NCERT Solutions for Class 10 Science Chapter 3 Metals and Non-metals

Answer: These four parts test the exceptions that break the general picture of a hard, shiny, solid metal. Most metals are hard solids, but the atomic structure of a few metals breaks that pattern — so the textbook names specific examples for each property.

Part (i): Mercury is a liquid at room temperature — the only metal that is.

Part (ii): Sodium (and the other alkali metals, lithium and potassium) is so soft that it can be cut with a knife.

Part (iii): Silver is the best conductor of heat; copper is the second best.

Part (iv): Lead and mercury are comparatively poor conductors of heat (NCERT, p. 2).

Answer: Both words describe how metals can be reshaped without breaking; the difference is the shape they are given. These properties exist because the layers of atoms in a metal can slide over each other when force is applied.

• Malleable: a metal that can be beaten into thin sheets is said to be malleable. Iron, zinc, lead and copper flatten when struck with a hammer, and gold and silver are the most malleable metals (NCERT, p. 2).
• Ductile: a metal that can be drawn into thin wires is said to be ductile. Gold is the most ductile metal — a wire of about 2 km length can be drawn from just one gram of gold (NCERT, p. 2).

Answer: Sodium is so reactive that it combines vigorously with the oxygen and moisture in ordinary air — the reaction is so violent that sodium can catch fire if kept in the open (NCERT, p. 5). Immersing it in kerosene oil cuts it off from both air and moisture, which protects the metal and prevents accidental fires.

Potassium is stored the same way for the same reason.

Answer: The violence of a metal–water reaction depends on where the metal sits in the reactivity series. Potassium reacts explosively with cold water; calcium reacts less violently; iron is so unreactive towards water that it needs steam, and the product is the oxide, not the hydroxide.

Part (i): Iron with steam gives the mixed oxide Fe3O4 and hydrogen gas:

\[ 3\text{Fe(s)} + 4\text{H}_2\text{O(g)} \xrightarrow{\text{Heat}} \text{Fe}_3\text{O}_4\text{(s)} + 4\text{H}_2\text{(g)} \]

Part (ii): Calcium reacts with cold water to give calcium hydroxide and hydrogen; the heat released is not enough for the hydrogen to catch fire:

\[ \text{Ca(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)} \]

Potassium reacts violently with cold water, and the reaction is so exothermic that the hydrogen released catches fire:

\[ 2\text{K(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{KOH(aq)} + \text{H}_2\text{(g)} + \text{heat energy} \]

The figure below shows the apparatus used to pass steam over a metal sample — the flask boils water and the steam is led over the metal in the heated tube.

Answer: A displacement happens only when the added metal is more reactive than the metal whose salt is in the solution (NCERT, p. 9). So each ‘Displacement’ cell tells you the added metal is above the salt’s metal in the series, and each ‘No reaction’ cell tells you it is below.

• A displaces copper from CuSO4 → A is above Cu; A does not react with FeSO4 → A is below Fe. So Fe > A > Cu.
• B displaces iron from FeSO4 → B is above Fe; B does not react with ZnSO4 → B is below Zn. So Zn > B > Fe.
• C displaces silver from AgNO3 → C is above Ag; C does not react with CuSO4 → C is below Cu. So Cu > C > Ag.
• D reacts with nothing → D is the least reactive of the four.

Part (i): B is the most reactive metal.

Part (ii): B is more reactive than copper (it is above Fe, and Fe is above Cu), so B displaces copper: the blue colour of copper(II) sulphate fades and a reddish-brown deposit of copper appears on B.

Part (iii): Decreasing reactivity: B > A > C > D.

Answer: Metals above hydrogen in the reactivity series displace hydrogen from dilute acids, so the gas produced is always hydrogen, \( \text{H}_2 \). The metal atom replaces the hydrogen of the acid to form a salt and hydrogen gas.

Iron is above hydrogen, so with dilute sulphuric acid it gives iron(II) sulphate and hydrogen:

\[ \text{Fe(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{H}_2\text{(g)} \]

Answer: Zinc sits above iron in the reactivity series, so zinc displaces iron from its salt solution — this is a displacement reaction. The green colour of iron(II) sulphate fades as iron is displaced, and a grey deposit of iron forms on the zinc.

\[ \text{Zn(s)} + \text{FeSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Fe(s)} \]

The photograph below shows how such metal-versus-salt-solution tests are set up: an iron nail in copper sulphate solution and a copper wire in iron sulphate solution. Only one test tube reacts, because only one added metal is more reactive than the metal in the solution.

Answer: An electron-dot structure shows only the valence (outermost) electrons of an atom, drawn as dots around its symbol. Metals have one or two valence electrons to lose; a non-metal like oxygen needs electrons to complete its octet. In an ionic compound, the electrons lost by the metal are exactly the electrons gained by the non-metal (NCERT, p. 11).

Part (i): Using the shell configurations (Na: 2,8,1; Mg: 2,8,2; O: 2,6):

• Sodium: Na· — one valence electron.
• Magnesium: ·Mg· — two valence electrons.
• Oxygen: ·O· with two dots above and two below the symbol — six valence electrons, drawn as two lone pairs and two unpaired electrons.

Part (ii): Formation by electron transfer. For Na2O, two sodium atoms each lose one electron to form Na+; oxygen gains both electrons to complete its octet as O2−:

\[ 2\text{Na} \rightarrow 2\text{Na}^+ + 2e^- \qquad \text{O} + 2e^- \rightarrow \text{O}^{2-} \]

For MgO, magnesium loses both its valence electrons and oxygen accepts them:

\[ \text{Mg} \rightarrow \text{Mg}^{2+} + 2e^- \qquad \text{O} + 2e^- \rightarrow \text{O}^{2-} \]

The two figures below show the same electron transfer in the formation of sodium chloride and magnesium chloride — a metal atom’s electrons pass to non-metal atoms, and the resulting oppositely charged ions attract each other and form the compound.

Part (iii): In Na2O, the ions present are two sodium cations, Na+, and one oxide anion, O2−. In MgO, the ions are the magnesium cation, Mg2+, and the oxide anion, O2−.

Answer: Melting requires breaking the forces that hold a solid together, and those forces are strong in ionic compounds. The oppositely charged ions are locked in a rigid lattice by strong electrostatic forces of attraction, so a considerable amount of energy is needed to separate them (NCERT, p. 12).

That is why NaCl melts only at 1074 K and CaO at 2850 K (Table 3.4).

Answer: The three terms describe one idea at three stages: a natural substance in the earth, the special version of it worth mining, and the waste that comes with it.

• Mineral: the elements or compounds that occur naturally in the earth’s crust — for example, sodium chloride and magnesium chloride, which occur as soluble salts in seawater (NCERT, p. 13).
• Ore: a mineral that contains a very high percentage of a particular metal, so high that the metal can be profitably extracted from it. Every ore is a mineral, but not every mineral is an ore.
• Gangue: the impurities such as soil and sand that contaminate the ore and must be removed before the metal is extracted (NCERT, p. 14).

Answer: Gold, silver, platinum and copper are found in the free (native) state because they sit at the bottom of the reactivity series. Being the least reactive metals, they do not combine readily with oxygen or other elements, so they survive uncombined in the earth’s crust (NCERT, p. 13). Two examples: gold and silver.

Answer: Reduction. The metal oxide must lose its oxygen to become the metal, and losing oxygen is reduction. For moderately reactive metals, a suitable reducing agent such as carbon (coke) is used. For highly reactive metals like sodium, magnesium and aluminium, carbon cannot reduce their oxides, so electrolytic reduction of the molten compound is used (NCERT, pp. 15-16).

Do not confuse the steps: roasting and calcination convert sulphide and carbonate ores into oxides; the step that turns an oxide into the metal is reduction. For the least reactive metals, the oxide is reduced by heating alone.

The flow diagram below summarises the whole route from ore to pure metal — enrichment of the ore, conversion to oxide, reduction, and refining.

Answer: A metal can displace another metal from its oxide only if it is more reactive — that is, above it in the reactivity series: Mg > Zn > Cu. Working through all nine cells:

• Zinc oxide + magnesium: Mg is above Zn → displacement occurs: \( \text{Mg} + \text{ZnO} \rightarrow \text{MgO} + \text{Zn} \).
• Zinc oxide + zinc / + copper: zinc cannot displace itself, and Cu is below Zn → no reaction.
• Magnesium oxide + zinc / + copper / + magnesium: no reaction in all three — zinc and copper are below Mg, and Mg cannot displace itself.
• Copper oxide + zinc: Zn is above Cu → displacement occurs: \( \text{Zn} + \text{CuO} \rightarrow \text{ZnO} + \text{Cu} \).
• Copper oxide + magnesium: Mg is above Cu → displacement occurs: \( \text{Mg} + \text{CuO} \rightarrow \text{MgO} + \text{Cu} \).
• Copper oxide + copper: same metal → no reaction.

So displacement takes place in three cases: Mg with zinc oxide, Zn with copper oxide, and Mg with copper oxide. In each, the added metal is above the metal of the oxide in the activity series.

Answer: The least reactive metals — gold, silver and platinum — do not corrode easily because they do not combine with the oxygen, moisture or sulphur compounds in the air.

Aluminium also resists corrosion in practice, but for a different reason: it is reactive, yet the thin, tough layer of aluminium oxide that forms on its surface seals the metal underneath from further attack (NCERT, p. 5).

Answer: (d) AgNO3 solution and copper metal.

A displacement reaction happens only when the added metal is more reactive than the metal in the solution. Check each pair against the reactivity series:

• (a) Na is far above Cu, so copper cannot displace sodium. No reaction.
• (b) Mg is above Al, so aluminium cannot displace magnesium. No reaction.
• (c) Fe is above Ag, so silver cannot displace iron. No reaction.
• (d) Cu is above Ag, so copper displaces silver from silver nitrate: \( \text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + 2\text{Ag} \). Reaction occurs.

Answer: (d) All of the above.

Rusting needs both air and moisture (NCERT, p. 17), so every method that seals the iron surface from air and water prevents rusting. Grease and paint form a barrier coat; a zinc coating is galvanisation, which protects the iron even if the coating is scratched. Each method works, so the correct option is (d).

Answer: (a) calcium.

Calcium burns to calcium oxide, CaO, which is an ionic compound — ionic compounds have high melting points, and CaO dissolves in water to give an alkaline solution of calcium hydroxide. Check the distractors: carbon gives CO2, a gas; silicon gives SiO2, which is insoluble in water; iron gives Fe2O3 or Fe3O4, which are insoluble oxides.

Only calcium’s oxide has both the properties named in the question.

Answer: (c) zinc is more reactive than tin.

Tin sits below zinc in the reactivity series, so it is less reactive and does not react with the food acids in the can. Zinc, being more reactive, could react with the food contents and spoil them, so food cans are coated with the less reactive metal — tin.

Zinc’s greater reactivity is exactly why galvanised iron uses zinc to protect iron, but the same property makes zinc unsuitable against food.

Answer: The two tests use the physical properties of malleability and electrical conductivity.

Part (a): Use the hammer to strike each sample on a hard block. Metals are malleable, so they flatten into thin sheets without breaking; non-metals are brittle and shatter. Use the battery, bulb, wires and switch to build a simple circuit, and place each sample between the terminals one by one — the bulb glows when the sample conducts electricity.

Metals conduct; most non-metals do not.

Part (b): These tests are quick and work for most samples, but they are not conclusive on their own because of the exceptions: graphite is a non-metal that conducts electricity, iodine is a lustrous non-metal, and sodium is a soft metal that can be cut with a knife.

So a final verdict needs chemical tests too — for example, whether the element’s oxide is basic (metal) or acidic (non-metal) (NCERT, p. 4).

Answer: Most metal oxides are basic, but a few react with both acids and bases to produce salt and water. Such metal oxides are called amphoteric oxides (NCERT, p. 5). The two textbook examples are aluminium oxide and zinc oxide.

Aluminium oxide reacts with hydrochloric acid as a base and with sodium hydroxide as an acid:

\[ \text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O} \]

\[ \text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O} \]

Zinc oxide behaves the same way — it reacts with HCl to give zinc chloride and water, and with NaOH to give sodium zincate and water.

Answer: The dividing line in the reactivity series is hydrogen itself: metals above hydrogen can displace it from dilute acids; metals below hydrogen cannot (NCERT, p. 19).

• Displace hydrogen: zinc and iron (also magnesium and aluminium — all above H). Example: \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \).
• Will not displace hydrogen: copper and silver (also mercury and gold — all below H). Copper does not react with dilute HCl at all.

Answer: Electrolytic refining uses electricity to transfer pure metal onto a pure strip, leaving impurities behind (NCERT, p. 16).

• Anode: a thick block of the impure metal M.
• Cathode: a thin strip of pure metal M.
• Electrolyte: a solution of a salt of metal M — for copper, acidified copper sulphate solution.

On passing current, pure metal from the anode dissolves into the electrolyte, and an equivalent amount of pure metal from the electrolyte is deposited on the cathode. Soluble impurities go into the solution, while insoluble impurities settle below the anode as anode mud.

Answer: Heating sulphur in air burns it to sulphur dioxide, a non-metal oxide. Non-metal oxides are acidic, but the acidity shows only when water is present to form an acid — that is the whole point of comparing dry and moist litmus.

Part (a)(i): Dry litmus paper shows no change — the gas alone has no H+ ions in the absence of water.

Part (a)(ii): Moist blue litmus turns red — SO2 dissolves in the moisture to form sulphurous acid, which is acidic (NCERT, p. 4, Activity 3.8).

Part (b): The balanced equation for the burning of sulphur in air:

\[ \text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)} \]

Answer: Rusting needs air and moisture together, so every prevention method cuts the iron off from one or both (NCERT, p. 18). Any two of these are correct:

• Painting, oiling or greasing: a protective coat that stops air and moisture reaching the iron.
• Galvanising: coating with a thin layer of zinc; the zinc protects the iron even if the coating is broken.
• Chrome plating or anodising: a surface layer that shields the metal from the surroundings.
• Alloying: mixing iron with nickel and chromium gives stainless steel, which is hard and does not rust.

Answer: Non-metals form acidic oxides — for example, sulphur burns to SO2, which dissolves in water to form sulphurous acid. Some non-metals form neutral oxides, which show neither acidic nor basic behaviour. This is the chemical opposite of metals, which form basic oxides (NCERT, p. 4).

Answer: All four parts are the reactivity series in action — each metal’s behaviour toward air, water and heat is fixed by its position in the series.

Part (a): Platinum, gold and silver sit at the bottom of the reactivity series. They are the least reactive metals, so they do not tarnish or react with air and moisture, and their shine lasts. That makes them suitable for jewellery.

Part (b): Sodium, potassium and lithium sit at the top of the series. They react vigorously with oxygen and moisture — so vigorously that they can catch fire — so they are kept immersed in oil to isolate them from air and water.

Part (c): Aluminium is reactive, but it forms a thin, tough layer of aluminium oxide (Al2O3) on its surface when exposed to air. This protective oxide layer prevents further corrosion, so the metal underneath stays safe — which is why aluminium utensils are safe to use (NCERT, p. 5).

Part (d): It is easier to obtain a metal from its oxide than from its sulphide or carbonate (NCERT, p. 15). So sulphide ores are first heated strongly in excess air (roasting) and carbonate ores in limited air (calcination) to convert them into oxides; the oxide is then reduced to the metal, usually with carbon.

Answer: The dull green coating on old copper is basic copper carbonate, formed when copper reacts with moist carbon dioxide in the air over time (NCERT, p. 17). Lemon and tamarind juices contain acids. The acid reacts with and dissolves the basic copper carbonate layer, so the fresh, shiny copper surface underneath is exposed and the vessel looks clean again.

Answer: Chemical properties separate metals from non-metals more reliably than physical properties, because physical properties have exceptions. Compare the five chemical behaviours below.

Answer: The solution was aqua regia — a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3:1 (NCERT, p. 8).

Here is the detective’s chain of reasoning:

• Ordinary acids cannot clean gold, because gold is the least reactive metal and does not react with them.
• Aqua regia is one of the few reagents that can dissolve gold and platinum, even though neither acid alone can do so.
• When the bangles were dipped in aqua regia, the dull surface layer of gold dissolved, so the bangles looked bright and new.
• But real gold from the bangles had also dissolved in the solution, so their weight dropped sharply.
• The goldsmith’s hasty retreat confirms the trick: the ‘glitter’ cost the lady actual gold.

Answer: Copper is placed below iron (and below hydrogen) in the reactivity series, so it is much less reactive than the iron inside steel. Copper does not react with hot water or steam, and it does not rust when exposed to water and air.

Steel, being an alloy of iron, contains the reactive metal iron — hot water and steam would slowly corrode the steel, and it would rust. Copper’s resistance to corrosion and good heat conduction make it the safer, longer-lasting choice for hot water tanks.