not forget that CO₂ turns lime water milky due to CaCO₃ formation.
- Remember that pH below 7 is acidic and pH above 7 is basic.
- Do not write that dry HCl gas changes dry blue litmus red.
- Balance chemical equations before submitting your answer.
- For assertion–reason and reasoning questions, explain the scientific cause rather than giving only a one-word answer.
14. Quick Revision – Acids, Bases and Salts
One-Minute Revision Sheet
- Acid: Produces H⁺ / H₃O⁺ ions in aqueous solution.
- Base: Produces OH⁻ ions in aqueous solution.
- Litmus: Acid turns blue litmus red; base turns red litmus blue.
- Acid + metal: Salt + hydrogen gas.
- Acid + carbonate: Salt + CO₂ + water.
- Acid + base: Salt + water (neutralisation).
- Metal oxide + acid: Salt + water.
- pH: Below 7 acidic; 7 neutral; above 7 basic.
- Acid rain: Rainwater pH below 5.6.
- Tooth decay: Begins when mouth pH falls below 5.5.
- Common salt: NaCl.
- Baking soda: NaHCO₃.
- Washing soda: Na₂CO₃·10H₂O.
- Bleaching powder: Represented in the textbook as Ca(ClO)₂.
- Gypsum: CaSO₄·2H₂O.
- Plaster of Paris: CaSO₄·½H₂O.
- Hydrated copper sulphate: CuSO₄·5H₂O; blue crystals.
- Chlor-alkali process: Electrolysis of brine produces NaOH, Cl₂ and H₂.
- Water of crystallisation: Fixed water molecules in a salt's formula unit.
15. How to Prepare This Chapter for the CBSE Board Exam
- Learn definitions: Acid, base, alkali, indicator, neutralisation, pH and water of crystallisation.
- Practise reactions: Write and balance the important equations without looking at the notes.
- Revise comparisons: Acids vs bases, baking soda vs washing soda, gypsum vs Plaster of Paris.
- Practise reasoning questions: Understand why dry HCl does not affect dry litmus, why acid is added to water and why CO₂ turns lime water milky.
- Memorise applications: Learn the uses of bleaching powder, baking soda, washing soda and Plaster of Paris.
- Test yourself: Attempt NCERT exercise questions and additional practice papers under timed conditions.
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