CBSE Class 10 Science Metals and Non Metals Assignment Set 07

This CBSE Class 10 Science assignment for Chapter 3, Metals and Non-metals, has 19 questions of 2 and 3 marks with solved answers. The questions deal with amphoteric oxides, alloys, corrosion of aluminium and copper, the activity series, calcination and roasting of ores, and the formation of magnesium chloride.

Short Answer Questions (2 Marks)

Question 1. Metal oxides are basic in nature, but some show both acidic and basic behaviour. What are these oxides called? Name one such oxide and write its reactions with an acid and a base. (2 marks)
Answer:
Such oxides are called amphoteric oxides. Example: aluminium oxide.
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (sodium aluminate)
In simple words: Amphoteric oxides react with both acids and bases.

Teacher's Note:
1. Zinc oxide is another example.
2. Both reactions give a salt and water.

Question 2. List any two observations when a highly reactive metal is dropped in water. (2 marks)
Answer:
1. A large amount of heat is evolved.
2. The metal starts floating and moves on the surface, and the hydrogen gas formed may catch fire.
In simple words: The metal fizzes, floats and gives off heat.

Teacher's Note:
1. Hydrogen gas is formed.
2. Sodium and potassium show this strongly.

Question 3. Name the constituents of bronze and write two of its uses. (2 marks)
Answer:
Bronze is an alloy of copper and tin.
Uses: 1. Making statues, medals and coins. 2. Making machine parts such as bearings and propellers.
In simple words: Bronze = copper + tin.

Teacher's Note:
1. Do not confuse bronze (Cu + Sn) with brass (Cu + Zn).
2. Bronze is harder than pure copper.

Question 4. Explain how the properties of an alloy are different from those of the constituent metals. (2 marks)
Answer:
1. An alloy is usually harder and stronger than its constituent metals.
2. It is more resistant to corrosion.
3. Its melting point is usually lower and its electrical conductivity is lower. For example, brass conducts less than copper.
In simple words: Alloys are stronger, rust less and conduct less than the pure metals.

Teacher's Note:
1. Alloys are made to improve properties.
2. Stainless steel is an example.

Question 5. Give reasons: (i) Aluminium sheets do not corrode easily. (ii) A copper vessel gets a green coating in the rainy season. (2 marks)
Answer:
(i) Aluminium is covered with a thin protective layer of aluminium oxide formed on exposure to air, which stops further reaction.
(ii) Copper reacts with moist carbon dioxide in air and loses its shiny surface, forming a green coating of basic copper carbonate.
In simple words: A film of oxide protects aluminium, while copper turns green in damp air.

Teacher's Note:
1. Green coat: CuCO₃.Cu(OH)₂.
2. Rainy season means more moisture.

Question 6. What happens when magnesium ribbon is burnt in air? What is the state of the metal oxide formed? (2 marks)
Answer:
Magnesium burns with a dazzling white light and forms white magnesium oxide, which is a solid: 2Mg(s) + O₂(g) → 2MgO(s).
In simple words: A white solid magnesium oxide is formed with a bright flame.

Teacher's Note:
1. Clean the ribbon before burning.
2. Magnesium oxide is a solid.

Question 7. What prevents metals such as magnesium, aluminium, zinc and lead from oxidation at ordinary temperature? (2 marks)
Answer:
At ordinary temperature the surface of these metals is covered with a thin layer of oxide. This protective oxide layer stops further oxidation of the metal.
In simple words: A thin oxide layer protects them.

Teacher's Note:
1. The layer is tightly attached.
2. Iron does not form such a layer.

Question 8. You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective. (2 marks)
Answer:
Lemon and tamarind juice contain acids (organic acids). They dissolve the coating of copper oxide or basic copper carbonate on the tarnished vessel, so the red-brown shine of copper returns.
In simple words: Acids in lemon and tamarind dissolve the dull coating.

Teacher's Note:
1. Metal oxides are basic and react with acids.
2. Do not use strong acids on utensils.

Question 9. Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron). (2 marks)
Answer:
Copper does not react with water or steam. Iron, the main constituent of steel, reacts with steam and also rusts, so a steel tank would corrode.
In simple words: Copper does not react with hot water, but iron does.

Teacher's Note:
1. Copper is below hydrogen in the series.
2. Hot water tanks must resist steam.

Question 10. Name two metals which will displace hydrogen from dilute acids and two metals which will not. (2 marks)
Answer:
Magnesium and aluminium displace hydrogen from dilute acids because they are very reactive. Gold and silver do not, because they lie below hydrogen in the reactivity series.
In simple words: Metals above hydrogen displace it; metals below do not.

Teacher's Note:
1. Copper and mercury also do not.
2. Zinc and iron do.

Short Answer Questions (3 Marks)

Question 11. How does the activity series of metals help us in predicting the relative reactivity of metals? (3 marks)
Answer:
1. The most reactive metal is placed at the top. It does not occur free and is highly electropositive.
2. The least reactive metal is placed at the bottom. It generally occurs free and is less electropositive.
3. A metal displaces from solutions of their salts the metals placed below it in the series.
4. Metals above hydrogen displace hydrogen from dilute acids.
In simple words: The position in the series tells which metal displaces which and which reacts with acids.

Teacher's Note:
1. Learn the series in order.
2. Position also decides the method of extraction.

Question 12. (a) Why are metals not generally found in the free state? (b) If a strip of aluminium with a clean, scratched surface is dipped in copper sulphate solution for a little time, the surface becomes brownish. Why? Write the balanced equation. (3 marks)
Answer:
(a) Most metals are reactive and readily combine with other elements, so they are found as compounds.
(b) Aluminium is more reactive than copper and displaces it. The copper deposited on the strip makes it brownish.
2Al + 3CuSO₄ → Al₂(SO₄)₃ + 3Cu
In simple words: Reactive metals form compounds, and aluminium displaces copper.

Teacher's Note:
1. The strip is scratched to remove the oxide layer.
2. Aluminium sulphate is colourless.

Question 13. Write the equations for the reactions when (i) cinnabar is heated and (ii) copper(I) sulphide, Cu₂S, is heated in air. (3 marks)
Answer:
(i) 2HgS + 3O₂ → 2HgO + 2SO₂; then 2HgO → 2Hg + O₂
(ii) 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂; then 2Cu₂O + Cu₂S → 6Cu + SO₂
In simple words: Roasting gives the oxide, and then the metal is obtained.

Teacher's Note:
1. In the copper case the sulphide itself reduces the oxide.
2. Check atom balance.

Question 14. Which method can be used in place of reduction by carbon? (3 marks)
Answer:
Displacement reactions can be used. Highly reactive metals such as sodium, calcium and aluminium are used as reducing agents because they can displace metals of lower reactivity from their compounds. For example, 3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat, and Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat (thermite reaction).
In simple words: A more reactive metal such as aluminium can reduce the oxide.

Teacher's Note:
1. The thermite reaction gives molten iron.
2. Used for welding rail tracks.

Question 15. Describe the calcination of a zinc ore. (3 marks)
Answer:
1. Calcination is heating a carbonate ore strongly in a limited supply of air (or in the absence of air).
2. Zinc carbonate is converted into zinc oxide: ZnCO₃ → ZnO + CO₂
3. Zinc oxide is then reduced to zinc by carbon: ZnO + C → Zn + CO
In simple words: A carbonate ore is heated to give the oxide, which is then reduced.

Teacher's Note:
1. Calcination is for carbonates.
2. Roasting is for sulphides.

Question 16. What happens when molten sodium chloride is electrolysed? (3 marks)
Answer:
Sodium metal is deposited at the cathode (negative electrode) and chlorine gas is liberated at the anode (positive electrode).
At cathode: Na⁺ + e⁻ → Na
At anode: 2Cl⁻ → Cl₂ + 2e⁻
In simple words: Sodium collects at the cathode and chlorine at the anode.

Teacher's Note:
1. Cations move to the cathode.
2. This is how sodium is manufactured.

Question 17. (a) Name the main ore of mercury. How is mercury obtained from its ore? Give balanced equations. (b) Name the method used to extract metals of high reactivity. (3 marks)
Answer:
(a) Cinnabar (HgS) is the main ore. It is roasted in air to form HgO, which on further heating gives mercury.
2HgS + 3O₂ → 2HgO + 2SO₂; 2HgO → 2Hg + O₂
(b) Highly reactive metals are extracted by electrolytic reduction of their molten ores.
In simple words: Mercury is got by heating cinnabar; reactive metals are got by electrolysis.

Teacher's Note:
1. Mercury does not need a reducing agent.
2. Electrolysis is used for Na, K, Ca and Al.

Question 18. Give reasons: (i) Ionic compounds usually have high melting and boiling points. (ii) Metals are good conductors of electricity whereas non-metals are not. (3 marks)
Answer:
(i) A large amount of energy is needed to break the strong electrostatic attraction between the oppositely charged ions.
(ii) Metals have free electrons which carry current. Non-metals have no free electrons, so they are insulators (graphite is an exception).
In simple words: Strong ionic forces give high melting points and free electrons give conduction.

Teacher's Note:
1. Link every property to its cause.
2. Graphite conducts because of free electrons.

Question 19. The atomic number of magnesium is 12 and that of chlorine is 17. Show the formation of magnesium chloride by the transfer of electrons. (3 marks)
Answer:
Mg (2, 8, 2) → Mg²⁺ (2, 8) + 2e⁻
2Cl (2, 8, 7) + 2e⁻ → 2Cl⁻ (2, 8, 8)
Mg²⁺ + 2Cl⁻ → MgCl₂
Each magnesium atom gives its two valence electrons to two chlorine atoms; the ions are held by electrostatic attraction (ionic bond).
In simple words: Magnesium gives two electrons, two chlorine atoms take one each.

Teacher's Note:
1. Write the electron arrangements before and after.
2. The compound is MgCl₂.