In short: Acids, Bases and Salts carries 8-10 marks in CBSE Class 10 and is one of the most formula-rich Chemistry chapters. Master the pH scale, know all indicators and their color changes, memorize the preparation and properties of important salts (baking soda, washing soda, Plaster of Paris, bleaching powder), and learn the chlor-alkali process. Chemical equations are key — write balanced equations for every reaction.
Acids, Bases and Salts is a chapter where memorization and understanding go hand in hand. You need to know the chemical formulae, the reactions, the pH values, and the practical applications. The good news is that the question patterns are predictable — indicators, pH scale, salt preparation, and chemical equations appear every year.
This guide covers every concept with chemical equations, tables, and exam tips. For marks distribution, check the CBSE Class 10 Science chapter-wise marks weightage.
What Are Acids and Bases?
Acids are substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. They taste sour, turn blue litmus red, and conduct electricity in solution.
Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃), Acetic acid (CH₃COOH), Citric acid.
Bases are substances that produce hydroxide ions (OH⁻) when dissolved in water. They taste bitter, feel soapy, and turn red litmus blue.
Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide (Ca(OH)₂), Ammonium hydroxide (NH₄OH).
A base that dissolves in water is called an alkali. All alkalis are bases, but not all bases are alkalis (for example, copper hydroxide is a base but is insoluble in water, so it is not an alkali).
Indicators and Their Color Changes
Indicators are substances that show different colors in acidic and basic solutions.
| Indicator | In Acid | In Base | In Neutral |
|---|---|---|---|
| Litmus (natural) | Red | Blue | Purple |
| Phenolphthalein | Colorless | Pink | Colorless |
| Methyl orange | Red | Yellow | Orange |
| Turmeric | Yellow | Reddish-brown | Yellow |
Olfactory indicators change smell in acids or bases. Examples: onion (loses smell in base), vanilla (loses smell in base), clove oil (loses smell in base).
Board Exam Tip: "Name the indicator that remains colorless in both acidic and neutral solutions but turns pink in basic solution" — the answer is phenolphthalein. This is a very common 1-mark question.
Chemical Properties of Acids
Reaction with Metals
Acid + Metal → Salt + Hydrogen gas
Examples:
- 2HCl + Zn → ZnCl₂ + H₂↑
- H₂SO₄ + Fe → FeSO₄ + H₂↑
The hydrogen gas produced burns with a "pop" sound when a burning splinter is brought near it. This is the standard test for hydrogen gas.
Reaction with Metal Carbonates and Bicarbonates
Acid + Metal Carbonate → Salt + Water + Carbon dioxide Acid + Metal Hydrogen Carbonate → Salt + Water + Carbon dioxide
Examples:
- 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑
- HCl + NaHCO₃ → NaCl + H₂O + CO₂↑
The CO₂ produced turns lime water milky. This is the standard test for carbon dioxide.
Reaction with Bases (Neutralization)
Acid + Base → Salt + Water
Examples:
- HCl + NaOH → NaCl + H₂O
- H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Neutralization is an exothermic reaction (it releases heat).
Reaction with Metal Oxides
Acid + Metal Oxide → Salt + Water
Example: 2HCl + CuO → CuCl₂ + H₂O
This reaction shows that metal oxides are basic in nature.
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Chemical Properties of Bases
Reaction with Metals
Some bases react with metals to produce salt and hydrogen gas.
Example: 2NaOH + Zn → Na₂ZnO₂ (sodium zincate) + H₂↑
Reaction with Non-metal Oxides
Base + Non-metal Oxide → Salt + Water
Example: 2NaOH + CO₂ → Na₂CO₃ + H₂O
This reaction shows that non-metal oxides are acidic in nature.
What Happens When Acids or Bases Are Mixed with Water?
Dissolving acids or bases in water is an exothermic process. The process of mixing acid or base with water is called dilution, and the resulting solution is called a dilute acid or dilute base.
Important safety rule: Always add acid to water, never water to acid. Adding water to concentrated acid releases so much heat that the mixture can splash out and cause burns. When acid is added slowly to water, the large volume of water absorbs the heat safely.
Why do acids produce H⁺ ions only in water? Acids like HCl exist as molecules in pure form. When dissolved in water, water molecules help break the bond between H and Cl, releasing H⁺ ions (which combine with water to form H₃O⁺). Without water, acids do not show acidic properties. This is why dry HCl gas does not turn blue litmus red, but HCl solution does.
The pH Scale
The pH scale measures how acidic or basic a solution is. It ranges from 0 to 14.
| pH Value | Nature | Examples |
|---|---|---|
| 0 | Strongly acidic | Battery acid (H₂SO₄) |
| 1 | Strongly acidic | Hydrochloric acid (gastric juice) |
| 2-3 | Acidic | Lemon juice (pH ≈ 2.2), vinegar (pH ≈ 3) |
| 4-5 | Weakly acidic | Tomato juice, acid rain (pH < 5.6) |
| 6 | Slightly acidic | Milk (pH ≈ 6.5) |
| 7 | Neutral | Pure water, blood (pH ≈ 7.4) |
| 8-9 | Weakly basic | Baking soda solution, sea water |
| 10-11 | Basic | Milk of magnesia, washing soda solution |
| 12-14 | Strongly basic | Soap solution, NaOH solution |
pH = −log[H⁺] (you do not need this formula for Class 10, but understanding that lower pH = more H⁺ ions = more acidic helps conceptually).
Importance of pH in Daily Life
Our body: Stomach produces HCl (pH ≈ 1.2) for digestion. When excess acid is produced, it causes acidity, which is treated with antacids like milk of magnesia (Mg(OH)₂).
Tooth decay: Bacteria in the mouth produce acids (pH below 5.5) that corrode tooth enamel (made of calcium phosphate). Using basic toothpaste neutralizes the acid.
Soil pH: Plants grow best at specific pH ranges. Farmers add quicklime (CaO) to acidic soil or organic matter to basic soil to adjust pH.
Acid rain: When pH of rainwater drops below 5.6 (due to SO₂ and NO₂ dissolving in rain), it becomes acid rain, which damages buildings, monuments (like the Taj Mahal), and aquatic life.
Self-defence by animals: Bee sting injects formic acid (methanoic acid) — applying baking soda (basic) neutralizes it. Wasp sting is basic — applying vinegar (acidic) helps.
Salts — Preparation and Properties of Important Salts
A salt is the product of a neutralization reaction between an acid and a base. The family of a salt depends on the acid and base used.
| Salt | Acid + Base | Formula |
|---|---|---|
| Common salt | HCl + NaOH | NaCl |
| Washing soda | — | Na₂CO₃·10H₂O |
| Baking soda | — | NaHCO₃ |
| Bleaching powder | — | CaOCl₂ |
| Plaster of Paris | — | CaSO₄·½H₂O |
Common Salt (Sodium Chloride — NaCl)
Obtained from sea water by evaporation. It is the raw material for many chemicals. The chlor-alkali process uses NaCl to produce NaOH, Cl₂, and H₂.
Chlor-Alkali Process
When electricity is passed through a concentrated solution of NaCl (brine):
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
Products formed at electrodes:
- At anode (positive): Chlorine gas (Cl₂)
- At cathode (negative): Hydrogen gas (H₂)
- Near cathode: Sodium hydroxide (NaOH) solution
Uses of products: Cl₂ is used for water purification, PVC, and disinfectants. NaOH is used for degreasing metals, soaps, and detergents. H₂ is used for fuels, margarine, and ammonia.
Baking Soda (Sodium Hydrogen Carbonate — NaHCO₃)
Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Properties:
- Mild non-corrosive basic salt
- Produces CO₂ when heated: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Uses: Baking powder (NaHCO₃ + tartaric acid) — the CO₂ released makes bread and cakes fluffy. Used as antacid to relieve acidity. Used in soda-acid fire extinguishers.
Washing Soda (Sodium Carbonate Decahydrate — Na₂CO₃·10H₂O)
Preparation: By heating baking soda and recrystallizing: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
Uses: In glass, soap, and paper industries. Water softener (removes permanent hardness). Cleaning agent for household purposes.
Bleaching Powder (Calcium Oxychloride — CaOCl₂)
Preparation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Dry slaked lime reacts with chlorine gas to form bleaching powder.
Uses: Bleaching cotton, linen, and wood pulp in textile and paper industries. Disinfecting drinking water. Oxidizing agent in chemical industries.
Plaster of Paris (Calcium Sulphate Hemihydrate — CaSO₄·½H₂O)
Preparation: By heating gypsum (CaSO₄·2H₂O) at 373 K:
CaSO₄·2H₂O → (373 K) → CaSO₄·½H₂O + 1½H₂O
Uses: Setting fractured bones (orthopaedic casts). Making decorative designs, toys, and statues. Making smooth surfaces (wall plastering).
Important property: When mixed with water, POP sets into a hard mass (gypsum) within 15-20 minutes: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. This is why it must be stored in moisture-proof containers.
Water of Crystallization
Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Examples:
- CuSO₄·5H₂O (copper sulphate pentahydrate) — blue color
- FeSO₄·7H₂O (ferrous sulphate heptahydrate) — green color
- Na₂CO₃·10H₂O (washing soda) — transparent crystals
- CaSO₄·2H₂O (gypsum) — white crystite
On heating, CuSO₄·5H₂O loses its water of crystallization and turns from blue to white (anhydrous CuSO₄). Adding water to the white powder restores the blue color. This reversibility is a standard lab activity.
Board Exam Important Questions
- Write the chemical formula of baking soda, washing soda, bleaching powder, and Plaster of Paris (1-2 marks each — very common).
- Explain the chlor-alkali process with a diagram (3-5 marks).
- What is the pH of acids, bases, and neutral solutions? Give examples (2-3 marks).
- Why should you add acid to water and not water to acid? (2 marks).
- Write the chemical equation for the preparation of bleaching powder (1-2 marks).
- Explain why dry HCl gas does not change the color of dry litmus paper (2 marks).
For the complete formula reference across all chapters, see the CBSE Class 10 Science formulas complete list.
Frequently Asked Questions
How many marks does Acids, Bases and Salts carry in CBSE Class 10?
This chapter typically carries 8 to 10 marks in the board exam. Questions include 1-mark MCQs on indicators and pH, 2-3 mark questions on chemical properties and reactions, and 3-5 mark questions on salt preparation and the chlor-alkali process. Memorizing all chemical formulae and equations is essential.
How do I remember the pH values of common substances?
Group substances by their pH range: strong acids (0-2) include battery acid and gastric juice; weak acids (3-5) include lemon, vinegar, and tomato juice; neutral (7) is pure water; weak bases (8-10) include baking soda and sea water; strong bases (12-14) include NaOH. Associate each substance with its approximate pH and review this list weekly.
What is the difference between baking soda and washing soda?
Baking soda is sodium hydrogen carbonate (NaHCO₃) — a mild base used in cooking and as an antacid. Washing soda is sodium carbonate decahydrate (Na₂CO₃·10H₂O) — a stronger base used as a cleaning agent and water softener. Washing soda is actually made by heating baking soda and then recrystallizing with water. The key difference is that washing soda is more alkaline and not suitable for consumption.
Why does dry HCl gas not change litmus color but HCl solution does?
HCl shows acidic properties only in the presence of water. In dry form, HCl exists as molecules and does not produce H⁺ ions. When dissolved in water, H₂O molecules help dissociate HCl into H⁺ and Cl⁻ ions. It is the H⁺ ions (or H₃O⁺ ions) that are responsible for acidic behavior and color change in litmus. This is why dry HCl gas has no effect on dry litmus paper.
What is the chlor-alkali process and why is it important?
The chlor-alkali process is the electrolysis of brine (concentrated NaCl solution) to produce three important chemicals: NaOH (at cathode), Cl₂ (at anode), and H₂ (at cathode). It is called "chlor-alkali" because the main products are chlorine and an alkali (NaOH). This process is industrially very important as NaOH is used in soap-making, Cl₂ in water treatment, and H₂ as a fuel.
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