NCERT Solutions for Class 10 Science Chapter 3 Exercise

(i) is a liquid at room temperature.

Answer: Mercury.

(ii) can be easily cut with a knife.

Answer: Sodium.

(iii) is the best conductor of heat.

Answer: Silver.

(iv) is a poor conductor of heat.

Answer: Lead.

Answer: Malleable – A substance that is malleable, can easily be beaten into thin sheets or pressed into thin shape without breaking or cracking. The property of malleability is mostly found in metals. Examples are gold and silver.

Ductile – A material that is ductile, can easily be drawn into thin wires or stretched into threads without losing toughness. The property of ductility is mostly found in metals. Examples are gold and copper.

Answer: Sodium is a highly reactive metal. If kept open in the air, it will vigorously react with oxygen and catch fire easily. So, sodium is kept immersed in kerosene oil to cut off its contact with atmospheric oxygen and avoid fire accidents.

(i) iron with steam

Answer — Reaction of iron with steam –
3Fe + 4H₂O ⟶ Fe₃O₄ + 4H₂

(ii) calcium and potassium with water

Answer — Reaction of calcium with water –
Ca + 2H₂O ⟶ Ca(OH)₂ + H₂

Reaction of potassium with water –
2K + 2H₂O ⟶ 2KOH + 2H₂ + Heat

Use the Table above to answer the following questions about metals A, B, C and D.

(i) Which is the most reactive metal?

Answer: B is the most reactive metal.

(ii) What would you observe if B is added to a solution of Copper (II) sulphate?

Answer: B, being more reactive than iron, will displace copper from its copper sulphate solution.

(iii) Arrange the metals A, B, C and D in the order of decreasing reactivity.

Answer: Metals in decreasing order of reactivity ⟶ B > A > C > D

Answer: Hydrogen gas is produced when dilute hydrochloric acid is added to a reactive metal –

Chemical reaction : Fe + H₂SO₄ ⟶ FeSO₄ + H₂

Answer: Zinc will displace the iron from iron (II) sulphate solution and zinc sulphate is formed on adding zinc to a solution of iron sulphate –

Chemical reaction : Zn + FeSO₄ ⟶ ZnSO₄ + Fe

Answer (i) : Electron-dot structures for sodium, oxygen and magnesium —

Sodium (2, 8, 1) =

Oxygen (2, 6) =

Magnesium (2, 8, 2) =

Answer (ii) : Formation of Na₂O or sodium oxide –

Formation of MgO of magnesium oxide –

Answer (iii) : The ions present in sodium oxide compound (Na₂O) are sodium ion (Na⁺) and oxide ion (O^2–) and the ions present in magnesium oxide compound (MgO) are magnesium ion (Mg²⁺) and oxide ion (O^2–).

Answer: The ionic compounds are made up of positive and negative ions. By nature, oppositely charged ions attract each other with a very strong force. So, it requires a lot of heat energy to break this force and melt the ionic compound. For this reason, ionic compounds have high melting points.

(i) Mineral

Answer: Mineral – Elements or compounds, which are found naturally in the earth’s crust are called minerals. They can be dug out of the ground and used in a variety of ways.

(ii) Ore

Answer: Ore – A mineral, from which a valuable constituent like a metal, can be profitably extracted is called an ore. A metal which occurs in a very high percentage in a mineral, makes it economically worthwhile to extract.

(iii) Gangue

Answer: Gangue – Ores mined from the earth are naturally contaminated with impurities such as soil, sand, rock etc. These unwanted materials in an ore are called gangue and must be removed from the ore prior to extraction of the metal.

Answer: Gold and platinum.

Answer: Reduction process is used for obtaining a metal from its oxide. For example, zinc is present in ores as its oxide. So, zinc oxide is reduced to metallic zinc by heating with carbon –

ZnO + C ⟶ Zn + CO

In which cases will you find displacement reactions taking place?

Answer —

Answer: Least reactive metals such as gold and platinum, do not corrode easily.

Answer: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. For example, brass is an alloy of copper and zinc.

Answer: (d) AgNO₃ solution and copper metal.

Answer: (c) Applying a coating of zinc.

Answer: (a) calcium

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

(a) How could you use them to distinguish between samples of metals and non-metals?

Answer: Malleability and electrical conductivity are the two physical properties that help us to answer this question.

(i) The given hammer will be used to beat samples of metals and non-metals placed on a block of iron. The metal samples turn into thin sheets without breaking because metals are malleable, while the non-metal samples break into pieces when beaten.

(ii) Now, we will use a battery, a bulb, wires and a switch to set up an electric circuit. We will also place the samples one by one to complete the circuit. If the bulb glows then the sample placed in the circuit is a metal because metals are good conductors of electricity otherwise the sample is a non-metal.

(b) Assess the usefulness of these tests in distinguishing between metals and non-metals.

Answer: These tests are based on physical properties that we experience in our daily life. These tests help us recognize a metal sample easily and quickly as they do not involve a chemical reaction, keeping a record of observations or doing mathematical calculations.

Answer: Amphoteric oxides are metal oxides that react with both acids and bases to produce water and salts. Examples are aluminium oxide (Al₂O₃) and zinc oxide (ZnO).

Answer: Aluminium and zinc will displace hydrogen from dilute acids while gold and silver will not.

Answer: In the electrolytic refining of a metal M, impure metal M is taken as the anode, a strip of pure metal M is taken as the cathode and a salt solution of metal M is considered as the electrolyte.

(a) What will be the action of gas on
(i) dry litmus paper?
(ii) moist litmus paper?

Answer: (i) Sulphur dioxide gas is obtained when sulphur powder is heated in the air. There will be no action of the gas on dry litmus paper.

(ii) The moist litmus paper changes its colour from blue to red because sulphur dioxide (SO₂) gas reacts with water present in the litmus paper to form sulphurous acid which turns the litmus paper to red.

(b) Write a balanced chemical equation for the reaction taking place.

Answer: Chemical equation 1 –
S + O₂ ⟶ SO₂

Chemical equation 2 –
SO₂ + H₂O ⟶ H₂SO₃

Answer: We can prevent the rusting of iron by –

(i) Painting which makes the surface of the iron waterproof and thus, atmospheric oxygen and moisture do not come into direct contact with the iron.

(ii) Galvanising which is a process of coating the iron with a thin layer of zinc.

Answer: Acidic oxides or neutral oxides are formed when non-metals combine with oxygen.

(a) Platinum, gold and silver are used to make jewellery.

Answer: Platinum, gold and silver are used to make jewellery because they are very lustrous and highly resistant to corrosion.

(b) Sodium, potassium and lithium are stored under oil.

Answer: Sodium, potassium and lithium are highly reactive metals. They readily react with oxygen and catch fire easily when kept open in the air. So, these metals are stored under oil to avoid fire accidents.

(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking

Answer: Being a highly reactive metal, aluminium reacts with atmospheric oxygen to form a layer of aluminium oxide on its surface. This oxide layer is very stable and prevents further reaction of aluminium with oxygen or any substance. So, aluminium utensils are safe for cooking.

(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.

Answer: Carbonate and sulphide ores are usually converted into oxides during the process of extraction because it is easier to extract metals from their oxides rather than from their carbonates and sulphides.

Answer: Copper reacts with moist carbon dioxide present in the air to form copper carbonate which is basic in nature. For this reason, copper vessels gain a green coat of copper carbonate on their surface over time and lose their shine. When the vessels are cleaned with lemon or tamarind juice, its citric acid neutralizes the basic copper carbonate and the green coat is removed. This helps the metal regain its luster.

Answer: We can differentiate between metals and non-metals on the basis of following chemical properties –

1. Metals are electropositive while non-metals are electronegative.
2. Metals react with oxygen to form basic oxides while non-metals react with oxygen to form acidic or neutral oxides.
3. Metals react with water to form oxides and hydroxides while non-metals do not react with water.
4. Metals replace/displace hydrogen from dilute acids. So, hydrogen gas is evolved and salt is obtained when metals react with dilute acids. But non-metals cannot replace hydrogen from dilute acids and thus no reaction occurs.
5. Metals act as reducing agents while non-metals act as oxidizing agents.

Answer: The particular solution must be “aqua regia” which is prepared by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO₃) in the ratio of 3:1. The mixture is a highly corrosive liquid and able to dissolve gold. The man comes to the lady with bad intentions and dips the gold bangles in the solution to steal the precious metal. The solution “aqua regia” absorbs some of the gold by dissolving it from its surface. In this way, the outer layer of the metal is removed and the inner layer becomes visible to eyes. It helps the ornaments to sparkle like new but their weight is reduced drastically.

Answer: Hot water tanks are made of copper and not steel to avoid corrosion of tanks because copper does not react with water at any temperature, whereas iron reacts with water on heating.