Chemical Reactions and Equations

A chemical reaction is a change in which one or more substances (the reactants) turn into new substances (the products) with different properties. Milk turning into curd, food being digested, iron rusting and magnesium burning are all chemical changes. Melting of ice, dissolving sugar and boiling water are not: no new substance forms. A 2025 MCQ asked exactly this, and the answer was the melting of glaciers.

Activity 1.1, burning of a magnesium ribbon. Clean a 2 cm piece with sandpaper, hold it with tongs and burn it over a watch-glass. It burns with a dazzling white flame and leaves a white powder, magnesium oxide: 2Mg(s) + O₂(g) → 2MgO(s). The ribbon is cleaned because magnesium reacts slowly with air to form a dull protective layer (magnesium oxide and basic magnesium carbonate) that stops the metal underneath from burning readily.

Activity 1.2, lead nitrate and potassium iodide. Two colourless solutions give a yellow precipitate of lead iodide: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).

Activity 1.3, zinc and dilute sulphuric acid. Zinc granules in dilute H₂SO₄ (or dilute HCl) give bubbles of hydrogen gas, and the test tube becomes warm: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).

From these, NCERT lists four observations that tell us a reaction has occurred: change in state, change in colour, evolution of a gas, and change in temperature. One is enough; often several appear together. In a 2-mark question, quote two observations and back each with the reaction, for example: bubbles of a colourless gas are seen and the tube gets warm when zinc is added to dilute sulphuric acid, so a gas has evolved and the temperature has changed.

A chemical equation is the shorthand in which chemists write a reaction. Start with the word equation for burning magnesium: Magnesium + Oxygen → Magnesium oxide. The substances on the left of the arrow are the reactants, the substance on the right is the product, and the arrow points from reactants to products and is read as gives. A plus sign between reactants means they react together; a plus sign between products means they are formed together.

Word equations are long, and they do not say how much of each substance is involved. So we replace the names with chemical formulae: Mg + O₂ → MgO. This is shorter and every chemist in the world reads it the same way. But it is a skeletal (unbalanced) equation: there are two oxygen atoms on the left and one on the right. The next section fixes that.

A complete chemical equation tells you four things, and a 1-mark question can ask for any of them: which substances react and which are formed; the physical states of each (solid, liquid, gas or aqueous); the relative numbers of atoms and molecules taking part, which is what the coefficients mean; and the conditions such as heat, light, pressure or a catalyst written over the arrow. Practise reading an equation aloud: CaO(s) + H₂O(l) → Ca(OH)₂(aq) reads as solid calcium oxide reacts with liquid water to give an aqueous solution of calcium hydroxide.

Mass can neither be created nor destroyed in a chemical reaction (law of conservation of mass). Atoms are only rearranged, so the number of atoms of each element must be the same before and after the reaction. That is the one-line reason the board wants when it asks why equations must be balanced.

Hit-and-trial method, with the NCERT example. Iron reacts with steam: Fe + H₂O → Fe₃O₄ + H₂.
Step 1: draw a box around each formula; you may only change the numbers in front of the boxes, never the subscripts inside them.
Step 2: count atoms. Left: Fe 1, H 2, O 1. Right: Fe 3, H 2, O 4.
Step 3: start with the compound having the most atoms (Fe₃O₄) and pick one element, say oxygen. Four O on the right, so put 4 in front of H₂O: Fe + 4H₂O → Fe₃O₄ + H₂.
Step 4: hydrogen is now 8 on the left, so write 4H₂ on the right.
Step 5: iron is 3 on the right, so write 3Fe on the left.
Step 6: recount. Fe 3 = 3, H 8 = 8, O 4 = 4. Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

States. Add (s) solid, (l) liquid, (g) gas and (aq) aqueous after each formula: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). Steam is written H₂O(g).

Conditions. Write them above or below the arrow: heat as Δ, a catalyst, pressure, sunlight or electricity. NCERT gives N₂(g) + 3H₂(g) → 2NH₃(g) with a catalyst at 200 atm, and photosynthesis, 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g), with sunlight and chlorophyll over the arrow.

Balance these before the exam; each has been an MCQ: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; 2Al + 3H₂O → Al₂O₃ + 3H₂; 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂; 3Zn + 2H₃PO₄ → Zn₃(PO₄)₂ + 3H₂.

A combination reaction is one in which two or more substances combine to form a single product. Activity 1.4 is the standard example: add water slowly to a few pieces of calcium oxide (quicklime) in a beaker and touch the beaker. It becomes hot, and the lumps turn into a white suspension of calcium hydroxide (slaked lime): CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat.

The slaked lime is used for whitewashing. It reacts slowly with carbon dioxide in the air over two to three days to form a thin layer of calcium carbonate, which gives the wall its shine: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). Marble has the same formula as this calcium carbonate. Both of these are combination reactions and the board has asked for both in one 3-mark question (identify the compound used for whitewashing, write its reaction with water, name the type).

Other NCERT combination reactions: burning of coal, C(s) + O₂(g) → CO₂(g), and formation of water, 2H₂(g) + O₂(g) → 2H₂O(l). Note that in a combination reaction the reactants may be elements or compounds, but the product is always a single compound.

Exothermic reactions. Reactions that release heat are exothermic. Quicklime with water is one. NCERT adds three more: burning of natural gas, CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + heat; respiration, in which glucose combines with oxygen in our cells, C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy; and the decomposition of vegetable matter into compost, which is also exothermic. The respiration one is asked almost every year: state that it is exothermic because energy is released when glucose is oxidised, and write the equation.

A decomposition reaction is one in which a single reactant breaks down into two or more simpler products. It is the opposite of combination, and it needs energy, which may be supplied as heat, electricity or light. Because energy is absorbed, decomposition reactions are endothermic.

Thermal decomposition (heat). Activity 1.5: heat 2 g of green ferrous sulphate crystals in a dry boiling tube. The crystals first lose their water of crystallisation and turn whitish, then decompose to reddish-brown ferric oxide with the smell of burning sulphur: 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g). Precautions the board has asked for: keep the mouth of the tube away from yourself and others, and hold the tube with a holder. Activity 1.6: heat lead nitrate in a boiling tube. You hear crackling, brown fumes of nitrogen dioxide come out, and a yellow residue of lead oxide is left: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g). Heating limestone gives quicklime, an important industrial reaction: CaCO₃(s) → CaO(s) + CO₂(g).

Electrolytic decomposition (electricity). Activity 1.7: water with a few drops of dilute sulphuric acid in a plastic mug, two carbon electrodes connected to a 6 V battery, an inverted water-filled test tube over each electrode. Bubbles form at both electrodes; the tube at the cathode fills twice as fast as the one at the anode. 2H₂O(l) → 2H₂(g) + O₂(g). Hydrogen collects at the cathode and oxygen at the anode in a 2:1 volume ratio because that is the ratio in the equation. The acid is added to make pure water conduct. A burning candle at the mouth of the hydrogen tube burns with a pop; at the oxygen tube it burns more brightly.

Photolytic decomposition (light). Activity 1.8: silver chloride kept in sunlight turns from white to grey because it decomposes to silver metal and chlorine: 2AgCl(s) → 2Ag(s) + Cl₂(g). Silver bromide behaves the same way, 2AgBr(s) → 2Ag(s) + Br₂(g), and this reaction is used in black and white photography.

In a displacement reaction a more reactive element takes the place of a less reactive element in its compound. With metals, the more reactive metal displaces the less reactive metal from its salt solution.

Activity 1.9, iron nail in copper sulphate solution. Tie a clean iron nail with thread, dip it in blue copper sulphate solution for about twenty minutes, and compare it with a nail kept aside. The blue colour of the solution fades to light green, and the nail is coated with a brownish layer of copper: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Iron is more reactive than copper, so it displaces copper from copper sulphate; the green colour is due to ferrous sulphate. Other metals that can do the same to copper sulphate are zinc, aluminium and magnesium, all more reactive than copper: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). NCERT also gives Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s).

The reverse never happens: a copper strip in ferrous sulphate or in zinc sulphate shows no change, because copper is less reactive. An aluminium strip in ferrous sulphate does react: 2Al(s) + 3FeSO₄(aq) → Al₂(SO₄)₃(aq) + 3Fe(s), and the green colour disappears. This pairing (Cu and Al strips in FeSO₄) was a 3-mark question in 2019.

Two applications worth remembering. In the refining of silver, copper displaces silver from silver nitrate solution: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s); the solution turns blue. For the same reason a silver nitrate solution must not be stirred with a copper spoon, and a copper sulphate solution must not be stored in an iron or zinc vessel. Zinc with silver nitrate, Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag, is the same idea.

In a double displacement reaction two compounds exchange their ions to form two new compounds. Think of the positive ions swapping partners.

Activity 1.10, sodium sulphate and barium chloride. Mix about 3 mL of each solution in a test tube: a white, insoluble substance settles at once. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). The white precipitate is barium sulphate, formed by the reaction of Ba²⁺ and SO₄²⁻ ions; the other product, sodium chloride, stays in solution. An insoluble solid that separates from a solution is a precipitate, and any reaction that produces one is a precipitation reaction. Since ions have been exchanged, it is also a double displacement reaction: give both names when the question asks for the type.

Other precipitation reactions from the chapter: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq), a yellow precipitate of lead iodide; and AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), a white precipitate of silver chloride.

Neutralisation is also double displacement. An acid and a base exchange ions to give a salt and water: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l); 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l). A 2023 question asked for one neutralisation and one precipitation example of double displacement, so keep one of each ready.

Displacement versus double displacement. In displacement one element displaces another from its compound, and an element is always a reactant (Fe + CuSO₄ → FeSO₄ + Cu). In double displacement two compounds exchange ions and no free element appears on either side (Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl). This comparison with one equation each is the standard 3-mark answer.

Activity 1.11, heating copper powder. Heat 1 g of copper powder in a china dish. The shiny brown surface turns black because copper combines with oxygen to form copper(II) oxide: 2Cu(s) + O₂(g) → 2CuO(s). If hydrogen gas is now passed over the hot black powder, the brown colour of copper returns: CuO(s) + H₂(g) → Cu(s) + H₂O(l). In the first reaction copper gains oxygen and is oxidised; in the second copper oxide loses oxygen and is reduced.

Definitions the examiner expects. Oxidation is the gain of oxygen or the loss of hydrogen by a substance. Reduction is the loss of oxygen or the gain of hydrogen. A reaction in which one reactant is oxidised while another is reduced is an oxidation-reduction or redox reaction. In CuO + H₂ → Cu + H₂O, copper oxide is reduced to copper and hydrogen is oxidised to water: one reaction, both changes.

Oxidising and reducing agents. The substance that gives oxygen (or removes hydrogen) is the oxidising agent; it is itself reduced. The substance that removes oxygen (or gives hydrogen) is the reducing agent; it is itself oxidised. So the substance oxidised is the reducing agent and the substance reduced is the oxidising agent. Students mix these up under pressure, so say it as a rule: oxidised = reducing agent.

Work through the NCERT set. ZnO + C → Zn + CO: zinc oxide loses oxygen (reduced, oxidising agent), carbon gains oxygen (oxidised, reducing agent). MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂: HCl loses hydrogen and becomes Cl₂, so HCl is oxidised and is the reducing agent; MnO₂ loses oxygen, so it is reduced and is the oxidising agent. 4Na + O₂ → 2Na₂O: sodium is oxidised, oxygen is reduced. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe: aluminium is oxidised (reducing agent) and iron oxide is reduced; this is also a displacement reaction. H₂S + Cl₂ → S + 2HCl: H₂S loses hydrogen, so it is oxidised.

When a metal is attacked by substances around it, such as moisture, acids and gases, it is said to corrode, and the process is corrosion. It is an oxidation of the metal by air and water. NCERT gives three everyday cases: the black coating on silver, which is silver sulphide formed by reaction with hydrogen sulphide in the air; the green coating on copper, which is basic copper carbonate formed in moist air containing carbon dioxide; and rust on iron, the reddish-brown, flaky hydrated iron(III) oxide, Fe₂O₃·xH₂O. A 2020 MCQ asked what the green coating on copper is; the answer is copper carbonate (basic copper carbonate), not copper oxide.

Rusting needs both oxygen and water together. Iron in dry air, or in boiled water with no dissolved oxygen, does not rust; the same nail in ordinary water open to air rusts within days. The equation you can quote: 4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s).

Corrosion is a serious problem: it weakens car bodies, bridges, railings, ships and every iron object exposed to weather, and huge sums are spent replacing damaged metal each year. The chapter says prevention is discussed in detail in the chapter on metals, but a 2-mark question here can still ask for two methods. Give any two: painting, applying oil or grease, galvanisation (coating iron with a layer of zinc), chrome plating, or alloying (stainless steel). Each method works by keeping air and moisture away from the metal surface.

Why do we paint iron articles? Because the paint layer stops air and moisture reaching the iron, so the oxidation that causes rusting cannot occur. That single sentence is the whole answer to NCERT exercise 18.

Fats and oils, and foods that contain them, are slowly oxidised by the oxygen in air when they are kept for a long time. Their smell and taste change and they become unpleasant; we say the food has turned rancid. The process is called rancidity. Butter, ghee, fried snacks, cooking oil and cake kept for weeks are the usual examples. Because rancidity is an oxidation, everything that prevents fat from meeting oxygen slows it down.

Ways to prevent rancidity, of which the board asks for two: add antioxidants, which are substances that prevent oxidation, to foods containing fats and oils; store food in airtight containers so that air cannot reach it; keep food in a refrigerator, since oxidation is slower at low temperature; keep food away from light; and flush packets of chips and other fried snacks with nitrogen gas. Nitrogen is unreactive, so it replaces the oxygen in the packet and the chips stay fresh for months. NCERT exercise 19 is exactly this: oil and fat containing food items are flushed with nitrogen. Why?

A typical 2-mark question runs: what changes would you observe in a rancid food item, and how can rancidity be prevented? Write two changes (unpleasant smell, changed taste, sometimes a changed colour) and two prevention methods with the reason that each keeps oxygen away or slows oxidation. Corrosion and rancidity are both effects of oxidation, and the chapter ends with that link: one damages metals, the other spoils food, and the chemistry underneath is the same gain of oxygen.