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Acids, Bases and Salts for Class 10: The Complete CBSE Guide (2026-27)

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Every year, the CBSE Class 10 Science board paper reserves 8-10 marks for Acids, Bases and Salts — making it one of the highest-weightage chapters in the chemistry section. The 2024-25 NCERT textbook structures this chapter into three core topics: chemical properties of acids and bases, the pH scale and its importance, and salts (preparation methods and uses). You will encounter this chapter as early as October in most CBSE schools, and it reappears in every pre-board, sample paper, and final exam. Understanding acids, bases and salts for Class 10 is also critical for Class 11 Equilibrium and Class 12 Electrochemistry. This guide walks you through every NCERT concept, formula, and question type, using the exact terminology and examples from your textbook, so you can confidently score full marks in both theory and practicals.

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Key takeaways

✓Acids, Bases and Salts Class 10 carries 8-10 marks in the CBSE board exam, with questions spanning VSA (1 mark), SA (3 marks), and case-based (4-5 marks) formats.
✓The pH scale quantifies acidity and basicity from 0-14; human blood maintains pH 7.35-7.45, while gastric juice sits at pH 1.2, demonstrating real-world relevance.
✓Chemical properties of acids include reaction with metals (producing H₂), bases (neutralisation), metal carbonates (CO₂ release), and metal oxides — all testable as balanced equations.
✓Salts are classified as acidic, basic, or neutral based on parent acid-base strength; sodium chloride (neutral), sodium carbonate (basic), and ammonium chloride (acidic) are NCERT examples.
✓Common salt (NaCl) is the starting material for sodium hydroxide, baking soda, washing soda, and bleaching powder — a fact-based link tested in 3-mark application questions.
✓Indicators like litmus, methyl orange, and phenolphthalein change colour at specific pH ranges; CBSE expects you to predict colour in given solutions.
✓Water of crystallisation in salts (e.g. CuSO₄·5H₂O, Na₂CO₃·10H₂O) is a recurring 1-2 mark definition and example question on the board paper.

Why Acids, Bases and Salts Class 10 Matters for CBSE Boards

Acids, Bases and Salts Class 10 is Chapter 2 in the NCERT Science textbook and contributes roughly 8-10 marks to the 80-mark theory paper. In the 2024 and 2025 CBSE board exams, this chapter appeared as two 1-mark MCQs, one 2-mark very short answer (definition or example), one 3-mark short answer (chemical equation with observation), and one 4-5 mark case-based integrated question linking pH, salts, and daily life. The chapter also feeds directly into the Class 10 practical on studying properties of acids and bases using indicators, pH paper, and reaction with metals — worth 2 marks in the 20-mark practical exam. Beyond boards, concepts like pH, buffer action, and salt hydrolysis form the foundation for Class 11 Ionic Equilibrium (NCERT Chemistry Part I, Chapter 7) and Class 12 Electrochemistry. Competitive exams (JEE Main, NEET) test acid-base strength, salt analysis, and pH calculations at a higher level, so mastering this chapter now saves months of catch-up later. CBSE marking schemes award full marks only when you write balanced chemical equations with states of matter, correct observations (effervescence, colour change), and proper chemical names — not formulae alone. This precision starts here in Class 10.

8-10 marks per year across MCQ, VSA, SA, and case-based question formats in the CBSE Class 10 board paper.
Directly tested in the mandatory Class 10 Science practical on properties of acids and bases (2 marks in viva, 2 marks in observation table).
Foundational for Class 11 Chapter 7 (Equilibrium) and Class 12 Chapter 3 (Electrochemistry) in NCERT Chemistry.
2024 board paper included a 5-mark case on antacid pH and neutralisation; 2025 paper tested plaster of Paris preparation from gypsum.
Common mistakes: writing H₂SO₄ instead of 'dilute sulphuric acid', omitting (aq) or (s) states, or using 'sodium carbonate' when NCERT specifies Na₂CO₃·10H₂O.

NCERT Structure: Three Core Topics in Acids, Bases and Salts Class 10

The 2024-25 NCERT Class 10 Science textbook divides Acids, Bases and Salts into three clearly demarcated topics. Topic 1 covers chemical properties of acids and bases: reaction of acids with metals (Zn + H₂SO₄ → ZnSO₄ + H₂), metal carbonates and hydrogen carbonates (producing CO₂), metal oxides (neutralisation), and bases; plus reaction of bases with metals and non-metallic oxides. Topic 2 introduces the pH scale and its importance, defining pH as the negative logarithm of H⁺ concentration, explaining the 0-14 range, and linking pH to daily-life contexts like soil pH for plant growth, pH of digestive juices, and pH in tooth decay. Topic 3 examines salts: their preparation (neutralisation, direct combination, displacement), classification (acidic, basic, neutral), the family of salts derived from common salt (NaOH, Na₂CO₃, NaHCO₃, bleaching powder), water of crystallisation (with examples CuSO₄·5H₂O and gypsum CaSO₄·2H₂O), and uses of everyday salts like baking soda, washing soda, and plaster of Paris. Every CBSE question maps to one of these three topics, so structuring your notes this way mirrors both the textbook and the exam blue-print.

Topic 1: Chemical properties — five classes of reactions for acids, two for bases, all with balanced equations and observations.
Topic 2: pH scale — definition (pH = –log[H⁺]), universal indicator colour chart, neutral (pH 7), acidic (<7), basic (>7), and biological/environmental pH examples.
Topic 3: Salts — four preparation methods, three classification types, the NaCl family tree (NaOH, Na₂CO₃·10H₂O, NaHCO₃, Ca(OCl)Cl), and water of crystallisation concept.
NCERT in-text questions (pages 18, 22, 25, 28) and end-exercises (page 33) are high-yield for board MCQs and 1-2 mark definitions.
Practical activities (testing acids/bases with litmus, pH paper, and zinc) directly translate to the mandatory Class 10 lab experiment.

Chemical Properties of Acids: Reactions You Must Master

NCERT Class 10 Science lists five key chemical properties of acids, each tested as a 3-mark 'write the balanced equation and observation' question. (1) Reaction with metals: Dilute acids react with active metals (Zn, Mg, Fe, Al — not Cu, Ag, Au) to produce a salt and hydrogen gas. Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑. Observation: effervescence, colourless gas burns with a pop sound (hydrogen test). (2) Reaction with metal carbonates and hydrogen carbonates: Acids react with carbonates (CaCO₃, Na₂CO₃) and hydrogen carbonates (NaHCO₃) to give salt, water, and carbon dioxide. Example: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)↑. Observation: brisk effervescence, gas turns lime water milky (CO₂ test). (3) Reaction with metal oxides: Metal oxides are basic; they neutralise acids to form salt and water only. Example: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l). Observation: black copper oxide dissolves, solution turns blue-green. (4) Reaction with bases (neutralisation): Acid + Base → Salt + Water. Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). This is exothermic; beaker feels warm. (5) Reaction with indicators: Acids turn blue litmus red, methyl orange pink, and phenolphthalein colourless. These five properties appear in every CBSE sample paper; you must write formulae, states, and observations correctly to earn full marks.

Chemical Properties of Bases: Two Key Reactions

NCERT describes bases as substances that release OH⁻ ions in aqueous solution and react with acids to form salts. While bases share the neutralisation property with acids, only two other chemical properties are emphasised for CBSE Class 10. (1) Reaction with metals: Strong bases like NaOH react with certain metals (aluminium, zinc) to produce a salt and hydrogen gas. Example: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)↑. Observation: hydrogen gas evolved. This is less commonly tested than acid-metal reactions but appeared in the 2019 board paper. (2) Reaction with non-metallic oxides: Non-metallic oxides (CO₂, SO₂, SO₃) are acidic in nature; they react with bases to form salt and water. Example: CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l). This reaction is used in the lab test for CO₂: when CO₂ is passed through lime water (Ca(OH)₂), it turns milky due to formation of insoluble CaCO₃. Both reactions illustrate that bases neutralise acidic species — whether H⁺ from acids or acidic oxides. The CBSE marking scheme expects you to classify oxides (metallic = basic, non-metallic = acidic) and write correct products. Additionally, bases turn red litmus blue, methyl orange yellow, and phenolphthalein pink — a 1-mark MCQ favourite. Strong alkalis (NaOH, KOH) feel soapy and are corrosive, while weak bases (Ca(OH)₂, Mg(OH)₂) are used in antacids and toothpaste, connecting chemistry to daily life.

Reaction with metals (Zn, Al): Base + Metal → Salt + H₂. Example: 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑.
Reaction with non-metallic oxides: Base + Acidic oxide → Salt + Water. Example: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (lime water test).
Indicator behaviour: Bases turn red litmus blue, phenolphthalein pink, and methyl orange yellow (unchanged from its alkaline colour).
Neutralisation: Base + Acid → Salt + Water (exothermic). Example: NaOH + HCl → NaCl + H₂O.
Strong vs weak: NaOH (strong, pH~14) used in soap; Mg(OH)₂ (weak, pH~9) used in milk of magnesia antacid.

The pH Scale and Its Importance in Acids, Bases and Salts Class 10

The pH scale is a logarithmic scale from 0 to 14 that measures the acidity or basicity of a solution. pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = –log₁₀[H⁺]. A solution with pH < 7 is acidic, pH = 7 is neutral (pure water at 25°C), and pH > 7 is basic (alkaline). Each unit change in pH represents a tenfold change in [H⁺]; for example, pH 3 is ten times more acidic than pH 4. NCERT introduces the universal indicator, a mixture of dyes that shows a smooth colour gradient: red (pH 0-2, strongly acidic), orange-yellow (pH 3-6, weakly acidic), green (pH 7, neutral), blue (pH 8-11, weakly basic), and violet (pH 12-14, strongly basic). The importance of pH spans biology, agriculture, and industry. In the human body, blood pH is tightly regulated at 7.35-7.45; deviations cause acidosis or alkalosis. Gastric juice has pH ~1.2 (due to HCl) for protein digestion, while saliva is near-neutral (pH 6.5-7.5) to start carbohydrate breakdown. In agriculture, most plants grow best in slightly acidic to neutral soil (pH 6-7); acidic soil is treated with slaked lime Ca(OH)₂ to raise pH. In tooth decay, mouth bacteria produce acids (pH drops below 5.5) that dissolve enamel; toothpaste (basic, pH ~9) neutralises the acid. CBSE loves to ask: 'Why does tooth decay start when pH falls below 5.5?' or 'A sample of soil has pH 4. Suggest a substance to neutralise it.' Know these applications verbatim from NCERT.

Salts: Definition, Preparation, and Classification

A salt is defined as an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion. In the typical neutralisation reaction, Acid + Base → Salt + Water, the salt is the product that carries the cation from the base and the anion from the acid. For example, HCl + NaOH → NaCl + H₂O: here NaCl is the salt. NCERT describes four methods of preparing salts. (1) Neutralisation: Acid + Base → Salt + Water. Example: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O (sodium sulphate). (2) Acid + Metal: Example: Zn + H₂SO₄ → ZnSO₄ + H₂. (3) Acid + Metal carbonate: Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. (4) Direct combination (for a few salts): Example: Fe + S → FeS (iron sulphide). Salts are classified by pH of their aqueous solutions into three types. Neutral salts (pH 7) come from strong acid + strong base: NaCl (from HCl + NaOH), KNO₃. Acidic salts (pH < 7) come from strong acid + weak base: NH₄Cl (from HCl + NH₄OH), CuSO₄. Basic salts (pH > 7) come from weak acid + strong base: Na₂CO₃ (from H₂CO₃ + NaOH), CH₃COONa (sodium acetate). This classification is tested as a 1-mark MCQ ('Which salt will turn red litmus blue?') or 2-mark ('Classify the following salts as acidic, basic, or neutral: NaCl, Na₂CO₃, NH₄Cl'). Always link the salt to its parent acid and base to predict pH behaviour.

The Family of Salts Derived from Common Salt (NaCl)

NCERT dedicates a section to how common salt (sodium chloride, NaCl) — obtained from sea water or rock deposits — serves as the raw material for several important chemicals. This 'family of salts' is a favourite 3-5 mark question in CBSE papers. (1) Sodium hydroxide (NaOH, caustic soda): Produced by the chlor-alkali process, electrolysis of brine (concentrated NaCl solution). At the cathode, H₂ is released; at the anode, Cl₂ is released; NaOH remains in solution. Uses: soap and detergent manufacture, paper industry, petroleum refining. (2) Baking soda (sodium hydrogen carbonate, NaHCO₃): Prepared by passing CO₂ through a concentrated solution of sodium chloride saturated with ammonia (Solvay process, simplified in NCERT). Alternatively, NaCl + H₂O + CO₂ + NH₃ → NaHCO₃ + NH₄Cl. Uses: mild non-corrosive base in fire extinguishers, antacid, baking powder (releases CO₂ on heating). (3) Washing soda (sodium carbonate decahydrate, Na₂CO₃·10H₂O): Made by heating baking soda: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Then recrystallisation gives Na₂CO₃·10H₂O. Uses: removal of permanent hardness of water, glass and soap manufacturing, as a cleaning agent. (4) Bleaching powder (calcium oxychloride, Ca(OCl)Cl): Prepared by passing chlorine gas over dry slaked lime: Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O. Uses: disinfectant for drinking water, bleaching cotton and linen. Exam questions often ask 'Write the chemical formula and one use of bleaching powder' or 'How is washing soda prepared from baking soda? Give equation.' Always include the decahydrate form Na₂CO₃·10H₂O for washing soda — writing Na₂CO₃ alone loses marks.

Sodium hydroxide (NaOH): from electrolysis of brine; used in soap, paper, detergents.
Baking soda (NaHCO₃): from NaCl + CO₂ + NH₃; decomposes on heating to give CO₂ (baking, fire extinguisher).
Washing soda (Na₂CO₃·10H₂O): recrystallisation of heated NaHCO₃; removes water hardness, used in glass and soap.
Bleaching powder (Ca(OCl)Cl): from Cl₂ + Ca(OH)₂; disinfectant and bleaching agent.
All four chemicals trace back to NaCl, illustrating the industrial importance of common salt beyond the kitchen.

Water of Crystallisation: Definition and Examples

Water of crystallisation is the fixed number of water molecules chemically bound in the crystal structure of a salt. These water molecules are an integral part of the crystal lattice and are represented in the chemical formula. For example, copper sulphate pentahydrate is written as CuSO₄·5H₂O: each formula unit of copper sulphate is associated with five water molecules. When you heat blue CuSO₄·5H₂O crystals, they lose water and turn into white anhydrous CuSO₄ powder. The reaction is: CuSO₄·5H₂O(s) [blue] --heat--> CuSO₄(s) [white] + 5H₂O(g). If you add water to the white powder, it turns blue again, proving the water was part of the structure, not just surface moisture. NCERT lists key examples you must memorise: (1) Gypsum (CaSO₄·2H₂O): used to make plaster of Paris. When heated to 373 K, it loses 1.5 molecules of water per formula unit to become CaSO₄·½H₂O (plaster of Paris). The equation is: CaSO₄·2H₂O --heat(373K)--> CaSO₄·½H₂O + 1½H₂O. Plaster of Paris, when mixed with water, re-hydrates and sets into a hard gypsum mass, used for casts, moulds, and building. (2) Washing soda (Na₂CO₃·10H₂O): contains ten water molecules; on exposure to air, it effloresces (loses water). (3) Ferrous sulphate (FeSO₄·7H₂O): green crystals with seven water molecules. On heating, it loses water and decomposes further. CBSE frequently asks 'What is water of crystallisation? Give two examples with formulae' (2 marks) or 'Write the equation for conversion of gypsum to plaster of Paris' (2 marks). Always write the dot notation (·) and the correct number of H₂O molecules; writing CuSO₄·5H₂O as CuSO₄(H₂O)₅ or CuSO₄5H₂O is incorrect notation and loses marks.

Important Formulas and Equations for Acids, Bases and Salts Class 10

CBSE Class 10 board exams and internal assessments expect you to recall and write balanced chemical equations with correct states of matter. Below are the must-know reactions for Acids, Bases and Salts Class 10, organized by topic. For Acids: (1) Acid + Metal → Salt + H₂. Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑. (2) Acid + Metal carbonate → Salt + H₂O + CO₂. Example: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)↑. (3) Acid + Metal hydrogen carbonate → Salt + H₂O + CO₂. Example: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)↑. (4) Acid + Base → Salt + Water (neutralisation). Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). (5) Acid + Metal oxide → Salt + Water. Example: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l). For Bases: (1) Base + Metal → Salt + H₂. Example: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)↑. (2) Base + Non-metallic oxide → Salt + Water. Example: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l). For Salts: (1) Thermal decomposition of metal carbonates: CaCO₃(s) --heat--> CaO(s) + CO₂(g). (2) Preparation of washing soda: 2NaHCO₃(s) --heat--> Na₂CO₃(s) + H₂O(g) + CO₂(g); then recrystallisation: Na₂CO₃(s) + 10H₂O(l) → Na₂CO₃·10H₂O(s). (3) Gypsum to plaster of Paris: CaSO₄·2H₂O(s) --heat(373K)--> CaSO₄·½H₂O(s) + 1½H₂O(g). (4) Plaster of Paris setting: CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s). (5) Chlor-alkali process (simplified): 2NaCl(aq) + 2H₂O(l) --electrolysis--> 2NaOH(aq) + Cl₂(g) + H₂(g). Memorise these with states; CBSE deducts 0.5 marks per missing or wrong state symbol in a 3-mark equation question.

Zn + H₂SO₄ → ZnSO₄ + H₂↑ (acid + metal, test for H₂ with pop sound)
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ (acid + carbonate, CO₂ turns lime water milky)
HCl + NaOH → NaCl + H₂O (neutralisation, exothermic)
CuO + 2HCl → CuCl₂ + H₂O (acid + metal oxide, black CuO dissolves, blue solution)
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (lime water test for CO₂, milky ppt)
2NaHCO₃ --heat--> Na₂CO₃ + H₂O + CO₂ (baking soda to washing soda)
CaSO₄·2H₂O --heat(373K)--> CaSO₄·½H₂O + 1.5H₂O (gypsum to plaster of Paris)
pH = –log₁₀[H⁺] (definition, not an equation to balance but know the formula)

Indicators and Their Colour Changes: A Quick Reference

Indicators are substances that show different colours in acidic and basic solutions by responding to changes in H⁺ concentration. NCERT Class 10 introduces natural indicators (litmus, turmeric, red cabbage juice) and synthetic indicators (methyl orange, phenolphthalein) plus the universal indicator. Litmus: Extracted from lichens. Red litmus turns blue in base; blue litmus turns red in acid. Neutral solutions cause no change. Methyl orange: In acidic solution (pH < 4.4) it is red/pink; in basic solution (pH > 6.2) it is yellow. It is used to titrate strong acids. Phenolphthalein: Colourless in acidic and neutral solutions (pH < 8.2); turns pink/magenta in basic solutions (pH > 8.2). It is the indicator of choice for titrating weak acids with strong bases. Universal indicator: A mixture that displays a spectrum of colours from red (pH 0) through orange, yellow, green (pH 7), blue, to violet (pH 14). CBSE often provides a table of solutions and asks you to predict the colour with a given indicator, or vice versa. For example, 'A solution turns phenolphthalein pink. What is its nature?' Answer: Basic (pH > 8.2). Or, 'Lemon juice turns blue litmus red. Is it acidic or basic?' Answer: Acidic. Turmeric is a natural indicator: it is yellow in neutral/acidic medium and turns reddish-brown in basic medium (used in the kitchen test). Olfactory indicators like onion or vanilla extract change smell in acid vs base, a fun demo but not tested in boards. In the Class 10 practical, you will test HCl, NaOH, and other solutions with litmus, methyl orange, and phenolphthalein, then record observations in a table — this exact table appears in the practical file and viva.

Daily-Life Applications of Acids, Bases and Salts: Case-Based Question Favourite

CBSE loves to test Acids, Bases and Salts Class 10 through case-based or assertion-reason questions that link chemistry to real-world scenarios. These carry 4-5 marks and require you to read a passage, then answer sub-questions on pH, reactions, and uses. Common themes: (1) Indigestion and antacids: The stomach produces HCl (pH ~1.2) for digestion. Overeating or stress leads to excess acid, causing pain. Antacids contain weak bases like Mg(OH)₂ (milk of magnesia) or Al(OH)₃ that neutralise excess HCl without making the stomach too alkaline. Equation: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. (2) Tooth decay: Mouth bacteria metabolise sugars to produce acids; when pH falls below 5.5, tooth enamel (calcium phosphate) begins to dissolve. Toothpaste contains mild bases (often NaHCO₃ or CaCO₃) that neutralise the acid and raise pH, preventing decay. (3) Soil treatment: Acidic soil (pH < 6) is treated with slaked lime Ca(OH)₂ or powdered limestone CaCO₃ to raise pH. Conversely, basic soil is treated with organic matter or gypsum (CaSO₄·2H₂O). (4) Bee and ant stings: Bee sting injects methanoic acid (formic acid); rubbing baking soda (NaHCO₃, a base) neutralises it and reduces pain. Ant sting also contains formic acid; same remedy. (5) Factory effluents: Acidic waste is neutralised with cheap bases like lime before release into rivers, preventing aquatic life damage. (6) Baking: Baking powder is a mixture of NaHCO₃ (base) and a weak acid (tartaric acid, citric acid). When moistened and heated, they react to produce CO₂, which makes the cake rise. Equation: NaHCO₃ + H⁺ (from tartaric acid) → Na⁺ + H₂O + CO₂↑. Know these applications with equations and pH values; they appear verbatim in CBSE sample papers.

Antacids (Mg(OH)₂, Al(OH)₃) neutralise excess stomach HCl; equation: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
Toothpaste (pH ~9, contains bases) neutralises mouth acid (pH <5.5) to prevent enamel decay (calcium phosphate dissolution).
Baking powder: NaHCO₃ + weak acid → CO₂ (makes cake fluffy); equation example with tartaric acid.
Soil pH correction: acidic soil + Ca(OH)₂ (slaked lime) → neutral; basic soil + organic matter or CaSO₄·2H₂O.
Bee sting (methanoic acid) relief: apply NaHCO₃ paste (base) to neutralise acid and reduce pain.
Water treatment: chlorination with bleaching powder Ca(OCl)Cl for disinfection, maintaining safe pH 6.5-8.5 for drinking.

Important Questions for Acids, Bases and Salts Class 10 Board Exam

Based on the 2024 and 2025 CBSE board papers and official sample papers, here are high-yield question types for Acids, Bases and Salts Class 10. (1 mark MCQ): 'Which of the following salts will turn blue litmus red when dissolved in water? (a) Na₂CO₃ (b) NaCl (c) NH₄Cl (d) KNO₃' — Answer: (c) NH₄Cl (acidic salt). (2 marks VSA): 'Define water of crystallisation. Give one example with chemical formula.' Answer: see earlier section, CuSO₄·5H₂O or CaSO₄·2H₂O. (2 marks): 'What happens when copper oxide reacts with dilute HCl? Write the equation.' Answer: black CuO dissolves in dilute HCl to form blue-green CuCl₂ solution; CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l). (3 marks SA): 'Write the chemical equations for the reactions of (a) zinc with dilute sulphuric acid, (b) sodium carbonate with dilute hydrochloric acid, (c) calcium hydroxide with carbon dioxide.' Answer: (a) Zn + H₂SO₄ → ZnSO₄ + H₂↑; (b) Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑; (c) Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. (3 marks): 'How is plaster of Paris prepared? Write the equation and give one use.' Answer: by heating gypsum to 373 K; CaSO₄·2H₂O → CaSO₄·½H₂O + 1.5H₂O. Use: making casts for broken limbs, statues, moulds. (5 marks case-based): A passage on pH of rainwater, soil, and neutralisation, followed by four sub-questions (1+1+1+2 marks). Practice such cases from CBSE sample papers. (5 marks): 'Explain how common salt (NaCl) is used as the starting material to produce (a) sodium hydroxide, (b) baking soda, (c) washing soda, (d) bleaching powder. Write one use of each.' Answer: outline the chlor-alkali process, Solvay process (simplified), heating and recrystallisation for washing soda, and Cl₂ + Ca(OH)₂ for bleaching powder, with uses. Assertion-Reason (1 mark): 'Assertion: Plaster of Paris should be stored in a moisture-proof container. Reason: Plaster of Paris absorbs moisture to form gypsum, making it useless for casting.' Answer: Both A and R true, R is correct explanation of A. Past toppers report that writing clean, balanced equations with states and clear observations fetches full marks even if theory explanation is brief. Practice writing equations fast and accurately.

1-mark MCQ on salt classification (acidic/basic/neutral) or indicator colour change.
2-mark definition questions: water of crystallisation, pH, neutralisation, with one example.
3-mark equation-based: write balanced equation + observation for acid-metal, acid-carbonate, neutralisation.
3-mark preparation method: plaster of Paris from gypsum, washing soda from baking soda (with equations).
5-mark case study: read a passage on pH in daily life, answer 4 sub-questions (MCQ, VSA, reasoning).
5-mark long answer: NaCl family (four products, equations, one use each) or chemical properties of acids/bases with examples.

How CBSETUTOR.ai Helps You Master Acids, Bases and Salts Class 10

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Practice unlimited MCQs and 3/5-mark questions on chemical properties, pH scale, salts, and daily-life applications, auto-graded with explanations.
Compare concepts side-by-side: washing soda vs baking soda, acidic vs basic salts, with formula, preparation, and uses in one table.
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Frequently asked questions

How many marks does Acids, Bases and Salts Class 10 carry in the CBSE board exam?+

Acids, Bases and Salts typically carries 8-10 marks in the 80-mark CBSE Class 10 Science theory paper. This breaks down into 1-2 MCQs (1 mark each), one 2-mark VSA (definition or example), one 3-mark SA (equation with observation), and one 4-5 mark case-based or long answer question linking pH, salts, and daily-life applications. The chapter also contributes 2 marks to the practical exam (properties of acids and bases using indicators and reactions).

What is the difference between washing soda and baking soda in terms of formula and use?+

Baking soda is sodium hydrogen carbonate, NaHCO₃, a mild base used in baking powder (releases CO₂ on heating), antacids, and fire extinguishers. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, a stronger base used to remove permanent hardness of water, in glass and soap manufacture, and as a cleaning agent. Washing soda is prepared by heating baking soda (2NaHCO₃ → Na₂CO₃ + H₂O + CO₂) followed by recrystallisation with water to get the decahydrate form.

Why does tooth decay begin when pH in the mouth falls below 5.5?+

Tooth enamel is made of calcium phosphate, which is insoluble at neutral pH but begins to dissolve (demineralise) when the pH drops below 5.5 due to acids produced by bacteria metabolising sugars. The acidic environment attacks the enamel, leading to cavities. Toothpaste contains mild bases (like NaHCO₃ or fluoride compounds) that neutralise the acid, raise the pH back above 5.5, and help remineralise the enamel, preventing decay.

How do I write the balanced equation for the reaction of sodium carbonate with dilute HCl?+

The balanced equation is: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)↑. Observation: brisk effervescence (bubbles of colourless CO₂ gas), and if you pass the gas through lime water (Ca(OH)₂), it turns milky due to formation of CaCO₃ precipitate. Always include states of matter: (s) solid, (aq) aqueous, (l) liquid, (g) gas, and use ↑ or ↓ to show gas evolution or precipitate formation for full marks.

What is water of crystallisation? Give two examples from the NCERT textbook.+

Water of crystallisation is the fixed number of water molecules chemically combined in the crystal structure of a salt, represented in the formula after a dot. Two NCERT examples: (1) Copper sulphate pentahydrate, CuSO₄·5H₂O (blue crystals; loses water on heating to form white anhydrous CuSO₄). (2) Gypsum, CaSO₄·2H₂O (used to make plaster of Paris by heating to 373 K, producing CaSO₄·½H₂O + 1.5H₂O). Washing soda (Na₂CO₃·10H₂O) and ferrous sulphate (FeSO₄·7H₂O) are also valid examples.

Why is NaCl classified as a neutral salt while Na₂CO₃ is a basic salt?+

NaCl is formed from strong acid (HCl) and strong base (NaOH); its aqueous solution has pH ~7 (neutral) because neither the Na⁺ cation nor the Cl⁻ anion hydrolyses water. Na₂CO₃ is formed from weak acid (H₂CO₃, carbonic acid) and strong base (NaOH); the carbonate ion (CO₃²⁻) hydrolyses water to produce OH⁻ ions, making the solution basic (pH ~11). The rule is: strong acid + strong base → neutral salt; weak acid + strong base → basic salt; strong acid + weak base → acidic salt.

How is plaster of Paris prepared from gypsum, and what is its chemical formula?+

Plaster of Paris is prepared by heating gypsum (CaSO₄·2H₂O) to 373 K (100°C). The equation is: CaSO₄·2H₂O(s) --heat(373K)--> CaSO₄·½H₂O(s) + 1½H₂O(g). The product, calcium sulphate hemihydrate (CaSO₄·½H₂O), is plaster of Paris. When mixed with water, it re-hydrates to form gypsum (CaSO₄·2H₂O) and sets into a hard mass, used for making casts for broken bones, statues, and decorative materials. Store it in a moisture-proof container to prevent premature setting.

What happens when you pass carbon dioxide gas through lime water? Write the equation and observation.+

When CO₂ is passed through lime water (calcium hydroxide solution, Ca(OH)₂), it reacts to form calcium carbonate, which is insoluble and makes the solution milky (turbid). Equation: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l). Observation: the clear lime water turns milky due to white CaCO₃ precipitate. If excess CO₂ is passed, the milkiness disappears because CaCO₃ reacts further to form soluble calcium hydrogen carbonate: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂(aq). This two-step test is used to confirm the presence of CO₂ gas.

Why does blue CuSO₄·5H₂O turn white on heating, and what is the white powder?+

Blue copper sulphate pentahydrate (CuSO₄·5H₂O) contains water of crystallisation. On heating, the five water molecules are driven off, and the compound loses its blue colour, forming white anhydrous copper sulphate (CuSO₄). Equation: CuSO₄·5H₂O(s) [blue] --heat--> CuSO₄(s) [white] + 5H₂O(g). If you add a few drops of water to the white powder, it turns blue again, proving the water was chemically bound (not just surface moisture) and the process is reversible. This is a standard test for water of crystallisation and often appears as a 2-mark question or practical viva.

What is the pH of human blood, and why is it important to maintain it within a narrow range?+

Human blood has a pH of 7.35-7.45, which is slightly basic (alkaline). This narrow range is maintained by buffer systems (mainly bicarbonate buffer: H₂CO₃/HCO₃⁻) in the blood. If pH falls below 7.35 (acidosis), it can cause fatigue, confusion, and in severe cases, coma. If pH rises above 7.45 (alkalosis), it can lead to muscle twitching, nausea, and seizures. Enzymes and metabolic processes function optimally only within this pH range, so the body tightly regulates it through breathing (expelling CO₂) and kidney function (excreting H⁺ or HCO₃⁻). This concept links chemistry to biology and is a favourite in case-based questions.

How does baking powder make a cake rise? Explain with a chemical equation.+

Baking powder is a mixture of baking soda (NaHCO₃, a base) and a weak acid (usually tartaric acid or citric acid). When moistened and heated during baking, the acid reacts with NaHCO₃ to produce carbon dioxide gas, which forms bubbles in the batter, making the cake fluffy. A simplified equation with tartaric acid (C₄H₆O₆) is: NaHCO₃ + H⁺ (from tartaric acid) → Na⁺ (salt) + H₂O + CO₂(g)↑. The CO₂ gas expands on heating, causing the cake to rise. This reaction must occur during baking, not before, so baking powder is stored dry and activates only with moisture and heat.

Why do CBSE examiners deduct marks if I write 'H₂SO₄' instead of 'dilute sulphuric acid' in an equation?+

CBSE marking schemes are strict about using the terms from NCERT. 'Dilute sulphuric acid' or 'dilute H₂SO₄' specifies the concentration, which affects the reaction. For example, dilute H₂SO₄ reacts with zinc to give ZnSO₄ + H₂, but concentrated H₂SO₄ reacts differently (produces SO₂, not H₂) and acts as an oxidising agent. Writing just 'H₂SO₄' is ambiguous. Similarly, always write 'sodium carbonate' as Na₂CO₃ (or specify Na₂CO₃·10H₂O for washing soda) and include states of matter (s, l, g, aq). Each missing detail can cost 0.5 marks in a 3-mark question. Follow NCERT language exactly, and you will never lose marks for terminology.

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