CBSE Class 10 Science Metals and non metals Sure Shot Questions Set 03

This is Set 03 of sure shot questions for CBSE Class 10 Science Chapter 3, Metals and Non-metals: 16 board-style questions that appear again and again in 2 to 5 mark papers, each with a step-wise answer and a Teacher's Note. It covers amphoteric oxides, extraction and refining, ionic bonding with electron dot diagrams, corrosion and its prevention, and the comparison of metals with non-metals.

Short Answer Questions

Question 1. What are amphoteric oxides? Give an example. Write balanced chemical equations to justify your answer.
Answer:
Amphoteric oxides are metal oxides that react with both acids and bases to form salt and water, so they behave as basic as well as acidic oxides.
Example: aluminium oxide, Al₂O₃ (zinc oxide, ZnO, is another).
With an acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
With a base: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (sodium aluminate)
In simple words: An amphoteric oxide reacts with an acid and also with an alkali.

Teacher's Note:
1. Always give one equation for the acid and one for the base.
2. ZnO + 2NaOH gives Na₂ZnO₂ + H₂O.

Question 2. (a) Define corrosion. (b) What is corrosion of iron called? (c) How will you recognise the corrosion of silver? (d) Why corrosion of iron is a serious problem? (e) How can we prevent corrosion of iron?
Answer:
(a) Corrosion is the slow eating up of metals when they are changed into oxides, carbonates, sulphides, etc., by the action of atmospheric gases and moisture.
(b) The corrosion of iron is called rusting. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O.
(c) Silver articles become black after some time when exposed to air, because a coating of black silver sulphide (Ag₂S) forms on them by the action of hydrogen sulphide gas present in the air.
(d) Corrosion of iron is a serious problem because a large amount of money is spent every year to replace damaged iron objects such as car bodies, bridges, railings and ships.
(e) Rusting is prevented by painting, oiling or greasing, galvanising (coating with zinc), electroplating with tin, nickel or chromium, and making alloys such as stainless steel.
In simple words: Corrosion eats away metals; rusting is corrosion of iron; silver turns black; paint, grease, zinc coating and alloying protect iron.

Teacher's Note:
1. Write the answer part by part.
2. Rusting needs both oxygen and moisture.

Question 3. What is meant by galvanisation ? Why is it done ?
Answer:
The process of depositing a thin layer of zinc metal on iron objects is called galvanisation.
It is done to protect iron objects from rusting. Zinc forms a thin layer of zinc oxide on exposure to damp air which does not corrode further, and, since zinc is more reactive than iron, it protects iron even when the layer is scratched.
In simple words: A coat of zinc keeps iron from rusting.

Teacher's Note:
1. Mention that zinc is more reactive than iron.
2. Galvanised iron is used for pipes and buckets.

Question 4. Differentiate between : (i) Mineral and Ore (ii) Corrosion and Rancidity (iii) Malleability and Ductility
Answer:
(i) A mineral is an element or compound that occurs naturally in the earth’s crust. An ore is a mineral from which the metal can be extracted profitably. All ores are minerals, but not all minerals are ores (bauxite is an ore of aluminium; clay is only a mineral).
(ii) Corrosion is the tarnishing of metals by the attack of moisture and acids in the air. Rancidity is the change in smell and taste of food when the fats and oils in it are oxidised.
(iii) Malleability is the property of a metal to be beaten into thin sheets. Ductility is the property of a metal to be drawn into thin wires.
In simple words: A paired answer: ore versus mineral, metal rusting versus food going bad, sheets versus wires.

Teacher's Note:
1. Give one example for each term.
2. Compare both terms in the same sentence.

Question 5. What is the difference between calcination and roasting?
Answer:
Calcination: the ore (usually a carbonate) is heated strongly in a limited supply or absence of air. It removes moisture and volatile impurities and gives the metal oxide and carbon dioxide: ZnCO₃ → ZnO + CO₂.
Roasting: the ore (usually a sulphide) is heated strongly in excess of air. It oxidises the ore to the metal oxide and gives off sulphur dioxide: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
In both cases the heating is below the melting point of the ore and the product is the metal oxide, which is then reduced to the metal.
In simple words: Calcination: carbonate ore, little air, CO₂ given off. Roasting: sulphide ore, excess air, SO₂ given off.

Teacher's Note:
1. Tabulate the type of ore, the air supply and the gas evolved.
2. Both steps convert the ore into its oxide.

Long Answer Questions

Question 6. (i) Write the steps involved in the extraction of pure metals in middle of the activity series from carbonate ores. (ii) How is copper extracted from its sulphide ore? Explain the various steps supported by chemical equations. Draw labelled diagram for electrolytic refining of copper.

Answer:
(i) Steps for carbonate ores of metals in the middle of the series (for example zinc carbonate):
Step 1 (calcination): the carbonate ore is heated strongly in the absence of air to give the oxide: ZnCO₃ → ZnO + CO₂.
Step 2 (reduction): the oxide is heated with carbon (coke) to give the metal: ZnO + C → Zn + CO.
(ii) Extraction of copper from copper(I) sulphide, Cu₂S:
Step 1 (roasting): 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
Step 2 (reduction of the oxide by the sulphide): 2Cu₂O + Cu₂S → 6Cu + SO₂
Step 3 (electrolytic refining): The impure copper is made the anode, a thin strip of pure copper is made the cathode and acidified copper sulphate solution is the electrolyte. On passing current, copper dissolves from the anode (Cu → Cu²⁺ + 2e⁻) and an equal amount of pure copper is deposited on the cathode (Cu²⁺ + 2e⁻ → Cu). The insoluble impurities settle below the anode as anode mud.

In simple words: Roast or calcine the ore, reduce the oxide, then refine the metal by electrolysis.

Teacher's Note:
1. Label the anode, cathode, electrolyte and anode mud in the diagram.
2. Metals in the middle of the series are reduced with carbon.

Teacher's Note:
1. Write at least three rows in the table.
2. Mention ‘molten’ and ‘exothermic’ in part (b)(ii).

Teacher's Note:
1. Give an example equation for at least two rows.
2. Use the words ‘electropositive’ and ‘electronegative’.

Question 9. You are provided with magnesium ribbon and sulphur powder. Explain with the help of an activity that metal oxides are basic and non-metal oxide are acidic in nature.
Answer:
Steps (with teacher’s supervision):
1. Hold a clean magnesium ribbon with tongs and burn it in a flame. It burns with a dazzling white flame to give white magnesium oxide: 2Mg + O₂ → 2MgO.
2. Collect the ash and dissolve it in warm water: MgO + H₂O → Mg(OH)₂. Test the solution with red litmus paper.
3. Burn sulphur powder in a deflagrating spoon in air to get sulphur dioxide: S + O₂ → SO₂. Dissolve the gas in water: SO₂ + H₂O → H₂SO₃. Test it with blue litmus paper.
Observation: The magnesium oxide solution turns red litmus blue, so the metal oxide is basic. The sulphur dioxide solution turns blue litmus red, so the non-metal oxide is acidic.
In simple words: Magnesium oxide solution turns red litmus blue; sulphur dioxide solution turns blue litmus red.

Teacher's Note:
1. Do the sulphur burning in a fume cupboard or open air.
2. Always name the litmus used for each test.

Question 10. The way, metals like sodium, magnesium and iron react with air and water is an indication of their relative positions in the ‘reactivity series’. Is this statement true? Justify your answer with examples.
Answer:
Yes, the statement is true.
Sodium reacts violently even with cold water and catches fire in air, so it is very reactive (near the top).
Magnesium burns in air with a dazzling white flame but reacts with water only when the water is hot, so it is less reactive than sodium.
Iron rusts slowly in moist air and reacts only with steam, so it is the least reactive of the three.
So the order Na > Mg > Fe from their reactions matches their positions in the reactivity series.
In simple words: The more violent the reaction with air and water, the higher the metal is in the series.

Teacher's Note:
1. Compare the conditions needed: cold water, hot water, steam.
2. Use the reactions as evidence for the order.

Question 11. Give reason why (i) metals are good conductors, whereas non-metals are bad conductors of electricity. (ii) metals replace hydrogen from acids whereas non-metals do not. (iii) an iron knife dipped in a blue copper sulphate solution turns the blue solution light green. (iv) sodium is kept under kerosene. (v) carbon cannot reduce the oxides of sodium or aluminium.
Answer:
(i) Metals have free electrons (or free ions) that carry current; non-metals do not have free electrons.
(ii) A metal above hydrogen loses electrons, and H⁺ ions of the acid accept them to form hydrogen: Zn + 2H⁺ → Zn²⁺ + H₂. Non-metals do not have a tendency to lose electrons.
(iii) Iron is more reactive than copper, so it displaces copper and the blue copper sulphate changes to light green iron(II) sulphate: Fe + CuSO₄ → FeSO₄ + Cu.
(iv) Sodium catches fire in air and reacts violently with water, so it is kept under kerosene to prevent accidents.
(v) Sodium and aluminium are at the top of the series and have more affinity for oxygen than carbon has.
In simple words: Free electrons, tendency to lose electrons and position in the reactivity series explain all five.

Teacher's Note:
1. Give one reason in a single line each.
2. Link every part to the reactivity series.

Question 12. (a) Name the following : (i) Metal that can be cut by knife (ii) Lustrous non-metal (iii) Metal that exists in liquid state at room temperature (iv) Most malleable and ductile metal (v) Metal that is best conductor of electricity (vi) Non-metal that can exist in different forms (b) How are alloys better than metals? Give composition of solder and amalgam.
Answer:
(a) (i) Sodium (ii) Iodine (iii) Mercury (iv) Gold (v) Silver (vi) Carbon (diamond, graphite and fullerene).
(b) Alloys are harder and stronger than the metals from which they are made, more resistant to corrosion and have a lower melting point; their electrical conductivity is lower.
Solder is an alloy of lead and tin. An amalgam is an alloy of mercury with another metal.
In simple words: Alloys are stronger and rust-resistant; solder is lead and tin.

Teacher's Note:
1. Learn the six names as a list.
2. Mention that alloys conduct less than pure metals.

Question 13. 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: hit each sample with the hammer. A metal can be beaten into a thin sheet (malleable); a non-metal breaks into pieces or powder (brittle).
Circuit test: connect the battery, bulb, switch and wires in series with the sample between two terminals. If the bulb glows, the sample conducts and is a metal; if the bulb does not glow, it is a non-metal.
(b) These tests are useful because most metals are malleable and good conductors while most non-metals are brittle and bad conductors. But they have exceptions: graphite (a non-metal) conducts electricity, and sodium and potassium (metals) are soft. So these tests alone are not enough, and chemical properties like the nature of the oxide should also be checked.
In simple words: Hammering and a bulb circuit separate most metals from non-metals, but graphite and soft metals are exceptions.

Teacher's Note:
1. Mention both the test and the observation.
2. Name at least one exception in part (b).

Ionic Compounds

Question 14. (i) Write the 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) Electron-dot structures: sodium (2, 8, 1) has 1 valence electron, oxygen (2, 6) has 6 and magnesium (2, 8, 2) has 2.

(ii) Formation of Na₂O: two sodium atoms each give one electron to one oxygen atom, forming 2Na⁺ and O²⁻.

Formation of MgO: a magnesium atom gives its two valence electrons to an oxygen atom, forming Mg²⁺ and O²⁻.

(iii) Ions present: Na₂O contains Na⁺ and O²⁻ ions; MgO contains Mg²⁺ and O²⁻ ions.
In simple words: Metals hand over their valence electrons to oxygen and the oppositely charged ions attract each other.

Teacher's Note:
1. Show the dots (or crosses) clearly and write the charge on every ion.
2. Each ion gets a stable octet.

Question 15. (i) Write electron dot diagram for chlorine (At No. 17) and calcium (At No. 20). Show the formation of calcium chloride by transfer of electrons. (ii) Identify the nature of above compound and explain three physical properties of such compound.

Answer:
(i) Chlorine (atomic number 17) is 2, 8, 7 with 7 valence electrons; calcium (atomic number 20) is 2, 8, 8, 2 with 2 valence electrons. Calcium gives one electron to each of two chlorine atoms:
Ca → Ca²⁺ + 2e⁻ and 2Cl + 2e⁻ → 2Cl⁻

(ii) Calcium chloride is an ionic compound. Three physical properties: (1) It is a hard, brittle crystalline solid because of the strong electrostatic attraction between Ca²⁺ and Cl⁻ ions. (2) It has high melting and boiling points because a lot of energy is needed to break this attraction. (3) It conducts electricity in the molten state and in aqueous solution (not in the solid state) because ions are then free to move; it is soluble in water and insoluble in kerosene and petrol.
In simple words: Calcium gives two electrons to two chlorine atoms; the ionic crystal is hard, high-melting and conducts when molten.

Teacher's Note:
1. Show both the electron transfer and the final ions.
2. State three physical properties with their reasons.

Question 16. Write down important properties of ionic compounds.
Answer:
1. Physical state: ionic compounds are generally hard, brittle crystalline solids, because of the strong force of attraction between the oppositely charged ions.
2. Melting and boiling points: they are high, because a large amount of energy is needed to overcome the strong attraction between the ions.
3. Solubility: they are generally soluble in water and insoluble in solvents such as kerosene and petrol.
4. Conduction of electricity: they do not conduct electricity in the solid state because the ions cannot move, but they are good conductors in the molten state and in aqueous solution, when the ions are free to move.
In simple words: Ionic compounds are hard, high-melting, water-soluble and conduct only when molten or dissolved.

Teacher's Note:
1. Give a reason with every property.
2. Common salt (NaCl) is the standard example.