Chemical Reactions, Acids, Bases and Salts
Four reaction patterns
Class 10 sorts reactions into four patterns by what happens to the partners. Learn the pattern, one standard example and the observation that goes with it.
| Type | Pattern | Standard example | Observation |
|---|---|---|---|
| Combination | A + B → AB | CaO + H₂O → Ca(OH)₂ + heat | Quicklime slakes with a hiss; the mixture becomes hot |
| Decomposition (thermal) | AB → A + B | CaCO₃ → CaO + CO₂ (heat) | Limestone gives quicklime and carbon dioxide |
| Decomposition (thermal) | AB → A + B | 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ | Brown fumes of nitrogen dioxide |
| Decomposition (thermal) | AB → A + B | 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ | Green crystals turn brown; smell of burning sulphur |
| Decomposition (electrolytic) | AB → A + B | 2H₂O → 2H₂ + O₂ | Hydrogen and oxygen in volume ratio 2 : 1 |
| Decomposition (photolytic) | AB → A + B | 2AgCl → 2Ag + Cl₂ (sunlight) | White silver chloride turns grey |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu | Blue solution fades to pale green; reddish-brown copper deposits on iron |
| Double displacement | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl | White precipitate of barium sulphate |
| Double displacement | AB + CD → AD + CB | Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃ | Yellow precipitate of lead iodide |
Reactions are also exothermic (release heat) or endothermic (absorb heat). Burning of natural gas, respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy), slaking of lime and the decay of vegetable matter into compost are exothermic. Thermal decomposition of calcium carbonate is endothermic. Whitewashing uses the slaked lime: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O, and the thin calcium carbonate coat gives a shiny finish after two to three days.
Oxidation, reduction and everyday redox
At Class 10 level, oxidation is gain of oxygen or loss of hydrogen, and reduction is loss of oxygen or gain of hydrogen. They always occur together, so such a reaction is a redox reaction. The substance that supplies oxygen (or removes hydrogen) is the oxidising agent and is itself reduced; the substance that removes oxygen (or supplies hydrogen) is the reducing agent and is itself oxidised.
| Reaction | Oxidised | Reduced | Oxidising agent |
|---|---|---|---|
| CuO + H₂ → Cu + H₂O | H₂ | CuO | CuO |
| ZnO + C → Zn + CO | C | ZnO | ZnO |
| 2Cu + O₂ → 2CuO | Cu | O₂ | O₂ |
| Fe₂O₃ + 2Al → 2Fe + Al₂O₃ (thermit) | Al | Fe₂O₃ | Fe₂O₃ |
Two everyday effects of oxidation are examinable. Corrosion is the slow attack of air, moisture or chemicals on metals: iron rusts to a brown coat, silver turns black (silver sulphide) and copper acquires a green coat. Rancidity is the oxidation of fats and oils in food, giving a bad smell and taste. It is slowed by antioxidants, airtight containers, refrigeration and by flushing packets of chips with nitrogen. A magnesium ribbon is cleaned with sandpaper before burning to remove its oxide coat; it then burns with a dazzling white flame to give white magnesium oxide, 2Mg + O₂ → 2MgO.
Acids, bases and indicators
An acid gives H⁺ ions in water (as H₃O⁺); a base that dissolves in water gives OH⁻ ions and is called an alkali. All alkalis are bases but not all bases are alkalis. Strong acids are HCl, H₂SO₄ and HNO₃; weak acids are ethanoic, citric and carbonic acid. NaOH and KOH are strong alkalis; NH₄OH is weak. HCl gas does not change dry litmus because it needs water to produce H⁺. Always add acid to water slowly, never water to acid, because dilution releases a great deal of heat.
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | turns red | stays blue |
| Red litmus | stays red | turns blue |
| Phenolphthalein | colourless | pink |
| Methyl orange | red | yellow |
| Turmeric | yellow (unchanged) | red-brown |
| Onion, vanilla, clove oil (olfactory) | smell retained | smell lost |
Four reactions to know.
(1) Acid + metal → salt + hydrogen, e.g. Zn + 2HCl → ZnCl₂ + H₂; hydrogen burns with a pop.
(2) Acid + metal carbonate or hydrogencarbonate → salt + water + carbon dioxide, e.g. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂; the gas turns lime water milky, Ca(OH)₂ + CO₂ → CaCO₃ + H₂O, and the milkiness vanishes in excess CO₂ because soluble Ca(HCO₃)₂ forms.
(3) Acid + base → salt + water, a neutralisation, e.g. NaOH + HCl → NaCl + H₂O; the same happens with a metal oxide, CuO + 2HCl → CuCl₂ + H₂O, so metal oxides are basic.
(4) A base with a non-metal oxide: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, so non-metal oxides are acidic. NaOH also liberates hydrogen with zinc, 2NaOH + Zn → Na₂ZnO₂ + H₂.
| Source | Acid |
|---|---|
| Vinegar | Ethanoic (acetic) acid |
| Orange, lemon | Citric acid |
| Tamarind, grapes | Tartaric acid |
| Tomato | Oxalic acid |
| Sour milk (curd) | Lactic acid |
| Ant or nettle sting | Methanoic (formic) acid |
The pH scale
The pH scale measures the concentration of hydrogen ions: pH = −log₁₀[H⁺], with [H⁺] in mol per litre. The ‘p’ stands for the German word potenz, ‘power’. On the usual 0 to 14 scale, pH below 7 is acidic, 7 is neutral (pure water at 25 °C) and above 7 is basic. The scale is logarithmic: a fall of one pH unit means ten times more H⁺. Universal indicator shows a different colour at each pH, red for strong acid, green near 7 and violet for strong base.
- Worked example 1. [H⁺] = 1 × 10⁻⁴ mol/L gives pH = 4, so the solution is acidic.
- Worked example 2. pH 6 to pH 3 is a fall of 3 units, so [H⁺] rises 10³ = 1000 times.
- Worked example 3. pH 2 against pH 5: the pH 2 solution has 1000 times more H⁺ and is the stronger acid.
| Liquid | Approximate pH | Nature |
|---|---|---|
| Gastric juice (hydrochloric acid) | 1 to 2 | strongly acidic |
| Lemon juice | about 2 | acidic |
| Vinegar | about 3 | acidic |
| Pure water | 7 | neutral |
| Human blood | about 7.4 | slightly basic |
| Milk of magnesia | about 10 | basic |
| Sodium hydroxide solution | about 14 | strongly basic |
Where pH matters: tooth enamel (calcium phosphate, the hardest material in the body) begins to corrode when the mouth pH falls below 5.5, so bacteria acting on sugar cause decay and a basic toothpaste neutralises the acid. Rain with pH below 5.6 is acid rain. Indigestion from excess stomach acid is relieved by an antacid such as milk of magnesia. The sting of a bee or an ant contains methanoic acid and is soothed by baking soda; soil of unsuitable pH is corrected with lime or organic matter.
Salts and their uses
A salt is formed when an acid and a base neutralise each other. Salts with the same positive or negative ion form a family: NaCl and Na₂SO₄ are sodium salts. The pH of a salt solution depends on the parents. Strong acid with strong base (NaCl, Na₂SO₄) gives a neutral solution, pH 7. Strong acid with weak base (NH₄Cl) gives pH below 7. Strong base with weak acid (sodium ethanoate, sodium carbonate) gives pH above 7.
| Name | Formula | Preparation | Uses |
|---|---|---|---|
| Common salt | NaCl | Sea water, or mined as rock salt | Food; raw material of the chlor-alkali industry |
| Bleaching powder | CaOCl₂ | Cl₂ + Ca(OH)₂ → CaOCl₂ + H₂O (chlorine on dry slaked lime) | Bleaching cotton and linen, disinfecting drinking water, oxidising agent |
| Baking soda | NaHCO₃ | NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ | Antacid, baking powder (with tartaric acid), soda-acid fire extinguisher |
| Washing soda | Na₂CO₃·10H₂O | Heat baking soda to get Na₂CO₃, then recrystallise | Glass, soap and paper industries; removes permanent hardness of water |
| Plaster of Paris | CaSO₄·½H₂O | Gypsum CaSO₄·2H₂O heated at 373 K | Supporting fractured bones, toys, decoration, smoothing surfaces |
Chlor-alkali process. Electrolysis of brine gives three products: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Chlorine collects at the anode, hydrogen at the cathode and sodium hydroxide forms near the cathode. Baking soda on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂; the CO₂ makes cakes rise. Plaster of Paris is stored in a moisture-proof container because it re-forms gypsum with water, CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O, and sets hard.
Key takeaways
- Balance by coefficients only, never subscripts; a balanced equation conserves atoms, and its coefficients give the molecule ratio. Fe + H₂O → Fe₃O₄ + H₂ balances as 3, 4, 1, 4.
- Four patterns: combination, decomposition (thermal, electrolytic, photolytic), displacement and double displacement; an insoluble product makes the last a precipitation reaction.
- Oxidation is gain of oxygen or loss of hydrogen; reduction is the reverse. The oxidising agent is reduced and the reducing agent is oxidised; rusting and rancidity are oxidation.
- Litmus: acid turns blue red, base turns red blue. Phenolphthalein: colourless in acid, pink in base. Methyl orange: red in acid, yellow in base. Acid + carbonate gives CO₂, acid + metal gives H₂.
- pH = −log[H⁺]; 7 is neutral; each unit is a tenfold change in H⁺, so pH 6 to pH 3 means 1000 times more H⁺. Enamel decays below 5.5; acid rain is below 5.6.
- Salts: bleaching powder CaOCl₂, baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O, plaster of Paris CaSO₄·½H₂O from gypsum at 373 K; brine electrolysis gives NaOH, Cl₂ and H₂.
Practice questions (18)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
In the balanced equation 3Fe + 4H₂O → Fe₃O₄ + xH₂, what is the value of x?
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Answer: C — 4
Four water molecules carry 8 hydrogen atoms, so 4 H₂ molecules are needed on the right; the 4 oxygen atoms also match Fe₃O₄. A value of 2 would leave half the hydrogen unaccounted for, which is the tempting choice if you balance only the iron.CaO + H₂O → Ca(OH)₂ + heat is an example of which type of reaction?
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Answer: D — Combination reaction
Two substances join to give a single product, so it is a combination reaction, and it is exothermic as slaking of quicklime shows. It is not a decomposition because nothing breaks down into simpler substances.When lead nitrate is heated strongly in a dry test tube, brown fumes are seen. Which gas is responsible for the brown colour?
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Answer: B — Nitrogen dioxide
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂: nitrogen dioxide is the brown gas. Oxygen is also released but is colourless, and sulphur trioxide does not form because there is no sulphur in the compound.For CuO + H₂ → Cu + H₂O, the statements are: P: CuO is reduced. Q: H₂ is oxidised. R: H₂ is the oxidising agent. S: CuO is the reducing agent. Which are correct?
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Answer: D — P and Q only
CuO loses oxygen (reduced) and H₂ gains oxygen (oxidised), so P and Q hold. The agent is the opposite of the change: H₂ is the reducing agent and CuO the oxidising agent, which makes R and S wrong.What is the sum of all coefficients when C₃H₈ + O₂ → CO₂ + H₂O is balanced with the smallest whole numbers?
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Answer: C — 13
The balanced form is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, and 1 + 5 + 3 + 4 = 13. The sum 12 is the tempting answer for those who forget that the unwritten coefficient of propane is 1.In the photolytic decomposition 2AgCl → 2Ag + Cl₂ in sunlight, the colour of silver chloride changes from white to what?
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Answer: B — Grey
Finely divided silver is formed and makes the solid look grey. Yellow is the colour of lead iodide and green is seen in iron(II) sulphate crystals, neither of which is produced here.An iron nail is kept in copper sulphate solution for about 30 minutes. What is observed?
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Answer: B — The solution turns pale green and a reddish-brown coat forms on the nail
Iron is more reactive than copper, so Fe + CuSO₄ → FeSO₄ + Cu: copper deposits as a reddish-brown layer and iron sulphate makes the solution pale green. The no-change choice reverses the reactivity order.Which indicator is colourless in acid and turns pink in a base?
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Answer: D — Phenolphthalein
Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base, and turmeric is yellow in acid and turns red-brown, not pink, in base.Which of the following acid and natural source pairs is NOT correctly matched?
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Answer: A — Tomato: tartaric acid
Tomato contains oxalic acid, while tartaric acid is the acid of tamarind and grapes. The other three pairs are standard: curd has lactic acid, orange has citric acid and a nettle sting has methanoic acid.The pH of a solution falls from 6 to 3. By what factor does the hydrogen ion concentration change?
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Answer: C — It rises 1000 times
pH is logarithmic: a fall of 3 units means 10³ = 1000 times more H⁺. A threefold rise treats pH as a linear scale, which is the classic error, and a fall would mean the solution had become less acidic.What is the pH of pure water at 25 °C?
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Answer: D — 7
Pure water is neutral and has pH 7, with [H⁺] = 10⁻⁷ mol/L. pH 14 belongs to a strong base such as sodium hydroxide, and pH 0 to a very strong acid.What is the formula of plaster of Paris?
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Answer: C — CaSO₄·½H₂O
Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O, made by heating gypsum at 373 K. CaSO₄·2H₂O is gypsum itself, the tempting alternative.On heating, baking soda (NaHCO₃) decomposes to give which set of products?
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Answer: B — Na₂CO₃, H₂O and CO₂
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ makes baked foods rise. NaCl cannot form because the compound contains no chlorine, and a product list with only NaOH and CO₂ does not balance the atoms.Electrolysis of concentrated sodium chloride solution (brine) gives which three products?
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Answer: B — NaOH, Cl₂ and H₂
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂: chlorine at the anode, hydrogen at the cathode, NaOH in solution. Sodium metal is not liberated from aqueous solution because it would react with water at once.An aqueous solution of which of these salts has a pH greater than 7?
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Answer: D — Sodium ethanoate
Sodium ethanoate comes from a strong base (NaOH) and a weak acid (ethanoic acid), so its solution is basic. NaCl and KNO₃ come from strong acid and strong base and are neutral; NH₄Cl, from a weak base, is acidic.Which statement about bleaching powder is NOT true? P: Its formula is CaOCl₂. Q: It is made by passing chlorine over dry slaked lime. R: It is used to disinfect drinking water. S: It is made by heating limestone.
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Answer: D — S
Heating limestone gives quicklime (CaO) and CO₂, not bleaching powder. Bleaching powder forms when chlorine acts on dry slaked lime, Cl₂ + Ca(OH)₂ → CaOCl₂ + H₂O, so P, Q and R are all true.Arrange these in increasing order of approximate pH: P: gastric juice, Q: pure water, R: milk of magnesia, S: vinegar.
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Answer: A — P, S, Q, R
Gastric juice (pH 1 to 2) is more acidic than vinegar (about 3), then neutral water (7), then basic milk of magnesia (about 10). Swapping gastric juice and vinegar is the usual slip.Which pair correctly names a precipitation reaction and its observation?
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Answer: A — Pb(NO₃)₂ + 2KI: yellow solid PbI₂
Lead nitrate with potassium iodide gives yellow PbI₂ as an insoluble product, a double displacement precipitation. Barium sulphate is white, not green, and the other two are decomposition and gas-evolving displacement.