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Acids, Bases and Salts

NCERT Class 10 · Science Based on NCERT Class 10 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the chemical properties of acids and bases, how they react with metals, carbonates, and each other through neutralisation reactions. It explains the concept of pH and its importance in everyday life, from digestion to tooth decay. The chapter also covers the formation, properties, and uses of various salts like baking soda, washing soda, bleaching powder, and Plaster of Paris. The one big idea a student must walk away with is that acids produce H⁺ ions in water, bases produce OH⁻ ions, and their interaction leads to the formation of salts and water, with pH being a measure of their strength.
Key Concepts (19)
Acid
A substance that is sour in taste, turns blue litmus red, and produces H⁺ (or H₃O⁺) ions in aqueous solution.
e.g. Hydrochloric acid (HCl) in the stomach helps in digestion.
Base
A substance that is bitter in taste, turns red litmus blue, and produces OH⁻ ions in aqueous solution.
e.g. Sodium hydroxide (NaOH) is used in soap making.
Indicator
A dye or mixture of dyes used to distinguish between acids and bases by a change in colour.
e.g. Litmus solution, extracted from lichen, turns red in acid and blue in base.
Olfactory indicator
A substance whose odour changes in acidic or basic media.
e.g. Onion, vanilla, and clove oil can be used as olfactory indicators.
Neutralisation reaction
The reaction between an acid and a base to give a salt and water.
e.g. NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)
Salt
A compound formed by the neutralisation reaction of an acid and a base.
e.g. Sodium chloride (NaCl) is a salt formed from HCl and NaOH.
pH scale
A scale for measuring hydrogen ion concentration in a solution, ranging from 0 (very acidic) to 14 (very alkaline), with 7 being neutral.
e.g. Lemon juice has a pH of about 2, while soap solution has a pH of about 10.
Universal indicator
A mixture of several indicators that shows different colours at different concentrations of hydrogen ions in a solution.
e.g. Universal indicator paper turns red in strong acid and violet in strong base.
Alkali
A base that is soluble in water.
e.g. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are alkalis.
Water of crystallisation
The fixed number of water molecules present in one formula unit of a salt.
e.g. Copper sulphate pentahydrate (CuSO₄·5H₂O) has 5 water molecules.
Brine
An aqueous solution of sodium chloride (NaCl) used in the chlor-alkali process.
e.g. Electrolysis of brine produces NaOH, Cl₂, and H₂.
Chlor-alkali process
The process of electrolysing aqueous sodium chloride (brine) to produce sodium hydroxide, chlorine gas, and hydrogen gas.
e.g. 2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)
Bleaching powder
A compound produced by the action of chlorine on dry slaked lime, represented as Ca(ClO)₂, used for bleaching and disinfection.
e.g. Used to make drinking water free from germs.
Baking soda
Sodium hydrogencarbonate (NaHCO₃), a mild non-corrosive basic salt used in cooking and as an antacid.
e.g. Used in baking powder to make cakes soft and spongy.
Washing soda
Sodium carbonate decahydrate (Na₂CO₃·10H₂O), a basic salt used as a cleaning agent and for removing permanent hardness of water.
e.g. Used in glass, soap, and paper industries.
Plaster of Paris
Calcium sulphate hemihydrate (CaSO₄·½H₂O), a white powder that forms a hard solid when mixed with water, used for supporting fractured bones.
e.g. Doctors use Plaster of Paris for plastering fractured bones.
Gypsum
Calcium sulphate dihydrate (CaSO₄·2H₂O), which on heating at 373 K gives Plaster of Paris.
e.g. Gypsum is used in making cement and fertilisers.
Acid rain
Rain water with a pH less than 5.6, caused by acidic gases in the atmosphere.
e.g. Acid rain lowers the pH of river water, making survival of aquatic life difficult.
Antacid
A mild base used to neutralise excess acid in the stomach.
e.g. Magnesium hydroxide (Milk of magnesia) is used as an antacid.
Formulas & Equations (14)
Acid + Metal → Salt + Hydrogen gas
Reads as: Acid plus metal gives salt plus hydrogen gas
Variables: Acid (e.g., HCl, H₂SO₄), Metal (e.g., Zn, Mg, Fe, Al), Salt (e.g., ZnCl₂, MgSO₄), Hydrogen gas (H₂)
Use when: When a metal reacts with an acid, e.g., Zn + H₂SO₄ → ZnSO₄ + H₂
⚠ Students forget to balance the equation or write the correct valency of the metal in the salt.
2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)
Reads as: Two moles of aqueous sodium hydroxide plus solid zinc gives solid sodium zincate plus hydrogen gas
Variables: NaOH (sodium hydroxide), Zn (zinc), Na₂ZnO₂ (sodium zincate), H₂ (hydrogen gas)
Use when: When a base reacts with a metal (e.g., zinc with sodium hydroxide)
⚠ Students think only acids react with metals; they forget that some bases also react with metals like zinc and aluminium.
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
Reads as: Metal carbonate or metal hydrogencarbonate plus acid gives salt plus carbon dioxide plus water
Variables: Metal carbonate (e.g., Na₂CO₃), Metal hydrogencarbonate (e.g., NaHCO₃), Acid (e.g., HCl), Salt (e.g., NaCl), CO₂ (carbon dioxide), H₂O (water)
Use when: When an acid reacts with a carbonate or hydrogencarbonate, e.g., Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
⚠ Students forget to write the correct state symbols or omit water in the products.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Reads as: Aqueous sodium hydroxide plus aqueous hydrochloric acid gives aqueous sodium chloride plus liquid water
Variables: NaOH (sodium hydroxide), HCl (hydrochloric acid), NaCl (sodium chloride), H₂O (water)
Use when: When an acid and a base neutralise each other
⚠ Students write the products incorrectly, e.g., forgetting water or writing the wrong salt.
Metal oxide + Acid → Salt + Water
Reads as: Metal oxide plus acid gives salt plus water
Variables: Metal oxide (e.g., CuO), Acid (e.g., HCl), Salt (e.g., CuCl₂), Water (H₂O)
Use when: When a metal oxide (basic oxide) reacts with an acid
⚠ Students think metal oxides do not react with acids; they forget that basic oxides neutralise acids.
HCl + H₂O → H₃O⁺ + Cl⁻
Reads as: Hydrochloric acid plus water gives hydronium ion plus chloride ion
Variables: HCl (hydrogen chloride), H₂O (water), H₃O⁺ (hydronium ion), Cl⁻ (chloride ion)
Use when: To show that acids produce H⁺ ions only in the presence of water
⚠ Students write H⁺ instead of H₃O⁺; exam expects H⁺(aq) or H₃O⁺.
NaOH(s) → Na⁺(aq) + OH⁻(aq)
Reads as: Solid sodium hydroxide dissociates in water to give sodium ions and hydroxide ions
Variables: NaOH (sodium hydroxide), Na⁺ (sodium ion), OH⁻ (hydroxide ion)
Use when: To show that bases produce OH⁻ ions in water
⚠ Students forget to write the state symbols or think bases do not dissociate.
H⁺(aq) + OH⁻(aq) → H₂O(l)
Reads as: Hydrogen ion plus hydroxide ion gives water
Variables: H⁺(aq) (hydrogen ion), OH⁻(aq) (hydroxide ion), H₂O(l) (water)
Use when: To represent the net ionic equation for neutralisation
⚠ Students write the full molecular equation instead of the net ionic form when asked.
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
Reads as: Two moles of aqueous sodium chloride plus two moles of liquid water gives two moles of aqueous sodium hydroxide plus chlorine gas plus hydrogen gas
Variables: NaCl (sodium chloride), H₂O (water), NaOH (sodium hydroxide), Cl₂ (chlorine), H₂ (hydrogen)
Use when: In the chlor-alkali process for electrolysis of brine
⚠ Students forget to balance the equation or write the wrong products.
2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O
Reads as: Two moles of calcium hydroxide plus two moles of chlorine gives calcium hypochlorite plus calcium chloride plus two moles of water
Variables: Ca(OH)₂ (slaked lime), Cl₂ (chlorine), Ca(ClO)₂ (bleaching powder), CaCl₂ (calcium chloride), H₂O (water)
Use when: For the manufacture of bleaching powder
⚠ Students write the formula of bleaching powder incorrectly as CaOCl₂ instead of Ca(ClO)₂.
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Reads as: Sodium chloride plus water plus carbon dioxide plus ammonia gives ammonium chloride plus sodium hydrogencarbonate
Variables: NaCl (sodium chloride), H₂O (water), CO₂ (carbon dioxide), NH₃ (ammonia), NH₄Cl (ammonium chloride), NaHCO₃ (sodium hydrogencarbonate)
Use when: For the production of baking soda
⚠ Students forget the reactants or products, especially ammonia.
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Reads as: Two moles of sodium hydrogencarbonate on heating gives sodium carbonate plus water plus carbon dioxide
Variables: NaHCO₃ (sodium hydrogencarbonate), Na₂CO₃ (sodium carbonate), H₂O (water), CO₂ (carbon dioxide)
Use when: When baking soda is heated during cooking
⚠ Students forget to write the heat symbol above the arrow.
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
Reads as: Sodium carbonate plus ten water molecules gives sodium carbonate decahydrate
Variables: Na₂CO₃ (sodium carbonate), H₂O (water), Na₂CO₃·10H₂O (washing soda)
Use when: For recrystallisation of sodium carbonate to obtain washing soda
⚠ Students forget the 10H₂O or think it makes the salt wet.
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Reads as: Calcium sulphate hemihydrate plus one and a half water molecules gives calcium sulphate dihydrate
Variables: CaSO₄·½H₂O (Plaster of Paris), H₂O (water), CaSO₄·2H₂O (gypsum)
Use when: When Plaster of Paris is mixed with water to form a hard solid
⚠ Students write the formula of Plaster of Paris as CaSO₄·H₂O instead of CaSO₄·½H₂O.
How It Works
Reaction of acids with metals
  1. Take a metal (e.g., zinc granules) in a test tube.
  2. Add dilute acid (e.g., H₂SO₄) to the metal.
  3. Observe bubbles of hydrogen gas on the surface of the metal.
  4. Pass the gas through soap solution to form bubbles.
  5. Bring a burning candle near a bubble; the gas burns with a pop sound, confirming hydrogen.
  6. The metal displaces hydrogen from the acid to form a salt and hydrogen gas.
💡 The gas produced is hydrogen, which burns with a pop sound. Not all metals react with all acids; reactivity series matters.
Reaction of bases with metals
  1. Place a few pieces of granulated zinc metal in a test tube.
  2. Add 2 mL of sodium hydroxide solution and warm the contents.
  3. Observe the evolution of hydrogen gas.
  4. Test the gas with a burning candle; it burns with a pop sound.
  5. The reaction produces sodium zincate (Na₂ZnO₂) and hydrogen gas.
💡 Only some metals like zinc and aluminium react with bases to produce hydrogen. Most metals do not react with bases.
Reaction of metal carbonates and hydrogencarbonates with acids
  1. Take sodium carbonate (Na₂CO₃) in test tube A and sodium hydrogencarbonate (NaHCO₃) in test tube B.
  2. Add dilute HCl to both test tubes.
  3. Observe effervescence due to carbon dioxide gas.
  4. Pass the gas through lime water (calcium hydroxide solution).
  5. Lime water turns milky due to formation of calcium carbonate (CaCO₃).
  6. If excess CO₂ is passed, the milkiness disappears due to formation of soluble calcium hydrogencarbonate.
💡 The gas is carbon dioxide, which turns lime water milky. Excess CO₂ makes the milkiness disappear.
Neutralisation reaction
  1. Take dilute NaOH solution in a test tube.
  2. Add two drops of phenolphthalein solution; the solution turns pink.
  3. Add dilute HCl drop by drop; the pink colour disappears, indicating neutralisation.
  4. Add a few drops of NaOH again; the pink colour reappears.
  5. The reaction between an acid and a base gives salt and water.
💡 Phenolphthalein is pink in basic solution and colourless in acidic/neutral solution. The colour change indicates the endpoint of neutralisation.
Reaction of metallic oxides with acids
  1. Take a small amount of copper oxide (CuO) in a beaker.
  2. Add dilute hydrochloric acid slowly while stirring.
  3. Observe the colour of the solution turning blue-green.
  4. Copper oxide dissolves, forming copper(II) chloride (CuCl₂) and water.
  5. Metallic oxides are basic oxides because they react with acids to give salt and water.
💡 Copper oxide is black; the blue-green colour of the solution confirms the formation of copper(II) chloride.
Reaction of non-metallic oxides with bases
  1. Pass carbon dioxide gas through lime water (calcium hydroxide solution).
  2. Lime water turns milky due to formation of calcium carbonate.
  3. This reaction is similar to an acid-base reaction, so non-metallic oxides are acidic in nature.
💡 Non-metallic oxides like CO₂ are acidic; they react with bases to form salt and water.
Conduction of electricity by acids and bases
  1. Fix two nails on a cork and place it in a beaker.
  2. Connect the nails to a 6V battery through a bulb and switch.
  3. Pour dilute HCl into the beaker and switch on the current.
  4. The bulb glows, indicating that the solution conducts electricity.
  5. Repeat with glucose and alcohol solutions; the bulb does not glow.
  6. Acids produce H⁺ ions in water, which carry electric current.
💡 Acids and bases conduct electricity only in aqueous solution due to the presence of ions. Glucose and alcohol do not ionise.
Formation of hydronium ions
  1. Take solid NaCl in a dry test tube and add concentrated H₂SO₄.
  2. A gas (HCl) is evolved.
  3. Test the gas with dry blue litmus paper; no colour change.
  4. Test the gas with wet blue litmus paper; it turns red.
  5. HCl gas produces H⁺ ions only in the presence of water, forming hydronium ions (H₃O⁺).
💡 Dry HCl gas does not show acidic behaviour; water is necessary for the formation of H⁺ ions.
Dilution of acids and bases
  1. Take 10 mL water in a beaker.
  2. Add a few drops of concentrated H₂SO₄ slowly with constant stirring.
  3. Touch the base of the beaker; it feels hot (exothermic process).
  4. Repeat with sodium hydroxide pellets; similar observation.
  5. Dilution decreases the concentration of H⁺ or OH⁻ ions per unit volume.
💡 Always add acid to water, not water to acid, to avoid splashing and burns due to excessive heat.
Testing pH using universal indicator
  1. Take the solution to be tested.
  2. Dip a universal indicator paper into the solution.
  3. Compare the colour change with the pH scale.
  4. pH 7 is neutral; pH < 7 is acidic; pH > 7 is basic.
  5. The colour indicates the strength of the acid or base.
💡 Lower pH means higher H⁺ ion concentration; higher pH means higher OH⁻ ion concentration.
Chlor-alkali process
  1. Take an aqueous solution of sodium chloride (brine).
  2. Pass electricity through the solution.
  3. Sodium hydroxide forms near the cathode.
  4. Chlorine gas is given off at the anode.
  5. Hydrogen gas is given off at the cathode.
  6. The products are all useful: NaOH, Cl₂, and H₂.
💡 The name 'chlor-alkali' comes from chlorine (chlor) and alkali (NaOH).
Manufacture of bleaching powder
  1. Take dry slaked lime [Ca(OH)₂].
  2. Pass chlorine gas over it.
  3. The reaction produces bleaching powder [Ca(ClO)₂], calcium chloride, and water.
💡 Bleaching powder is used for bleaching and disinfection; its formula is Ca(ClO)₂.
Manufacture of baking soda
  1. React sodium chloride with water, carbon dioxide, and ammonia.
  2. The products are ammonium chloride and sodium hydrogencarbonate (NaHCO₃).
  3. Sodium hydrogencarbonate is baking soda.
💡 Baking soda is a mild non-corrosive basic salt used in cooking and as an antacid.
Manufacture of washing soda
  1. Heat baking soda (NaHCO₃) to obtain sodium carbonate (Na₂CO₃).
  2. Recrystallise sodium carbonate from water to get washing soda (Na₂CO₃·10H₂O).
💡 Washing soda is used for cleaning and removing permanent hardness of water.
Water of crystallisation in copper sulphate
  1. Heat a few crystals of blue copper sulphate (CuSO₄·5H₂O) in a dry boiling tube.
  2. The blue colour fades and the salt turns white.
  3. Water droplets appear on the walls of the tube.
  4. Add 2-3 drops of water to the white sample; the blue colour reappears.
💡 The blue colour is due to water of crystallisation; heating removes it, and adding water restores it.
Preparation of Plaster of Paris
  1. Heat gypsum (CaSO₄·2H₂O) at 373 K.
  2. It loses water molecules and becomes calcium sulphate hemihydrate (CaSO₄·½H₂O), called Plaster of Paris.
  3. When mixed with water, it rehydrates to form gypsum, a hard solid.
💡 Plaster of Paris should be stored in a moisture-proof container because it reacts with moisture to form gypsum.
Exam Traps — Don't Make These Mistakes
⚠ Confusing acid and base with indicator colour change
✗ Acids turn red litmus blue.
→ Acids turn blue litmus red, not the other way around. Bases turn red litmus blue.
✓ Acids turn blue litmus red, and bases turn red litmus blue.
⚠ Writing H⁺ instead of H₃O⁺ or H⁺(aq)
✗ HCl gives H⁺ ions in water.
→ Hydrogen ions cannot exist alone; they combine with water to form hydronium ions (H₃O⁺). The correct representation is H⁺(aq) or H₃O⁺.
✓ HCl gives H⁺(aq) or H₃O⁺ ions in water.
⚠ Forgetting that dry HCl gas does not show acidic behaviour
✗ Dry HCl gas turns blue litmus red.
→ Dry HCl gas does not produce H⁺ ions in the absence of water; therefore, it does not change the colour of dry litmus paper.
✓ Dry HCl gas does not change the colour of dry litmus paper because H⁺ ions are produced only in the presence of water.
⚠ Mixing up the products of the chlor-alkali process
✗ Electrolysis of brine gives NaOH at the anode and Cl₂ at the cathode.
→ Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide forms near the cathode.
✓ In the chlor-alkali process, chlorine gas is produced at the anode, hydrogen gas at the cathode, and sodium hydroxide near the cathode.
⚠ Incorrect formula for bleaching powder
✗ Bleaching powder is CaOCl₂.
→ Though sometimes represented as CaOCl₂, the correct representation in NCERT is Ca(ClO)₂, as the actual composition is complex.
✓ Bleaching powder is represented as Ca(ClO)₂.
⚠ Confusing baking soda with washing soda
✗ Baking soda is Na₂CO₃.
→ Baking soda is sodium hydrogencarbonate (NaHCO₃), while washing soda is sodium carbonate decahydrate (Na₂CO₃·10H₂O).
✓ Baking soda is NaHCO₃, and washing soda is Na₂CO₃·10H₂O.
Compare & Contrast
Taste and litmus test
Acid
Base
Acids are sour and turn blue litmus red; bases are bitter and turn red litmus blue.
Number of H⁺ ions produced in solution
Strong acid
Weak acid
Strong acids (e.g., HCl) give rise to more H⁺ ions in solution, while weak acids (e.g., acetic acid) give rise to fewer H⁺ ions for the same concentration.
Chemical formula and use
Baking soda
Washing soda
Baking soda is NaHCO₃, used in cooking and as an antacid; washing soda is Na₂CO₃·10H₂O, used as a cleaning agent and for removing permanent hardness of water.
Water of crystallisation and formation
Gypsum
Plaster of Paris
Gypsum is CaSO₄·2H₂O; on heating at 373 K, it loses water to form Plaster of Paris (CaSO₄·½H₂O). Plaster of Paris rehydrates to form gypsum when mixed with water.
Exam Focus
Marks pattern: This chapter typically appears in board exams as: 2-3 MCQs (1 mark each), 1-2 Very Short Answer questions (1 mark each), 1-2 Short Answer questions (2-3 marks each), and 1 Long Answer question (5 marks). Total weightage is approximately 8-10 marks.
Most asked: Properties of acids and bases (taste, litmus test, reaction with metals, carbonates, and each other) · pH scale and its importance in everyday life (digestion, tooth decay, acid rain) · Neutralisation reaction and its applications · Chlor-alkali process and products (NaOH, Cl₂, H₂) · Water of crystallisation and Plaster of Paris · Uses of baking soda, washing soda, and bleaching powder
Don't skip: Write balanced chemical equations for reactions of acids and bases with metals, carbonates, and hydrogencarbonates.; Explain the pH scale and its importance in everyday life with examples.; Describe the chlor-alkali process and list the uses of its products.; Differentiate between baking soda and washing soda with their chemical formulae and uses.; Explain water of crystallisation with the example of copper sulphate and Plaster of Paris.
One-Line Revision (read aloud before the exam)
  1. Acids turn blue litmus red, bases turn red litmus blue.
  2. Acids produce H⁺(aq) or H₃O⁺ ions in water; bases produce OH⁻ ions in water.
  3. Neutralisation reaction: Acid + Base → Salt + Water.
  4. Reaction of acid with metal: Acid + Metal → Salt + Hydrogen gas.
  5. Reaction of acid with metal carbonate/hydrogencarbonate: Acid + Carbonate/Hydrogencarbonate → Salt + CO₂ + H₂O.
  6. Metal oxides are basic; non-metal oxides are acidic.
  7. pH scale ranges from 0 to 14; pH 7 is neutral, pH < 7 is acidic, pH > 7 is basic.
  8. Lower pH means higher H⁺ ion concentration; higher pH means higher OH⁻ ion concentration.
  9. Dry HCl gas does not show acidic behaviour because H⁺ ions are produced only in the presence of water.
  10. Always add acid to water slowly with constant stirring when diluting, never water to acid.
  11. Chlor-alkali process: Electrolysis of brine gives NaOH, Cl₂, and H₂.
  12. Bleaching powder is Ca(ClO)₂, used for bleaching and disinfection.
  13. Baking soda is NaHCO₃; washing soda is Na₂CO₃·10H₂O.
  14. Water of crystallisation is the fixed number of water molecules in one formula unit of a salt.
  15. Copper sulphate crystals (CuSO₄·5H₂O) are blue; on heating, they lose water and turn white.
  16. Plaster of Paris is CaSO₄·½H₂O; it forms a hard solid when mixed with water.
  17. Gypsum is CaSO₄·2H₂O; heating at 373 K gives Plaster of Paris.
  18. Antacids like milk of magnesia neutralise excess stomach acid.
  19. Tooth decay starts when mouth pH is below 5.5; toothpastes are basic to neutralise acid.
  20. Acid rain has pH less than 5.6 and harms aquatic life.

MCQs — 10 Questions with Answers

Q1. A student adds zinc granules to dilute hydrochloric acid in a test tube and observes vigorous bubbling. When a burning candle is brought near the gas collected, it burns with a 'pop' sound. What does this observation indicate about the product formed?

  • A. Hydrogen gas is produced because the metal displaces hydrogen from the acid ✓
  • B. Oxygen gas is produced because the acid oxidizes the metal
  • C. Carbon dioxide gas is produced from the reaction between acid and carbonate impurities
  • D. Chlorine gas is produced from decomposition of hydrochloric acid

Answer: A — The 'pop' sound is the characteristic test for hydrogen gas; zinc displaces hydrogen from HCl to form zinc chloride and H₂, not oxygen (no burning candle test) or CO₂ (no pop sound with burning candle).

Q2. When red litmus paper is dipped into an unknown solution and turns blue, while blue litmus paper shows no change, which conclusion can be drawn?

  • A. The solution is basic because red litmus turns blue only in basic conditions ✓
  • B. The solution is acidic because it affects one type of litmus paper
  • C. The solution is neutral because only one litmus changes color
  • D. The solution is a salt because it turns red litmus blue

Answer: A — Red litmus turning blue is the definitive test for a basic solution; blue litmus turning red would indicate acidic conditions, so no change in blue litmus confirms basicity, not neutrality or acidity.

Q3. Assertion (A): When granulated zinc is added to sodium hydroxide solution and the mixture is warmed, hydrogen gas is evolved. Reason (R): Bases react with all metals to produce hydrogen gas and a salt. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false ✓
  • D. A is false but R is true

Answer: C — The assertion is true (zinc reacts with NaOH to produce H₂ and sodium zincate), but the reason is false because bases do not react with all metals—only amphoteric metals like zinc and aluminum react with bases to produce hydrogen.

Q4. A homeowner uses turmeric powder solution to test whether the dishwashing liquid is basic or acidic. The solution changes from yellow to reddish-brown. What does this color change indicate?

  • A. The dishwashing liquid is basic, making turmeric an effective indicator in basic media ✓
  • B. The dishwashing liquid is acidic, and turmeric only works in acidic conditions
  • C. The dishwashing liquid is neutral, and turmeric changes color at neutral pH
  • D. The turmeric has decomposed due to the temperature of the solution

Answer: A — Turmeric turns reddish-brown in basic solutions; dishwashing liquids are indeed basic (alkaline), making this a correct application of natural indicators to identify basic substances in daily life.

Q5. Assertion (A): Litmus solution is purple in color when it is in a neutral solution. Reason (R): Litmus changes color only in the presence of acids or bases, not in neutral solutions. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Litmus solution is indeed purple in neutral conditions; this occurs precisely because litmus only changes color (to red in acid, blue in base) when pH deviates from neutral, making the reason the correct explanation.

Q6. Two cloth strips soaked in onion juice show different odors when exposed to dilute HCl and dilute NaOH respectively. After rinsing with water, both strips regain their original onion smell. What does this observation reveal about onion as an olfactory indicator?

  • A. Onion can be used as an olfactory indicator because its odor is reversibly modified in acidic and basic solutions ✓
  • B. Onion cannot be used as an olfactory indicator because the odor change is temporary
  • C. Onion is a permanent indicator because the odor change never returns to normal
  • D. Onion only works as an indicator in acidic solutions, not basic ones

Answer: A — The reversible change in odor upon exposure to acids/bases and recovery after rinsing demonstrates that onion is an effective olfactory indicator; permanence is not required for indicator use, only detectable change upon reaction.

Q7. Assertion (A): When carbon dioxide gas is passed through lime water, a white precipitate forms, but if excess carbon dioxide is continued to be passed, the precipitate dissolves. Reason (R): Calcium carbonate is insoluble in water, but calcium bicarbonate formed by excess CO₂ is soluble. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Both statements are true: Ca(OH)₂ + CO₂ → CaCO₃ (white ppt) + H₂O, and with excess CO₂, CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble); the reason correctly explains why the precipitate dissolves.

Q8. A student performs Activity 2.3 using three different acids: dilute HCl, dilute H₂SO₄, and dilute CH₃COOH. In all three cases, zinc granules produce bubbles that pop when a burning candle is brought near. What conclusion about acid strength can be drawn from identical observations?

  • A. All three acids have the same strength because they all produce hydrogen gas with zinc
  • B. Strong acids (HCl and H₂SO₄) are more reactive than weak acids (CH₃COOH), though all produce hydrogen
  • C. The observation alone cannot determine acid strength; identical gas production does not indicate identical reactivity rates ✓
  • D. Weak acids like acetic acid do not produce hydrogen gas, so the observation contradicts the results

Answer: C — While all three acids produce hydrogen gas (same product), the rate and vigor of the reaction differ based on acid strength; observation of gas production alone is insufficient to rank acid strengths without measuring reaction rates.

Q9. Assertion (A): Phenolphthalein is colorless in acidic solutions and pink in basic solutions. Reason (R): Phenolphthalein is a synthetic indicator that responds to pH changes because its molecular structure changes in different pH environments. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — The assertion correctly describes phenolphthalein's color changes; the reason correctly explains that structural changes in the indicator molecule at different pH values cause the color shift from colorless to pink.

Q10. A laboratory assistant accidentally mixed sodium carbonate and sodium bicarbonate solutions. When dilute HCl is added to the mixture, vigorous bubbling occurs and gas is evolved. After collecting the gas and passing it through lime water, a white precipitate forms. What is the gas produced and why does lime water confirm its identity?

  • A. Hydrogen gas is produced because metals in carbonates displace hydrogen from HCl
  • B. Carbon dioxide gas is produced; lime water test confirms it because CO₂ reacts with Ca(OH)₂ to form white CaCO₃ precipitate ✓
  • C. Oxygen gas is produced from decomposition of the carbonates at room temperature
  • D. Chlorine gas is produced from the reaction between HCl and carbonate salts

Answer: B — Both Na₂CO₃ and NaHCO₃ react with HCl to produce CO₂; the white precipitate with lime water (Ca(OH)₂ → CaCO₃) is the classic confirmatory test for CO₂, not H₂, O₂, or Cl₂.

Flashcards

What do you observe when a burning candle is brought near H₂ gas produced from metal + acid?

The candle burns with a pop sound because H₂ gas is flammable.

Why does phenolphthalein turn pink in a basic solution?

Phenolphthalein is a synthetic indicator that changes color in the presence of OH⁻ ions (bases).

Write the general reaction equation for metal + acid.

Acid + Metal → Salt + Hydrogen gas (e.g., HCl + Zn → ZnCl₂ + H₂).

What is the difference between a natural and synthetic acid-base indicator?

Natural indicators like turmeric and litmus come from plants, while synthetic indicators like phenolphthalein and methyl orange are chemically manufactured.

When Na₂CO₃ reacts with dilute HCl, what gas is produced and how is it identified?

Carbon dioxide gas (CO₂) is produced; it turns lime water milky white as a positive test.

Why does zinc react with both dilute HCl and dilute NaOH to produce H₂ gas?

Zinc is amphoteric—it reacts with acids to displace H⁺ ions and with bases to displace H⁻ ions, both producing H₂ gas.

What does an olfactory indicator do?

An olfactory indicator changes its odor in acidic or basic media (e.g., onion smell changes in acid/base).

If you have only red litmus paper and three unknown solutions (acid, base, water), how will you identify each?

The acid will turn red litmus red, the base will NOT change red litmus, and water will NOT change red litmus (same as base result, so test base with blue litmus separately).

What happens to the color of turmeric stain when basic soap is scrubbed on it?

Turmeric stain turns reddish-brown in basic solution and returns to yellow when washed with water.

Write the equation for the reaction of sodium hydroxide with zinc metal.

2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) (sodium zincate is formed).

Important Board Questions

What do you observe when dilute hydrochloric acid is added to zinc granules? Write the equation for this reaction and name the gas produced. [2 marks]

Observe bubbling/effervescence on zinc surface; identify the gas using burning candle (pop sound means H₂); write Metal + Acid → Salt + H₂.

Explain why a student who has heartburn (excess stomach acid) is advised to take a spoonful of baking soda solution instead of lemon juice. Write the relevant chemical equation. [3 marks]

Baking soda is basic and neutralizes excess acid; lemon juice is acidic and worsens the problem; use Acid + Base → Salt + H₂O equation (NaHCO₃ + HCl → NaCl + H₂O + CO₂).

You are provided with three test tubes containing: (A) dilute HCl, (B) dilute NaOH, and (C) distilled water. Using only phenolphthalein solution, explain how you would identify each solution. Also describe what happens when the solution from test tube A is added dropwise to the solution from test tube B. [5 marks]

Phenolphthalein turns pink in base (B) only; colorless in acid (A) and water (C); when A is added to B, neutralization occurs—pink color fades as OH⁻ ions are consumed by H⁺ ions, forming salt and water; write the neutralization equation.

Assertion-Reason Questions for Acids, Bases and Salts

Q1. Assertion (A):

Litmus is a natural indicator that changes colour in the presence of acids and bases.

Reason (R):

Litmus is a purple dye extracted from lichen and shows different colours in acidic and basic solutions.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. Litmus does change colour with acids and bases, and the reason correctly explains why – it is a natural dye extracted from lichen that responds to pH changes. The reason is the correct explanation for the assertion.

Q2. Assertion (A):

When zinc metal is added to dilute sulphuric acid, hydrogen gas is produced.

Reason (R):

Zinc is more reactive than hydrogen, so it displaces hydrogen from the acid to form hydrogen gas and zinc sulphate.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. Zinc does produce hydrogen gas when added to dilute sulphuric acid, and this occurs because zinc is more reactive than hydrogen and can displace it from the acid. The reason correctly explains the mechanism behind the assertion.

Q3. Assertion (A):

Carbon dioxide gas is produced when sodium carbonate reacts with dilute hydrochloric acid.

Reason (R):

The lime water test turns blue when carbon dioxide gas is passed through it.

Answer: C — A is true but R is false — The assertion is true – sodium carbonate does react with dilute HCl to produce CO₂ gas. However, the reason is false. Lime water turns white (not blue) due to the formation of white calcium carbonate precipitate. The white precipitate is the confirmatory test for CO₂, not a blue colour change.

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Case-Based Questions (CBQ) for Acids, Bases and Salts

Case Study:

A student conducted an experiment in the chemistry laboratory to test the properties of different household solutions. She took five test tubes containing solutions of: (1) lemon juice, (2) vinegar, (3) baking soda solution, (4) soap solution, and (5) distilled water. She added a few drops of methyl orange indicator to each test tube one by one. The results showed that lemon juice and vinegar turned the indicator red, baking soda solution turned it yellow, soap solution turned it orange-yellow, and distilled water remained orange. The student then added dilute sodium hydroxide solution to the acidic solutions and observed colour changes.

Q1. Which of the following solutions are acidic in nature based on the methyl orange test?

  • A. Only lemon juice
  • B. Only vinegar
  • C. Lemon juice and vinegar ✓
  • D. Lemon juice, vinegar, and soap solution

Answer: C

Q2. Why did the baking soda solution turn the methyl orange indicator yellow?

  • A. Because baking soda is acidic
  • B. Because baking soda is basic and changes the colour of methyl orange ✓
  • C. Because baking soda has no effect on indicators
  • D. Because the indicator was defective

Answer: B

Q3. What would be observed if the student added the baking soda solution to a mixture of lemon juice and vinegar?

  • A. The mixture would become more acidic
  • B. No reaction would occur
  • C. The mixture would become neutral or slightly basic as acids and base neutralize ✓
  • D. The colour would remain red

Answer: C

Q4. Which solution from the experiment could be used as a remedy for acidity according to the information given?

  • A. Baking soda solution ✓
  • B. Vinegar
  • C. Lemon juice
  • D. Distilled water

Answer: A

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Previous Year Questions (PYQ) for Acids, Bases and Salts

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 31/4/1 · 1 mark

An aqueous solution turns red litmus blue. Excess addition of which of the following solution would reverse the change?

  • Sodium hydroxide
  • Hydrochloric acid
  • Baking soda
  • Slaked lime

CBSE 2026 · Set 31/4/1 · 1 mark

While washing dirty clothes by soap, micelles are formed. Select the correct statement about micelles.

  • The hydrophobic tail of the soap molecule is in the interior of the cluster, whereas hydrophilic end is on the surface of the cluster.
  • The hydrophobic tail is on the surface of the cluster, whereas hydrophilic end is in the interior of cluster.
  • Both hydrophobic tail and hydrophilic end are in the interior of the cluster.
  • Both hydrophobic tail and hydrophilic end are on the surface of the cluster.

CBSE 2026 · Set 31/4/1 · 1 mark

How will you protect yourself from the heat generated while diluting a concentrated acid?

  • By adding acid to water with constant stirring.
  • By adding water to acid with constant stirring.
  • By adding water to acid followed by base.
  • By adding base to acid with constant stirring.

CBSE 2026 · Set 31/4/1 · 1 mark

(a) A compound 'A' is used in soda-lime fire-extinguisher and decomposes on heating to form compound 'B'. Compound 'B' is used for removing permanent hardness of water. Identify 'A' and 'B'.

CBSE 2026 · Set 31/4/1 · 4 marks

The pH plays an important role in our daily life. Strength of acids and bases depends on the number of H+ ions and OH– ions produced by these respectively. pH scale helps in predicting the strength of acids and bases by measuring hydrogen ion concentration in a solution.

(a) What type of indicator is used to predict how strong an acid or base is?

CBSE 2026 · Set 31/4/2 · 2 marks

Write the balanced chemical equations involved in the preparation of (a) Bleaching powder (b) Baking soda

CBSE 2026 · Set 31/4/3 · 1 mark

The volume ratio of hydrogen and oxygen gases liberated during electrolysis of water is

  • 1 : 1
  • 2 : 1
  • 4 : 1
  • 1 : 2

CBSE 2026 · Set 31/4/3 · 1 mark

Which one of the following chemicals is used in soda-acid fire-extinguishers?

  • Sodium carbonate
  • Sodium hydrogen carbonate
  • Sodium hydroxide
  • Sodium acetate

CBSE 2026 · Set 31/4/3 · 2 marks

Write the balanced chemical equation for the preparation of: (a) Sodium hydroxide by electrolysis of brine solution (b) Sodium carbonate

CBSE 2026 · Set 31/5/1 · 1 mark

Which of the following is an amphoteric oxide?

  • Na2O
  • K2O
  • CO2
  • Al2O3

CBSE 2026 · Set 31/5/1 · 1 mark

Study the following table and select the correct option:

  • NaCl, HCl, NaOH - Basic
  • Na2CO3, H2CO3, NaOH - Neutral
  • Na2SO4, H2SO4, NaOH - Acidic
  • CH3COONa, CH3COOH, NaOH - Basic

CBSE 2026 · Set 31/5/1 · 1 mark

Four solutions, namely fructose, ethanol, hydrochloric acid and sodium hydroxide filled in four separate beakers are connected one by one in an electric circuit with a bulb. The solutions in which the bulb will glow when current is passed are:

  • ethanol and hydrochloric acid
  • fructose and ethanol
  • fructose and sodium hydroxide
  • hydrochloric acid and sodium hydroxide

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Class 10 Science Formula Sheet

Every formula and chemical equation from the 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

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Class 10 Science Previous Year Questions, Chapter-wise

All 875 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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