NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text (Page-wise) Questions
Q1. Give an example of a metal which
(i) is a liquid at room temperature
(ii) can be easily cut with a knife
(iii) is the best conductor of heat
(iv) is a poor conductor of heat
Ans.
(i) Mercury (Hg) — the only metal that exists as a liquid at room temperature.
(ii) Sodium (Na) — it is so soft that it can be sliced with a knife. Potassium is also soft enough to cut.
(iii) Silver (Ag) — it has the highest thermal conductivity among all metals. Copper comes a close second.
(iv) Lead (Pb) and Mercury (Hg) — both are comparatively poor conductors of heat among metals.
Explain the meanings of malleable and ductile.
Answer
Malleable: A substance is malleable if it can be beaten or hammered into thin sheets without breaking. Most metals possess this property. Gold and silver are the most malleable metals — gold can be beaten into sheets as thin as 0.001 mm (gold leaf).
Ductile: A substance is ductile if it can be drawn or stretched into thin wires. Most metals are ductile. Gold is the most ductile metal, followed by silver and platinum. A single gram of gold can be drawn into a wire about 2 km long.
Why is sodium kept immersed in kerosene oil?
Answer
Sodium is an extremely reactive metal. It reacts vigorously with oxygen in the air and can catch fire spontaneously. It also reacts explosively with water, producing hydrogen gas and a large amount of heat — enough to ignite the hydrogen. To prevent these dangerous reactions, sodium is stored under kerosene oil, which acts as a protective layer, keeping the metal away from air and moisture.
Q4. Write equations for the reactions of
(i) iron with steam
(ii) calcium and potassium with water
Ans.
(i) Iron reacts with steam (not cold water) to form iron oxide and hydrogen gas:
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
(ii) Calcium reacts with cold water gently, forming calcium hydroxide and hydrogen:
Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)↑
(The calcium starts floating because H₂ gas bubbles stick to its surface.)
Potassium reacts violently with cold water, producing potassium hydroxide and hydrogen — the reaction is so exothermic that the hydrogen gas catches fire:
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)↑ + Heat
Samples of four metals A, B, C, and D were tested with solutions of various salts. Results showed that metal B displaces all others.
(i) Which is the most reactive metal?
(ii) What would happen if B is added to a solution of copper(II) sulphate?
(iii) Arrange A, B, C, D in decreasing order of reactivity.
Answer
(i) B is the most reactive metal because it can displace all other metals from their salt solutions.
(ii) Since B is more reactive than copper, B would displace copper from CuSO₄ solution. The blue colour of the solution would fade, and copper metal would deposit.
(iii) Decreasing order of reactivity: B > A > C > D
Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction of iron with dilute H₂SO₄.
Answer
Hydrogen gas (H₂) is produced when dilute HCl is added to a reactive metal.
Reaction of iron with dilute sulphuric acid:
Fe(s) + H₂SO₄(dil.)
⇒ FeSO₄(aq) + H₂(g)↑
The hydrogen gas can be tested by bringing a burning matchstick near it — it burns with a pop sound.
What would you observe when zinc is added to a solution of iron(II) sulphate? Write the chemical reaction that takes place.
Answer
Since zinc is more reactive than iron, it displaces iron from the iron(II) sulphate solution. You would observe that the green colour of the solution fades and iron metal deposits:
Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s)
This is a displacement reaction.
(i) Write electron-dot structures for sodium, oxygen, and magnesium.
(ii) Show the formation of Na₂O and MgO by the transfer of electrons.
(iii) What are the ions present in these compounds?
Answer
(i) Sodium has 1 valence electron (Na·), Oxygen has 6 valence electrons, Magnesium has 2 valence electrons (Mg:)
(ii) In Na₂O: Two sodium atoms each transfer 1 electron to one oxygen atom. Na loses 1e⁻
⇒ Na⁺; O gains 2e⁻
⇒ O²⁻
In MgO: One magnesium atom transfers 2 electrons to one oxygen atom. Mg loses 2e⁻
⇒ Mg²⁺; O gains 2e⁻
⇒ O²⁻
(iii) Na₂O contains Na⁺ and O²⁻ ions; MgO contains Mg²⁺ and O²⁻ ions.
Why do ionic compounds have high melting points?
Answer
Ionic compounds are made up of positively and negatively charged ions held together by strong electrostatic forces of attraction (ionic bonds). These forces operate in all directions, creating a tightly packed crystal lattice structure. A very large amount of energy is needed to overcome these strong inter-ionic forces. Therefore, ionic compounds have high melting and boiling points.
Q10. Define the following terms
(i) Mineral (ii) Ore (iii) Gangue
Ans.
(i) Mineral: The naturally occurring inorganic substances in the earth's crust that contain metals or their compounds are called minerals. All ores are minerals, but not all minerals are ores.
(ii) Ore: A mineral from which a metal can be extracted profitably and conveniently is called an ore. For example, bauxite (Al₂O₃·2H₂O) is the ore of aluminium.
(iii) Gangue: The unwanted impurities such as sand, rocky material, and earthy particles present in an ore are called gangue (or matrix). These must be removed before extracting the metal.
Name two metals that are found in nature in the free state.
Answer
Gold (Au) and Platinum (Pt) are found in the free (native) state in nature. These metals are at the bottom of the reactivity series and are very unreactive, so they do not combine easily with other elements. Silver is also sometimes found in the free state.
What chemical process is used for obtaining a metal from its oxide?
Answer
The process of reduction is used to obtain a metal from its metal oxide. Different reducing agents are used depending on the metal's reactivity:
• Heating alone — for oxides of metals low in the reactivity series (e.g., HgO
⇒ heat Hg + O₂)
• Reduction with carbon/coke — for moderately reactive metals (e.g., ZnO + C
⇒ Zn + CO)
• Electrolytic reduction — for highly reactive metals like Na, Al, Ca (e.g., electrolysis of molten Al₂O₃ to get Al)
Which metals do not corrode easily?
Answer
Metals that are least reactive (at the bottom of the reactivity series) do not corrode easily. These include gold, platinum, and silver. Gold and platinum are virtually corrosion-proof, which is why they retain their lustre for centuries. Silver does tarnish slowly (forms a black layer of Ag₂S) but does not corrode in the traditional sense.
What are alloys?
Answer
An alloy is a homogeneous mixture of two or more metals, or a metal with a non-metal. Alloys are prepared to enhance the properties of the base metal — making it harder, more resistant to corrosion, or changing its colour/appearance.
Examples:
• Brass = Copper + Zinc
• Bronze = Copper + Tin
• Stainless steel = Iron + Chromium + Nickel + Carbon
• Solder = Lead + Tin (used for welding)
Section B: NCERT Exercise Questions
Which of the following pairs of solution when mixed will give a displacement reaction?
Answer
(d) AgNO₃ solution and copper metal — Copper is more reactive than silver, so it displaces silver:
Cu(s) + 2AgNO₃(aq)
⇒ Cu(NO₃)₂(aq) + 2Ag(s)
In all other options, the metal is less reactive than the metal in the salt, so no displacement occurs.
Which of the following methods is suitable for preventing an iron frying pan from rusting?
Answer
(c) Applying a coating of zinc (galvanisation). Grease and paint would burn off at the high cooking temperatures. Zinc coating is heat-resistant and, being more reactive than iron, provides sacrificial protection even if the coating is scratched.
An element reacts with oxygen to give a compound with a high melting point. This compound is also soluble in water. The element is likely to be:
Answer
(a) Calcium. Calcium reacts with oxygen to form calcium oxide (CaO), an ionic compound with a high melting point. CaO dissolves in water to form Ca(OH)₂. Carbon and silicon oxides are covalent (not high melting); iron oxide is insoluble in water.
Food cans are coated with tin and not with zinc because
Answer
(c) Zinc is more reactive than tin. Though zinc can protect iron better from rusting, zinc is more reactive and can react with the acidic contents of food, making it toxic. Tin is less reactive and does not contaminate the food, making it safe for food storage.
You are given a hammer, a battery, a bulb, wires and a switch.
(a) How could you use them to distinguish between samples of metals and non-metals?
(b) Assess the usefulness of these tests in distinguishing between metals and non-metals.
Answer
(a) Hammer test: Strike each sample with the hammer. Metals are malleable — they flatten into sheets. Non-metals are brittle — they shatter into pieces.
Electrical conductivity test: Connect each sample in a circuit with the battery, bulb, wires, and switch. If the bulb glows, the sample is a metal (conductor). If it doesn't glow, it is a non-metal (insulator).
(b) These tests work for most metals and non-metals but have exceptions:
• Sodium, potassium, and lithium are metals but so soft they can be cut — they may behave unexpectedly with a hammer.
• Graphite (a form of carbon, which is a non-metal) conducts electricity, so the bulb test would wrongly classify it as a metal.
• Some metals like mercury (liquid) cannot be tested with a hammer.
What are amphoteric oxides? Give two examples of amphoteric oxides.
Answer
Amphoteric oxides are metal oxides that can react with both acids and bases to produce a salt and water. They show dual behaviour — acidic with bases and basic with acids.
Examples: Aluminium oxide (Al₂O₃) and Zinc oxide (ZnO)
Al₂O₃ + 6HCl
⇒ 2AlCl₃ + 3H₂O (reacts as a base with acid)
Al₂O₃ + 2NaOH
⇒ 2NaAlO₂ + H₂O (reacts as an acid with base)
Name two metals that will displace hydrogen from dilute acids, and two metals that will not.
Answer
Metals that displace hydrogen: Zinc (Zn) and Magnesium (Mg) — these are above hydrogen in the reactivity series.
Metals that do not displace hydrogen: Copper (Cu) and Silver (Ag) — these are below hydrogen in the reactivity series and are not reactive enough to replace hydrogen from acids.
In the electrolytic refining of a metal M, what would you take as the anode, the cathode and the electrolyte?
Answer
• Anode (positive electrode): A thick block of impure metal M
• Cathode (negative electrode): A thin strip of pure metal M
• Electrolyte: An aqueous solution of a soluble salt of metal M
When current is passed, the impure metal at the anode dissolves into the electrolyte as ions, which then deposit as pure metal on the cathode. The impurities settle at the bottom as anode mud.
Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube. (a) What will be the action of gas on (i) dry litmus paper (ii) moist litmus paper? (b) Write a balanced equation for the reaction.
Answer
(a)(i) No change — dry litmus paper is not affected because the gas needs moisture to show acidic behaviour.
(a)(ii) Moist blue litmus turns red — sulphur dioxide dissolves in the moisture to form sulphurous acid (H₂SO₃), which is acidic.
(b) S(s) + O₂(g) → SO₂(g)
SO₂(g) + H₂O(l) → H₂SO₃(aq)
State two ways to prevent the rusting of iron.
Answer
(i) Galvanisation: Coating the iron surface with a layer of zinc. Even if the zinc layer is scratched, zinc (being more reactive) gets oxidised preferentially, protecting the iron underneath — this is called sacrificial protection.
(ii) Painting or oiling: Applying a coat of paint, varnish, or oil creates a barrier that prevents oxygen and moisture from reaching the iron surface, stopping the rusting reaction.
What type of oxides are formed when non-metals combine with oxygen?
Answer
Non-metals form acidic oxides or sometimes neutral oxides when they react with oxygen.
Acidic oxides: SO₂, CO₂, P₂O₅, N₂O₅ — these dissolve in water to form acids.
Neutral oxides: CO, H₂O, N₂O — these are neither acidic nor basic.
Q12. Give reasons
(a) Platinum, gold and silver are used to make jewellery.
(b) Sodium, potassium and lithium are stored under oil.
(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.
(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.
Ans.
(a) These metals are used for jewellery because they are highly lustrous (shiny), resistant to corrosion (unreactive, at the bottom of the reactivity series), malleable, and ductile — they can be moulded into beautiful designs that last for generations.
(b) These metals are extremely reactive — they react vigorously with oxygen in air and explosively with water. Storing them under kerosene oil creates an inert environment that prevents contact with air and moisture.
(c) Although aluminium is reactive, it forms a thin, tough layer of aluminium oxide (Al₂O₃) on its surface when exposed to air. This oxide layer acts as a protective shield, preventing further reaction. Additionally, aluminium is an excellent conductor of heat, making it ideal for cooking vessels.
(d) It is easier to reduce oxides to metals (using carbon or other reducing agents) than it is to reduce carbonates or sulphides directly. So, as a preliminary step, carbonate ores are converted to oxides by calcination (heating in absence of air), and sulphide ores are converted by roasting (heating in presence of air).
You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these are effective at cleaning the vessels.
Answer
When copper vessels are exposed to air and moisture over time, a green coating of basic copper carbonate [CuCO₃·Cu(OH)₂] forms on their surface — this is tarnishing. Lemon juice contains citric acid and tamarind contains tartaric acid. These acids dissolve the basic copper carbonate through a neutralisation reaction, removing the green coating and restoring the original shine of the copper beneath.
Differentiate between metals and non-metals on the basis of their chemical properties.
Answer
(i) Reaction with oxygen: Metals form basic or amphoteric oxides (e.g., MgO, Al₂O₃). Non-metals form acidic or neutral oxides (e.g., CO₂, SO₂).
(ii) Reaction with water: Reactive metals react with water to form hydroxides and H₂. Non-metals generally do not react with water.
(iii) Reaction with acids: Metals above hydrogen in the reactivity series displace H₂ from dilute acids. Non-metals do not react with dilute acids.
(iv) Reaction with salt solutions: A more reactive metal displaces a less reactive one. Non-metals do not show displacement reactions with salt solutions.
(v) Electropositive/Electronegative: Metals tend to lose electrons (electropositive). Non-metals tend to gain electrons (electronegative).
A man went to a jeweller to get his gold bangles polished. He observed that the weights of the bangles were reduced on getting polished. Can you explain why?
Answer
The jeweller likely dipped the gold bangles in aqua regia — a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in a 3:1 ratio. Aqua regia is one of the very few chemicals that can dissolve gold. During the process, a thin layer of gold from the surface dissolves in the aqua regia, making the bangles lighter. The dissolved gold is later recovered by the jeweller, resulting in profit at the customer's expense.
Why is copper used for making hot water tanks rather than steel (an alloy of iron)?
Answer
Copper is used because it is a much better conductor of heat than steel. This means copper tanks heat water more quickly and efficiently, requiring less energy. Additionally, copper does not rust like iron/steel does when in constant contact with water, making copper tanks more durable for this purpose.
Section C: Important Extra Questions for Board Exam
What is thermite reaction? Where is it used?
Answer
The thermite reaction is a highly exothermic displacement reaction between aluminium powder and iron(III) oxide:
Fe₂O₃(s) + 2Al(s)
⇒ Al₂O₃(s) + 2Fe(l) + Heat
The reaction produces molten iron and immense heat (temperatures above 2000°C). It is used for welding railway tracks (thermite welding) and for welding broken machine parts where conventional welding is impractical.
What is anodising? Why is it done?
Answer
Anodising is the process of forming a thick, protective oxide layer on the surface of aluminium by making it the anode during electrolysis. The aluminium article is placed in dilute sulphuric acid, and electric current is passed. Oxygen released at the anode reacts with aluminium to form a thick, durable layer of Al₂O₃.
This oxide layer:
• Protects the metal from corrosion
• Can be dyed in attractive colours for decorative purposes
• Makes the surface scratch-resistant
What is the reactivity series of metals? List the first five and last five metals.
Answer
The reactivity series is an arrangement of metals in decreasing order of their reactivity. A more reactive metal can displace a less reactive metal from its salt solution.
Top 5 (most reactive): Potassium (K) > Sodium (Na) > Calcium (Ca) > Magnesium (Mg) > Aluminium (Al)
Bottom 5 (least reactive): Lead (Pb) > Copper (Cu) > Mercury (Hg) > Silver (Ag) > Gold (Au)
Hydrogen is placed in the middle as a reference point — metals above it can displace H₂ from acids; metals below it cannot.