Important Chemical Reactions

Two or more substances combine to form a single product. Combination reactions are always exothermic — write this in your answer.

One reactant breaks into two or more products. Also: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes — highly frequently asked).

Decomposition by light energy. Also: 2AgBr → 2Ag + Br₂. Used in black-and-white photography. 1-mark MCQ: 'what happens to AgCl in sunlight?' Answer: turns grey.

H₂ : O₂ volume ratio = 2 : 1. Test H₂ with burning splint (pop sound). Test O₂ with glowing splint (relights).

More reactive metal displaces less reactive metal from its salt solution. Iron is above copper in the reactivity series. Also: Zn + CuSO₄ → ZnSO₄ + Cu.

↓ = precipitate (insoluble solid). Always write state symbols — (aq), (s), (l), (g) — in board exams for full marks.

Acid + Base → Salt + Water (always). Heat produced = heat of neutralisation. Test endpoint with universal indicator or phenolphthalein.

Gain of oxygen = oxidation. Reverse: pass H₂ over black CuO → black disappears (Cu reduced back to reddish-brown). Classic redox demonstration.

CuO loses O → reduced → oxidising agent. H₂ gains O → oxidised → reducing agent. OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Both O₂ AND H₂O are required — iron won't rust in dry air or deoxygenated water alone. Salt water speeds rusting. Prevention: painting, galvanising, alloying.

Prevention: antioxidants (BHA, BHT), nitrogen flushing (chip packets), vacuum packing, refrigeration. A 1–2 mark question almost every year.

General: Metal + Dilute Acid → Salt + Hydrogen. Cu, Ag, Au do NOT react (below H in reactivity series). Alternate: Zn + 2HCl → ZnCl₂ + H₂↑.

Metal oxides are basic oxides → react with acid to form salt + water. Same pattern as acid + base. ZnO + H₂SO₄ → ZnSO₄ + H₂O is another common example.

CO₂ test: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (lime water turns milky). Excess CO₂ makes it clear again (CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂, which is soluble).

NaHCO₃ = baking soda. Reacts with acid to release CO₂ — used in baking (dough rises) and antacids (neutralises stomach acid). Always produces CO₂ like carbonate.

Acid + Base → Salt + Water (always). Product is common salt (NaCl). Heat released = heat of neutralisation. Detect endpoint using universal indicator.

Non-metal oxides are acidic oxides → react with base (just as acid reacts with base). This IS the standard CO₂ detection test. Excess CO₂ turns it clear again.

Electrolysis of brine (saturated NaCl). Cl₂ → bleaching, PVC. H₂ → fuel, margarine. NaOH → soap, paper, aluminium industry. All three products are industrially important.

Chemical name: Calcium oxychloride (CaOCl₂). Used to disinfect drinking water and bleach in paper and textile industries.

Baking soda (NaHCO₃) in baking powder releases CO₂ on heating → makes dough rise. Also used in soda-acid fire extinguishers. Chemical name: Sodium hydrogen carbonate.

POP is made from gypsum by heating at 100°C. It EXPANDS on setting (unusual!) — that's why it's perfect for filling casts, statues, and dental moulds without leaving gaps.

MgO is a basic oxide — dissolves in water to give Mg(OH)₂ (alkaline). A very reactive metal burns brilliantly. Always include observation of bright white flame in answers.

Reactivity with water: K, Na (cold water, vigorous) > Ca (cold water, less vigorous) > Mg (hot water only) > Fe (steam only). This is the reactivity series in action.

Iron reacts only with STEAM — not cold or hot water. Fe₃O₄ is the black magnetic oxide (different from rust Fe₂O₃). Write 'steam' not 'water' in the equation or lose marks.

All metals ABOVE H in reactivity series displace H₂ from dilute acids. Cu, Ag, Au (below H) do NOT react. Zn + 2HCl → ZnCl₂ + H₂ is the simplest common example.

Copper is more reactive than silver → displaces Ag from its salt. Reactivity series (high to low): K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au. Use Fe + CuSO₄ → FeSO₄ + Cu as alternate example.

Al is more reactive than Fe → displaces Fe from Fe₂O₃. Al is oxidised (reducing agent). Fe₂O₃ is reduced (oxidising agent). Type: both displacement AND redox reaction.

Non-metal oxides = ACIDIC oxides (opposite of metal oxides which are basic). SO₂ causes acid rain. CO₂ makes rainwater slightly acidic. Both react with bases.

Amphoteric = reacts with BOTH acids AND bases. Al₂O₃ and ZnO are the two common amphoteric oxides in CBSE. This makes them different from purely basic metal oxides.

ZnO also: ZnO + 2NaOH → Na₂ZnO₂ + H₂O (sodium zincate). Dual behaviour with both acid and base = definition of amphoteric. One of the most commonly tested distinctions.

Complete combustion = sufficient O₂ → CO₂ + H₂O + heat (blue flame). Incomplete combustion = insufficient O₂ → CO + soot + less heat (sooty yellow/orange flame). LPG burns blue = complete.

Ethanol (ethyl alcohol) burns cleanly and is used as a fuel. Balance: count C → CO₂, count H → H₂O, then balance O on right, adjust O₂ on left.

Conditions: conc. H₂SO₄ as catalyst + gentle heat. Acid + Alcohol → Ester + Water. The ⇌ arrow = reversible reaction. Esters are used in perfumes and artificial fruit flavours.

Saponification = alkaline hydrolysis of ester. Industrial: fat (ester of glycerol) + NaOH → soap + glycerol. Soap: hydrophilic head (−COO⁻Na⁺) + hydrophobic tail (carbon chain).

Unsaturated → saturated by adding H₂ (hydrogenation). Industrial use: vegetable oil (unsaturated) + H₂ → vanaspati ghee (saturated). Catalyst: Ni or Pt at high temperature.

Bromine water test: alkenes/alkynes decolourise Br₂ water instantly — alkanes do NOT. This is THE standard test for unsaturation in NCERT. Cl₂ addition: CH₂=CH₂ + Cl₂ → CH₂ClCH₂Cl.

Saturated compounds undergo SUBSTITUTION (not addition). Condition: sunlight (photochemical, free radical). H is replaced by Cl one at a time: CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄.

Oxidising agents for ethanol → ethanoic acid: alkaline KMnO₄ OR acidic K₂Cr₂O₇. Both act as oxygen donors. Alcohol (−OH) → Carboxylic acid (−COOH). Intermediate: ethanal (CH₃CHO).

H₂SO₄ acts as dehydrating agent (removes H₂O). At 170°C → ethene. At 140°C → diethyl ether instead. Confirm product with Br₂ water test (decolourises = unsaturated = ethene).

Carboxylic acids react with Na₂CO₃ AND NaHCO₃ — this distinguishes them from phenols (phenols do NOT react with NaHCO₃). Key identification test for −COOH group.

This reaction distinguishes carboxylic acids from phenols: phenol does NOT react with NaHCO₃, but carboxylic acid does. A classic NCERT distinguishing reaction — worth 1 mark in board exams.