Acids, Bases and Salts · Class 10 Science: 88 flashcards
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1. What happens to blue litmus in an acid, and to red litmus in a base?
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Blue litmus turns red in acid; red litmus turns blue in base.
2. What colour is phenolphthalein in (a) acid, (b) base?
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(a) Colourless in acid, (b) pink in base.
3. What colour is methyl orange in (a) acid, (b) base?
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(a) Red in acid, (b) yellow in base.
4. What are olfactory indicators? Give two examples.
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Substances whose smell changes in acidic or basic solutions — e.g., onion and vanilla essence (also clove oil); their smell fades in a base.
5. Write the general reaction of an acid with a metal, with an example.
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Acid + metal → salt + hydrogen gas. Example: Zn + 2HCl → ZnCl₂ + H₂↑.
6. How is hydrogen gas tested?
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Bring a burning candle or splint near the gas — hydrogen burns with a 'pop' sound.
7. Write the reaction of sodium carbonate with dilute HCl.
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Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑.
8. How is carbon dioxide gas tested?
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Pass it through limewater — the limewater turns milky due to formation of insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
9. What happens when EXCESS CO₂ is passed through milky limewater? Write the equation.
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The milkiness disappears because soluble calcium hydrogencarbonate forms: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂.
10. Write the reaction of copper oxide with hydrochloric acid, with observations.
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CuO + 2HCl → CuCl₂ + H₂O. The black copper oxide dissolves and the solution becomes blue-green.
11. What does the reaction of metal oxides with acids show about their nature?
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Metal oxides neutralise acids (giving salt + water) just like bases — so metal oxides are basic in nature.
12. What does the reaction of CO₂ with bases show about non-metal oxides?
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Non-metal oxides react with bases to give salt and water, like acids do — so non-metal oxides are acidic in nature.
13. Define neutralisation with the general equation.
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The reaction between an acid and a base to form salt and water: acid + base → salt + water. E.g., NaOH + HCl → NaCl + H₂O.
14. How do antacids work?
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They contain mild bases (like magnesium hydroxide) that neutralise the excess hydrochloric acid in the stomach, relieving acidity and pain.
15. How is acidic soil treated?
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By spreading quicklime (CaO), slaked lime (Ca(OH)₂) or chalk — bases that neutralise the excess acid in the soil.
16. A bee sting is acidic. What household remedy neutralises it?
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Rubbing a mild base — baking soda (sodium hydrogencarbonate) solution — on the sting.
17. What happens when zinc reacts with sodium hydroxide?
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Zn + 2NaOH → Na₂ZnO₂ (sodium zincate) + H₂↑ — one of the few metals that gives hydrogen with a base too.
18. Which metals do NOT give hydrogen with dilute HCl?
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Metals less reactive than hydrogen — like copper and silver — do not displace hydrogen from dilute acids.
19. A solution turns phenolphthalein pink. Adding HCl drop by drop makes the pink vanish. Why?
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The solution was basic (pink). Added acid neutralises the base; once the base is used up, the solution is no longer basic and phenolphthalein returns to colourless.
20. Why does a turmeric stain turn red-brown with soap?
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Soap solution is basic, and turmeric is a natural indicator that turns reddish-brown in bases.
21. Name the natural acids in: lemon, vinegar, curd, tamarind.
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Lemon — citric acid; vinegar — acetic acid; curd — lactic acid; tamarind — tartaric acid.
22. Name the base present in milk of magnesia and its use.
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Magnesium hydroxide, Mg(OH)₂ — used as an antacid to neutralise excess stomach acid.
23. What ion do all acids produce in water?
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The hydrogen ion H⁺, which combines with water to exist as the hydronium ion H₃O⁺.
24. What ion do all bases produce in water?
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The hydroxide ion, OH⁻.
25. Why can't H⁺ exist alone in water?
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It is a bare proton — extremely reactive — so it immediately attaches to a water molecule, forming the hydronium ion H₃O⁺.
26. Write neutralisation as an ionic equation.
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H⁺ + OH⁻ → H₂O — the acid's and base's signature ions combine into water.
27. Why does dilute HCl conduct electricity?
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In water it ionises into H₃O⁺ and Cl⁻ ions, which are mobile charge carriers.
28. Glucose and alcohol contain hydrogen. Why are they not acids?
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They do not ionise in water — no H⁺ ions are released — so they show no acidic properties and their solutions do not conduct electricity.
29. What happens when dry HCl gas meets dry blue litmus paper?
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No colour change — without water, HCl does not ionise, so no H⁺ ions exist and no acidic behaviour appears.
30. What happens when HCl gas meets MOIST blue litmus?
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It turns red — the moisture lets HCl ionise into H⁺ ions, switching on acidity.
31. Which drying agent is used to dry HCl gas in the classic experiment?
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Anhydrous calcium chloride, in a guard tube through which the gas passes.
32. Is dissolving an acid in water exothermic or endothermic?
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Highly exothermic — a large amount of heat is released (the same is true for bases like NaOH).
33. State the golden rule for diluting a concentrated acid.
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Always add the acid slowly to water with constant stirring — never add water to the concentrated acid.
34. Why is adding water to concentrated acid dangerous?
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The intense heat is released into a tiny volume of water, which can boil instantly and splash concentrated acid out of the container onto skin and eyes.
35. What happens to the ion concentration when an acid is diluted?
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The concentration of hydronium ions per unit volume decreases — the solution becomes less acidic (the ions are spread through more water).
36. Define an alkali.
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A base that is soluble in water — e.g., NaOH, KOH, Ca(OH)₂.
37. 'All alkalis are bases but…' — complete and give an example.
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…not all bases are alkalis. Example: copper hydroxide Cu(OH)₂ is a base but, being insoluble in water, is not an alkali.
38. Define a strong acid with three examples.
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An acid that ionises completely in water, giving all its hydrogen as H⁺ ions — HCl, H₂SO₄, HNO₃.
39. Define a weak acid with three examples.
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An acid that ionises only partially in water — most molecules remain un-ionised. Examples: acetic acid, citric acid, carbonic acid.
40. Name two strong bases and two weak bases.
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Strong: NaOH, KOH. Weak: NH₄OH (ammonium hydroxide), Mg(OH)₂.
41. Distinguish 'strong acid' from 'concentrated acid'.
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Strength = degree of ionisation (a strong acid ionises completely, however dilute). Concentration = amount of acid dissolved per volume. Dilute HCl is a dilute STRONG acid; glacial acetic acid is a concentrated WEAK acid.
42. Why does a battery use dilute sulphuric acid?
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Its ions (H₃O⁺, SO₄²⁻) conduct electricity between the plates — the electrolyte's job depends entirely on ionisation.
43. Why is pure water neutral?
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It self-ionises very slightly into equal numbers of H₃O⁺ and OH⁻ ions — neither dominates, so it is neutral (pH 7).
44. What would you observe on touching the beaker when NaOH pellets dissolve in water?
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The beaker becomes warm/hot — dissolution of bases, like acids, is exothermic.
45. What does the pH scale measure, and what is its range?
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The hydrogen-ion concentration of a solution — how acidic or basic it is — on a scale from 0 to 14.
46. Classify solutions by pH value.
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pH less than 7: acidic. pH exactly 7: neutral. pH greater than 7: basic.
47. How does pH relate to H⁺ concentration?
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Inversely — the higher the H⁺ concentration, the LOWER the pH. Each unit of pH is a ten-fold change in H⁺.
48. What is universal indicator?
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A mixture of several indicators that shows a different colour at each pH value, letting us estimate the pH of a solution from its colour.
49. What colour does universal indicator show in a neutral solution?
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Green — at pH 7.
50. State the approximate pH of: gastric juice, lemon juice, pure water, blood.
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Gastric juice ~1.2; lemon juice ~2.2; pure water 7; human blood ~7.4.
51. In what pH range do living cells/body processes work?
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A narrow range of about 7.0 to 7.8 — blood is buffered near 7.4.
52. What is acid rain?
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Rain with pH below 5.6, caused by oxides of sulphur and nitrogen (SO₂, NO₂) from burning fuels dissolving in rainwater as acids.
53. Why is acid rain harmful to rivers and lakes?
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It lowers the water's pH below the narrow range aquatic organisms can survive in, killing fish and other life; it also damages monuments and crops.
54. Why does natural, unpolluted rain still have pH slightly below 7?
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Carbon dioxide from air dissolves in the drops forming weak carbonic acid, giving natural rain a pH around 6.
55. Which acid does the stomach secrete and what is its role?
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Hydrochloric acid (pH ~1.2) — it kills germs in food and creates the acidic medium needed for the digestive enzyme pepsin to work.
56. How do antacids relieve indigestion? Name one antacid.
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They are mild bases that neutralise the excess hydrochloric acid in the stomach, raising the pH to normal. Example: milk of magnesia (magnesium hydroxide).
57. At what pH does tooth decay start, and why?
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When mouth pH falls below 5.5 — tooth enamel (calcium phosphate) begins to dissolve in the acid produced by mouth bacteria acting on sugar.
58. What is tooth enamel made of, and what attacks it?
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Calcium phosphate — nothing else in the body is as hard. It does not dissolve in water and is unaffected by bases, but it corrodes in acid (mouth pH below 5.5).
59. How does toothpaste prevent tooth decay?
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Toothpastes are generally basic — they neutralise the acid produced by mouth bacteria, keeping the mouth pH above the enamel-dissolving threshold.
60. How do farmers correct (a) acidic soil, (b) basic soil?
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(a) By spreading bases: quicklime (CaO), slaked lime (Ca(OH)₂) or chalk (CaCO₃). (b) By adding organic matter/compost, which releases mild acids as it decomposes.
61. Why is soil pH tested before treatment?
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Each crop grows best in a specific pH range; testing shows whether the soil is too acidic or too basic and how much corrective material is needed.
62. Which acid does a bee sting inject, and what is the remedy?
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Methanoic acid (formic acid) — the acidic sting is relieved by rubbing a mild base such as baking soda solution.
63. Why is baking soda rubbed on a bee sting?
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A bee sting injects methanoic acid. Baking soda is a mild base, so it neutralises that acid and relieves the pain.
64. Nettle leaves sting with methanoic acid. What is the traditional remedy?
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Rubbing the sting with a dock leaf — a plant that often grows beside nettles.
65. Solution A has pH 3 and solution B has pH 5. Which has more H⁺ and by how much?
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A — with pH two units lower, it has about 100 times (10 × 10) the hydrogen-ion concentration of B.
66. Arrange in increasing pH: NaOH solution, blood, lemon juice, pure water.
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Lemon juice (~2.2) < pure water (7) < blood (~7.4) < NaOH solution (~13-14).
67. How does a salt's pH depend on its parent acid and base?
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Strong acid + strong base → neutral salt (pH 7, e.g., NaCl); strong acid + weak base → acidic salt (NH₄Cl); weak acid + strong base → basic salt (Na₂CO₃).
68. What is brine?
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A concentrated aqueous solution of sodium chloride (common salt) — the raw material of the chlor-alkali process.
69. Write the chlor-alkali equation and name where each product appears.
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2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Chlorine is released at the anode, hydrogen at the cathode, and NaOH forms in solution near the cathode.
70. Why is the process called 'chlor-alkali'?
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Because of its two headline products — CHLORine gas and the ALKALI sodium hydroxide.
71. Give two uses each of NaOH, Cl₂ and H₂ from the chlor-alkali process.
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NaOH: making soaps/detergents, paper (also degreasing metals, artificial fibres). Cl₂: treating drinking/swimming water, making PVC (also disinfectants). H₂: fuel, making margarine (also ammonia for fertilisers).
72. How is bleaching powder prepared? Give its formula.
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By passing chlorine gas over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Formula: CaOCl₂ (calcium oxychloride).
73. State three uses of bleaching powder.
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(1) Bleaching cotton/linen in textiles and wood pulp in paper factories, (2) disinfecting drinking water, (3) as an oxidising agent in chemical industries.
74. Give the chemical name, formula and preparation of baking soda.
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Sodium hydrogencarbonate, NaHCO₃ — prepared from brine: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
75. What happens when baking soda is heated? Write the equation.
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It decomposes to sodium carbonate, water and carbon dioxide: 2NaHCO₃ --Δ--> Na₂CO₃ + H₂O + CO₂ — the CO₂ is why baked goods rise.
76. What is baking powder made of, and why the second ingredient?
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Baking soda plus a mild edible acid such as tartaric acid. On heating, the soda releases CO₂ (raising the cake); the tartaric acid neutralises the bitter sodium carbonate produced, protecting the taste.
77. Give three uses of baking soda.
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(1) In baking powder for cakes and bread, (2) as an antacid for acidity, (3) in soda-acid fire extinguishers (also mild household cleaning).
78. Give the chemical name, formula and preparation of washing soda.
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Sodium carbonate decahydrate, Na₂CO₃·10H₂O — made by heating baking soda to sodium carbonate, then recrystallising it with water: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
79. State three uses of washing soda.
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(1) In the glass, soap and paper industries, (2) as a household cleaning agent, (3) for removing the permanent hardness of water (also making borax).
80. Define water of crystallisation with two examples.
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The fixed number of water molecules chemically bound within one formula unit of a crystalline salt — e.g., CuSO₄·5H₂O (five) and Na₂CO₃·10H₂O (ten); also gypsum CaSO₄·2H₂O.
81. What is observed when blue copper sulphate crystals are heated, and then rewetted?
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On heating they turn white (losing their five waters of crystallisation; droplets condense on the tube's cool wall). Adding water to the white powder restores the blue colour — the hydrate reforms.
82. Give the chemical names and formulas of gypsum and plaster of Paris.
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Gypsum: calcium sulphate dihydrate, CaSO₄·2H₂O. Plaster of Paris: calcium sulphate hemihydrate, CaSO₄·½H₂O.
83. How is plaster of Paris prepared? Why is the temperature critical?
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By heating gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. The temperature must not exceed 373 K — stronger heating drives off ALL the water, giving 'dead burnt plaster' (anhydrous CaSO₄) that cannot set.
84. Write the setting reaction of plaster of Paris.
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CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O — mixing with water converts it back to hard gypsum within minutes.
85. Give three uses of plaster of Paris.
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(1) Casts supporting fractured bones, (2) moulds for toys, statues and decorative material, (3) filling gaps and smoothing surfaces/false ceilings.
86. Why must plaster of Paris be stored in moisture-proof containers?
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It sets by combining with water — even atmospheric moisture slowly converts it into hard gypsum, spoiling the powder.
87. What does the ½ in CaSO₄·½H₂O actually mean?
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That two formula units of CaSO₄ share one molecule of water in the crystal — on average, half a water molecule per CaSO₄.
88. Classify by pH: NaCl, NH₄Cl, Na₂CO₃, CH₃COONa.
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NaCl — neutral (strong + strong). NH₄Cl — acidic (strong acid HCl + weak base NH₄OH). Na₂CO₃ — basic (weak carbonic acid + strong NaOH). CH₃COONa — basic (weak acetic acid + strong NaOH).
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