NCERT Solutions Class 10 Science Chapter 1 Chemical Reactions and Equations

Surface-first thinking. Whenever a board question asks why a metal is cleaned, scraped or polished before a reaction, the marking point is almost always the same idea: a surface layer is stopping the real reaction, so we remove it first.

For magnesium the surface layer is its own oxide. Magnesium sits high in the reactivity series, so even at room temperature it slowly reacts with oxygen and the moisture in air. Over days this builds up a dull, powdery coat of magnesium oxide on the bright ribbon. Oxide layers like this are chemically stable and do not burn, so they act like a shield over the clean metal.

When we try to ignite an uncleaned ribbon, the flame meets this oxide shield first, not the metal, so the reaction either refuses to start or proceeds in a slow, patchy way. Rubbing with sandpaper removes the shield and uncovers fresh magnesium, which then meets oxygen directly. That is why a cleaned ribbon lights up at once with a bright white flame and gives clean white magnesium oxide.

Answer: We clean the ribbon to remove its unreactive magnesium-oxide coating, exposing fresh metal so combustion starts quickly and burns cleanly.

Order of balancing. A clean routine prevents most errors: balance metals first, then non-metal groups (like sulphate), then hydrogen, and finally oxygen. Leave free elements such as H₂ or O₂ for the last step, because their coefficients can soak up any leftover atoms.

In part (i) the count is symmetric, so only HCl needs a 2. In part (ii) aluminium sulphate brings two aluminium atoms and three sulphate groups in one formula, so the right approach is to fix three barium sulphate units and two aluminium chloride units, which forces three barium chloride units on the left. Always re-count at the end: 3 Ba, 6 Cl, 2 Al, 3 S, 12 O on each side.

Part (iii) is the classic odd-hydrogen case. Because hydrogen leaves as H₂ (an even number), we take two water molecules so the hydrogen comes out even, which then needs two NaOH and two Na. This metal-with-water pattern repeats for potassium and calcium, so learning it once pays off across the chapter.

Answer: (i) H₂ + Cl₂ → 2HCl; (ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃; (iii) 2Na + 2H₂O → 2NaOH + H₂.

Read the words, assign the states. Board questions like this hand you the physical states inside the sentence. "Solution in water" means (aq), "insoluble" means a solid (s) precipitate, and "water" as a product is liquid (l). Translate each phrase before you balance, and the state symbols fall into place.

Part (i) is a double displacement (precipitation) reaction: the ions of the two dissolved salts change partners. Barium pairs with sulphate to make insoluble barium sulphate, which leaves the solution as a white solid, while sodium pairs with chloride to make sodium chloride, which stays dissolved. The chloride count forces a 2 before sodium chloride.

Part (ii) is a neutralisation, a special double displacement between an acid and a base. The hydrogen of the acid joins the hydroxide of the base to form water, while sodium and chloride form common salt in solution. Both species are in a 1:1 ratio, so no extra coefficients are needed. A small down-arrow can also mark a precipitate and an up-arrow an escaping gas.

Answer: (i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq); (ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).

Trace the chain of reactions. This question is easy if you remember the three-stage lime story: quick lime, then slaked lime, then the carbonate finish. Many students name slaked lime as 'X', but 'X' is the starting substance used to make the whitewash solution, so it must be calcium oxide.

Calcium oxide is produced industrially by heating limestone, and it reacts so strongly with water that the mixture becomes hot enough to steam. This exothermic combination gives calcium hydroxide, a mild base that dissolves slightly in water to form lime water, which the painter brushes over the wall.

The finishing reaction happens on the wall itself: carbon dioxide from the air reacts with the calcium hydroxide layer to deposit calcium carbonate, a hard white solid that gives the bright, smooth finish. So three compounds appear in order, calcium oxide, calcium hydroxide and calcium carbonate, and naming all three with their roles secures full marks.

Answer: 'X' is calcium oxide, CaO; CaO(s) + H₂O(l) → Ca(OH)₂(aq), an exothermic combination reaction giving slaked lime.

Let the formula decide the ratio. The cleanest way to answer is to read the volumes straight off the balanced equation. Decomposing water gives two hydrogen molecules for each oxygen molecule, and for gases the molecule ratio equals the volume ratio, so hydrogen comes out at twice the volume of oxygen with no calculation needed.

This is a decomposition reaction driven by electricity, an electrolytic decomposition. Pure water barely conducts, so a few drops of dilute sulphuric acid are added to let the current pass. As the current flows, bubbles rise at both carbon electrodes, but the tube over the negative electrode fills roughly twice as fast.

Identifying the gases is the natural next step: a burning splint near the hydrogen tube gives a small "pop" sound, while the gas in the other tube relights a glowing splint, confirming oxygen. So the gas collected in double amount is hydrogen, released at the cathode, exactly as the 2:1 ratio in the equation predicts.

Answer: Hydrogen is collected in double the amount because water has hydrogen and oxygen atoms in a 2:1 ratio; the gas in larger amount is hydrogen.

Follow the more reactive metal. The key to every displacement question is the reactivity series. Iron stands above copper, which means iron holds on to sulphate more strongly than copper does, so when an iron nail sits in copper sulphate, iron pushes copper out of the salt and takes its place.

The visible change tracks the ions in solution. The blue of copper sulphate is caused by dissolved copper ions. As iron goes into solution as iron(II) ions and copper comes out as metal, the population of blue copper ions falls and pale green iron ions build up, so the liquid shifts from blue toward green. The copper that leaves the solution settles on the nail as a thin reddish-brown layer.

This single experiment shows three observations at once: a fading blue colour, a greenish tint appearing, and a brown coating on the iron. All three come from the one displacement reaction, and quoting them together makes the answer complete.

Answer: Because iron displaces copper from copper sulphate, copper ions (blue) are removed and iron sulphate (light green) forms, so the solution colour changes and copper coats the nail.

Pick a guaranteed precipitate. To answer safely, choose a pair of salts you know gives an insoluble product, because a visible precipitate is the surest proof of double displacement. Silver chloride and barium sulphate are the two classic "always insoluble" choices, so reactions producing them are reliable examples.

Take silver nitrate and sodium chloride, both freely soluble in water. When the solutions meet, the ions rearrange: silver joins chloride, and sodium joins nitrate. Silver chloride cannot stay dissolved, so it appears at once as a white solid that clouds the mixture, while sodium nitrate remains in solution.

Another fully acceptable answer is lead nitrate with potassium iodide, which gives a bright yellow precipitate of lead iodide. Both fit the ion-exchange shape AB + CD → AD + CB and both produce a clear precipitate, so either earns the marks.

Answer: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), a double displacement reaction giving a white precipitate of silver chloride.

Track the oxygen. For Class 10 the quickest route is to follow the oxygen atoms: whoever gains oxygen during the reaction is oxidised, and whoever loses oxygen is reduced. Apply this rule to each substance separately and the answer drops out.

In the first reaction sodium starts as a free metal and ends inside sodium oxide, so it has clearly gained oxygen and is oxidised. The oxygen molecule is the oxidising agent here, since it supplies the oxygen that sodium picks up.

In the second reaction, copper oxide begins with oxygen attached and ends as bare copper metal, so it has lost oxygen and is reduced. Meanwhile hydrogen begins as a free gas and ends combined in water, so it has gained oxygen and is oxidised. A cross-check by gain or loss of hydrogen gives the same answer: hydrogen is the reducing agent because it removes oxygen from copper oxide. Stating both the substance and the reason earns full marks.

Answer: (i) Sodium is oxidised. (ii) Copper oxide is reduced; hydrogen is oxidised. Both are redox reactions.

Decide the truth first, then judge each line. For a statement-checking question, never grade the options one by one without first settling the actual chemistry. Here oxygen moves from lead oxide to carbon, so lead oxide is reduced and carbon is oxidised. Hold these two facts and test each statement against them.

Statement (a) claims lead is getting reduced. The substance that loses oxygen is lead oxide; lead itself is the product of that reduction, so naming "lead" as the thing reduced is wrong, making (a) incorrect. Statement (d), which says lead oxide is reduced, is the correct version of the same idea.

Statement (b) says carbon dioxide is oxidised, but it is what forms after carbon picks up oxygen, so it is a product, not something being oxidised, making (b) incorrect. Statement (c), carbon is oxidised, is correct. So exactly two statements, (a) and (b), are wrong, giving option (i).

Answer: The incorrect statements are (a) and (b), so the answer is option (i).

Match the skeleton to the type. The fastest way to classify a reaction is to compare its skeleton with the four standard patterns. Here one free element (aluminium) reacts with one compound (iron oxide), and another element (iron) comes out free, which is precisely the displacement shape A + BC → AC + B, so the answer is option (d).

The driving force is the reactivity order: aluminium lies above iron, so aluminium has the stronger pull on oxygen. When the mixture is ignited, aluminium strips oxygen from iron(III) oxide to form aluminium oxide, leaving iron behind, and the energy released is enough to melt the iron.

It is worth knowing this is also a redox reaction, since aluminium is oxidised and iron is reduced, but the type asked for is based on structure, which is displacement. Industrially this is the thermite reaction, prized for welding rail tracks because it makes molten iron exactly where it is needed.

Answer: Displacement reaction, option (d); aluminium displaces iron in the exothermic thermite reaction.

Use the metal-acid rule. The reliable rule for any metal added to a dilute acid is: salt plus hydrogen gas, provided the metal lies above hydrogen in the reactivity series. Iron does lie above hydrogen, so it reacts and pushes hydrogen out of the acid. That single rule eliminates three of the four options.

The salt takes its name from the acid: hydrochloric acid gives a chloride, so the salt is iron chloride. The hydrogen displaced leaves as a gas, seen as a steady stream of bubbles. There is no way for chlorine gas or a hydroxide to form here, because chlorine stays locked in the chloride salt and no source of hydroxide is present.

Option (d), iron salt and water, fits an acid-plus-base neutralisation, not an acid-plus-metal reaction; water is not produced when a metal reacts with an acid. So the only consistent choice is (a).

Answer: Option (a): dilute hydrochloric acid reacts with iron filings to give iron chloride and hydrogen gas.

Tie the definition to the law. A complete answer here has two parts: what a balanced equation is, and why we insist on it. Keep them separate and support the second with the law of conservation of mass, which is the real reason behind balancing.

A balanced equation is simply one where the atom count of every element agrees on both sides of the arrow. We achieve this by adjusting coefficients only, never the chemical formulae, because changing a formula would change the substance itself. A quick way to verify is to list each element and tally its atoms on the left and the right.

The reason we must balance is physical, not just neat bookkeeping. Atoms are neither created nor destroyed in a chemical change; they are only rearranged. So the same atoms that enter as reactants must reappear among the products, which forces equal atom counts and hence equal mass. An unbalanced equation would imply atoms appearing from nowhere or vanishing.

Answer: A balanced equation has equal atoms of each element on both sides; we balance to satisfy the law of conservation of mass.

Translate, then balance. Treat each part in two clean stages. Stage one: turn the words into formulae and write a skeletal equation. Stage two: balance it by changing only the coefficients. Doing both stages carefully prevents the most common board-exam slips.

In part (a) remember that nitrogen and hydrogen exist as N₂ and H₂, so ammonia formation needs three hydrogen molecules and yields two ammonia molecules. In part (b) "burns in air" means reaction with oxygen, and balancing hydrogen and sulphur first then fixing oxygen gives the 2:3:2:2 ratio.

Part (c) is the same aluminium-sulphate balancing seen earlier: keep sulphate as one block, set three barium sulphate and two aluminium chloride units, which forces three barium chloride on the left. Part (d) is the alkali-metal-with-water pattern, identical in shape to sodium with water.

Answer: (a) N₂ + 3H₂ → 2NH₃; (b) 2H₂S + 3O₂ → 2H₂O + 2SO₂; (c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄; (d) 2K + 2H₂O → 2KOH + H₂.

Spot the polyatomic groups. Each of these equations is built around a polyatomic ion that stays intact, so the smart move is to balance those groups first: nitrate in (a) and (c), sulphate in (b) and (d). Keep them as single units and the counting becomes light.

In (a) the calcium nitrate product carries two nitrate groups, so two nitric acid molecules are needed, and the four hydrogens form two water molecules. In (b) sodium sulphate has two sodium atoms, so two sodium hydroxide units are required, and again two water molecules balance the hydrogen and oxygen.

Equation (c) is a quiet one: one of each species already balances sodium, chlorine, silver and nitrate, so no coefficient is needed; confirm rather than force a number. In (d) the single sulphate transfers cleanly to barium sulphate, but the two chlorides released require two hydrochloric acid molecules.

Answer: (a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O; (b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O; (c) NaCl + AgNO₃ → AgCl + NaNO₃; (d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl.

Name the type as you balance. Each part hides a reaction type, and spotting it helps you predict the products and balance with confidence. Part (a) is a combination, (b) and (c) are displacements, and (d) is a double displacement.

Part (a) needs no coefficients once written correctly, since calcium, carbon, oxygen and hydrogen all balance with one of each species. Part (b) is zinc displacing silver, where zinc nitrate carries two nitrate groups, so two silver nitrate units are consumed and two silver atoms are freed.

Part (c) is the trickier displacement because aluminium is trivalent: balancing chlorine across copper chloride and aluminium chloride needs three copper chloride and two aluminium chloride, freeing three copper atoms. Part (d) is a straightforward ion swap where barium pairs with sulphate to precipitate barium sulphate, and the two chlorides form two potassium chloride.

Answer: (a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; (b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag; (c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu; (d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl.

Balance and label together. Tackle each part by balancing and naming the type in one pass, using the number of reactants and products as your first clue. That count alone narrows the type, and the formulae confirm it.

In (a) two dissolved salts exchange partners, so potassium goes with iodide and barium with bromide; balancing bromine and iodine needs two potassium bromide and two potassium iodide, marking it a double displacement. In (b) only one reactant, zinc carbonate, appears, and it splits into two products on heating, the hallmark of decomposition.

In (c) two elements combine into a single compound, hydrogen chloride, the clean signature of a combination reaction, and a 2 before HCl balances it. In (d) a free metal, magnesium, reacts with an acid to release hydrogen and form a salt, a displacement; the two chlorides require two hydrochloric acid molecules.

Answer: (a) double displacement; (b) decomposition; (c) combination; (d) displacement, with balanced equations as shown.

Define by the direction of heat. The whole distinction rests on which way heat flows. If a reaction gives heat out to the surroundings, it is exothermic; if it pulls heat in from the surroundings, it is endothermic. Build your answer around this single idea and then attach examples.

Exothermic reactions are very common because most fuels burn and most combination reactions release energy. Burning natural gas and respiration both release energy that we use for heat and for living. Quick lime reacting with water is so strongly exothermic that the mixture can steam.

Endothermic reactions need a continuous supply of energy to proceed, usually as heat. Decomposing calcium carbonate requires constant heating in a kiln, and it stops the moment heating stops. The barium hydroxide and ammonium chloride mixture draws so much heat from its surroundings that the test tube turns cold to the touch.

Answer: Exothermic reactions release heat (burning of natural gas, respiration); endothermic reactions absorb heat (decomposition of calcium carbonate).

Connect food, oxygen and energy. The marking idea is that respiration gives out energy, and any energy-releasing reaction is exothermic. Build the explanation from food to glucose to energy, and the conclusion follows naturally.

Our body cannot run without a constant supply of energy, and that energy comes from the food we eat. The carbohydrates in our meals are first broken down during digestion into a simple sugar, glucose, the fuel that cells actually use.

Inside the cells, glucose reacts with oxygen and is broken down into carbon dioxide and water, and in doing so it releases energy. This release of energy is exactly what defines an exothermic reaction. The released energy keeps the body warm and drives movement, growth and repair.

Answer: Respiration oxidises glucose to release energy; because energy is given out, it is an exothermic reaction.

Contrast the structures. The clearest way to justify "opposite" is to put the two general patterns side by side. Combination is A + B → AB, many into one. Decomposition is AB → A + B, one into many. They are exact reverses, so the label fits perfectly.

The energy story reinforces the contrast. Combination reactions usually give out energy; calcium oxide combining with water releases so much heat that the mixture warms up. That is energy coming out as the new bonds form.

Decomposition reactions usually need energy put in to break the existing bonds. Calcium carbonate does not fall apart on its own; it must be heated strongly to split into calcium oxide and carbon dioxide. So one process assembles a larger molecule and releases energy, while the other dismantles a molecule and absorbs energy.

Answer: Decomposition AB → A + B reverses combination A + B → AB, so it is the opposite; e.g. CaCO₃ → CaO + CO₂ vs CaO + H₂O → Ca(OH)₂.

One clean example per energy type. The question wants exactly three equations, one for each energy source, so the best strategy is to pick a single, well-known decomposition for heat, light and electricity, and state the source in words rather than on the arrow.

For heat, the standard choice is the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide, an important industrial reaction used to make quick lime for cement. Heating supplies the energy that breaks the carbonate apart.

For light, silver chloride is the classic example: it turns from white to grey in sunlight as it decomposes into silver and chlorine, once used in black-and-white photography. For electricity, the electrolysis of water splits it into hydrogen and oxygen in a 2:1 volume ratio.

Answer: Heat: CaCO₃ → CaO + CO₂; Light: 2AgCl → 2Ag + Cl₂; Electricity: 2H₂O → 2H₂ + O₂.

Compare what gets swapped. The cleanest contrast is to ask: is a single element being replaced, or are two ions being exchanged? Displacement replaces one element; double displacement exchanges ions between two compounds. Anchor your answer on this and support it with one equation each.

In displacement, a free, more reactive element pushes a less reactive element out of its compound. Iron in copper sulphate is the textbook case: iron, being more reactive, takes the place of copper, forming iron sulphate and depositing copper. Only one element changes hands, and the reactivity series decides the outcome.

In double displacement, two compounds in solution trade their partner ions, often producing a precipitate. Sodium sulphate with barium chloride is the standard example: sulphate goes to barium and chloride goes to sodium, so barium sulphate falls out as a white solid while sodium chloride stays dissolved.

Answer: Displacement replaces one element Fe + CuSO₄ → FeSO₄ + Cu; double displacement exchanges ions of two compounds Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.

Apply the reactivity series. Recovering a metal from its salt solution by adding a more reactive metal is a standard displacement, and the reactivity series tells you whether it will work. Copper lies above silver, so copper can push silver out of silver nitrate, which is exactly what the refining step uses.

When copper metal is dipped into silver nitrate solution, copper atoms go into solution while silver ions are reduced and deposit as shining silver metal. The solution, originally containing silver nitrate, gradually turns blue as copper nitrate forms in its place.

Balancing requires care with the nitrate groups: copper nitrate contains two nitrate groups, so two silver nitrate molecules react and two silver atoms are released for every copper atom that dissolves. The net result is pure silver recovered as a solid and copper carried away in solution as copper nitrate.

Answer: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag; copper, being more reactive, displaces silver from the solution.

Lead with the insoluble product. The defining feature of a precipitation reaction is that a solid drops out of a clear solution, so frame your answer around that insoluble product. Begin by stating that two solutions react to give an insoluble precipitate, then support it with examples whose precipitates you can name.

These reactions are almost always double displacements: two dissolved salts swap their ions, and one of the new combinations happens to be insoluble. The insoluble combination cannot stay dissolved, so it appears as a cloud of fine solid that slowly settles to the bottom.

Sodium sulphate with barium chloride is the classic example, giving a white precipitate of barium sulphate while sodium chloride stays in solution. Silver nitrate with sodium chloride is another, giving a white precipitate of silver chloride. Naming the colour of the precipitate (white here) adds detail that examiners reward.

Answer: A precipitation reaction produces an insoluble precipitate from two solutions; e.g. Na₂SO₄ + BaCl₂ → BaSO₄(s) + 2NaCl and AgNO₃ + NaCl → AgCl(s) + NaNO₃.

Define by oxygen, then prove with examples. For Class 10 the oxygen definition is the simplest and the most expected: oxidation is gain of oxygen, reduction is loss of oxygen. State both definitions plainly, then back each with two equations where the oxygen change is obvious.

For oxidation, the burning of metals is ideal because the metal visibly takes up oxygen to form an oxide. Copper heated in air forms black copper oxide, and magnesium burns to form white magnesium oxide; in both, the element gains oxygen and is oxidised.

For reduction, choose reactions where an oxide loses its oxygen. Copper oxide heated with hydrogen turns back to copper, losing oxygen to form water. Zinc oxide heated with carbon gives zinc and carbon monoxide, again the oxide losing oxygen. Both processes accompany each other in a redox reaction.

Answer: (a) Oxidation is gain of oxygen 2Cu + O₂ → 2CuO, 2Mg + O₂ → 2MgO. (b) Reduction is loss of oxygen CuO + H₂ → Cu + H₂O, ZnO + C → Zn + CO.

Read the clues like a detective. Identification questions give you colour and behaviour clues that point to one metal. "Shiny brown" and "turns black on heating in air" together describe copper, whose reddish-brown surface darkens to black copper oxide when heated.

The chemistry behind the colour change is straightforward oxidation. On heating, copper combines with oxygen from the air, and oxygen is added to the metal, so the copper is oxidised. The product is copper(II) oxide, a black solid that forms a coating over the bright metal.

A neat way to confirm the identity is to recall the reverse reaction from the chapter: when this black coated copper oxide is heated in a stream of hydrogen, the black layer turns back to brown copper as the oxide is reduced. That round trip, brown to black to brown, is unique to copper among the common metals here.

Answer: 'X' is copper, and the black compound formed on heating in air is copper(II) oxide, CuO.

Stop the air and water reaching the metal. The single idea behind painting iron is to break contact between the metal and its attackers, oxygen and moisture. Rusting needs both air and water; if either is kept away from the iron, rust cannot form, and paint provides that barrier.

Left unprotected, iron reacts with oxygen and moisture to form a flaky reddish-brown rust. Rust does not cling tightly like some protective oxides; it keeps flaking off and exposing fresh metal, so the object slowly weakens and crumbles. For large structures such as bridges and railway tracks, this damage is both dangerous and expensive.

A coat of paint seals the surface so that air and water cannot touch the iron underneath, preventing the rusting reaction from starting. The paint must be renewed when it chips, because a break in the coat lets moisture reach the metal again. Oiling, greasing and galvanising work on the same principle.

Answer: Paint forms a protective layer that keeps air and moisture away from iron, preventing rusting (corrosion) and extending the life of the article.

Replace the reactive gas with an inert one. The reason chips and similar foods are packed in nitrogen is to keep oxygen away from the oils and fats inside. Oxygen is the gas that turns these foods rancid, so removing it protects both the flavour and the safety of the food.

When oils and fats are exposed to the oxygen in air, they undergo oxidation and become rancid. Rancid food smells and tastes unpleasant and is no longer good to eat. Because the spoilage is an oxidation reaction, it can be slowed simply by denying the food its supply of oxygen.

Nitrogen is ideal because it is chemically unreactive and harmless. When a packet is flushed with nitrogen, the gas pushes out the air, leaving almost no oxygen inside the sealed pack, so the oxidation cannot proceed. The puffed-up packet of chips is filled with nitrogen for exactly this reason, and it also cushions the contents from getting crushed. Antioxidants and airtight storage help too.

Answer: Nitrogen is unreactive and displaces oxygen from the pack, preventing the oxidation that causes rancidity and keeping oil and fat rich foods fresh.

Two oxidation effects, two materials. Corrosion and rancidity are both consequences of oxidation, but they act on different things, so the clearest answer defines each and gives a matching example. Corrosion attacks metals; rancidity spoils food.

Corrosion is the gradual damage to a metal caused by the air, moisture and chemicals around it. The most familiar case is the rusting of iron, where moist air turns the shiny metal into flaky reddish-brown rust. Silver tarnishing black and copper turning green are corrosion too, each metal forming its own coating.

Rancidity is what happens to oils and fats when they are oxidised on exposure to air over time. The food develops a foul smell and an off taste, as anyone who has smelt stale butter or old chips knows. Both are oxidation processes, but corrosion is tied to metals and rancidity to food.

Answer: (a) Corrosion is the slow damage of a metal by its surroundings (rusting of iron). (b) Rancidity is the oxidation of oils and fats that spoils smell and taste (stale butter).