CBSE Class 10 Chemistry Metals and Non-metals MCQs Set 41

Answer: (a) Strong electrostatic forces of attraction between the oppositely charged ions need a lot of energy to overcome

Explanation:
1. Ionic compounds are made of positive and negative ions held in a lattice.
2. A large amount of energy is needed to break the strong electrostatic attraction, so the melting point is high.

Teacher's Note:
1. Ionic solids do not conduct, but molten or dissolved ionic compounds do.
2. Covalent compounds have weak intermolecular forces and low melting points.

Answer: (c) zinc

Explanation:
1. Zinc is amphoteric: it reacts with steam, dilute acids and strong alkalies to give hydrogen.
2. Iron does not react with alkalies, mercury is below hydrogen, and calcium does not react with alkalies.

Teacher's Note:
1. Zn + 2NaOH → Na₂ZnO₂ + H₂.
2. Aluminium is another metal that reacts with alkalies.

Answer: (b) leaching

Explanation:
1. Bauxite is dissolved in hot concentrated NaOH solution (Bayer's process), which is leaching.
2. The Al₂O₃ dissolves and the impurities such as Fe₂O₃ and SiO₂ are filtered off.

Teacher's Note:
1. Leaching uses a chemical that dissolves the ore.
2. Magnetic separation is used for magnetic ores.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Hg, Cu and Pb are obtained from HgS, Cu₂S and PbS by partial roasting to oxide and then heating with the remaining sulphide with the air cut off.
2. For example 2Cu₂O + Cu₂S → 6Cu + SO₂, so both statements are true and R explains A.

Teacher's Note:
1. The metals are less reactive and their oxides are easily reduced.
2. No external reducing agent is needed.

Answer: (a) sulphuryl chloride, SO₂Cl₂

Explanation:
1. SO₂ + Cl₂ → SO₂Cl₂ in sunlight (or with a charcoal catalyst).
2. The product is sulphuryl chloride.

Teacher's Note:
1. Sulphuryl chloride is a colourless fuming liquid.
2. The reaction is an addition reaction.

Answer: (d) dehydration

Explanation:
1. Concentrated H₂SO₄ removes the elements of water from sugar (C₁₂H₂₂O₁₁), leaving black carbon.
2. This is dehydration.

Teacher's Note:
1. C₁₂H₂₂O₁₁ → 12C + 11H₂O.
2. The reaction is highly exothermic.

Answer: (d) calcination

Explanation:
1. Calcination is heating the ore strongly in a limited supply of air or none, to remove moisture and volatile matter.
2. Hydrated alumina loses water to give anhydrous Al₂O₃.

Teacher's Note:
1. Calcination is used for carbonate and hydrated ores.
2. Roasting uses excess air and is used for sulphide ores.

Answer: (d) moisture and volatile impurities are removed and sulphur escapes as SO₂ or SO₃

Explanation:
1. The passage states that moisture and volatile oxides of As, Sb and Se are removed and sulphur is expelled as SO₂ or SO₃.
2. So all the changes occur, which is option (d).

Teacher's Note:
1. The sulphide ore changes to oxide.
2. The SO₂ formed can be used to make sulphuric acid.

Answer: (a) they are very reactive and combine readily with oxygen and water

Explanation:
1. Alkali metals are at the top of the reactivity series and react with air and moisture.
2. So they occur only as compounds.

Teacher's Note:
1. They are stored under kerosene.
2. Gold and platinum are found free.

Answer: (a) The coin is covered with a shiny grey deposit of silver and the solution turns blue

Explanation:
1. Copper is more reactive than silver and displaces it: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag.
2. Silver is deposited on the coin and the solution turns blue because of Cu²⁺ ions.

Teacher's Note:
1. A more reactive metal displaces a less reactive one from its salt.
2. The deposit appears greyish-white.

Answer: (b) the impure metal

Explanation:
1. In electrolytic refining the impure metal is made the anode and a thin strip of pure metal the cathode.
2. The anode dissolves and pure metal is deposited at the cathode.

Teacher's Note:
1. Electrolyte: a soluble salt of the metal.
2. Impurities settle as anode mud.

Answer: (c) gold and platinum

Explanation:
1. Gold and platinum are the least reactive metals and do not form oxides in air.
2. The other pairs burn or form oxides readily.

Teacher's Note:
1. Noble metals are unaffected by air.
2. Reactive metals burn with bright flames.

Answer: (c) Zn only

Explanation:
1. Only a metal more reactive than copper can displace it from its salt.
2. Zinc is more reactive than copper: Zn + CuSO₄ → ZnSO₄ + Cu.
3. Platinum and silver are less reactive, so there is no reaction.

Teacher's Note:
1. The blue colour of the solution fades with zinc.
2. Pt and Ag are below Cu in the series.

Answer: (d) presence of oxygen, moisture and acids or salts

Explanation:
1. Corrosion needs oxygen and water; acids, salts (as in sea water) and carbon dioxide accelerate it.
2. In dry air or in an inert atmosphere iron does not corrode.

Teacher's Note:
1. Rusting is faster near the sea.
2. Zinc coating prevents it.

Answer: (a) Na reacts vigorously with cold water, Mg reacts with hot water or steam, and Cu does not react

Explanation:
1. Sodium reacts violently with cold water giving NaOH and H₂.
2. Magnesium reacts only with hot water (or steam) and copper does not react at all.

Teacher's Note:
1. This follows the reactivity series.
2. Na > Mg > Cu.

Answer: (b) the ore contains earthy impurities (gangue) which waste energy and reagents

Explanation:
1. Gangue increases the cost and complicates the extraction.
2. Removing it first makes the next steps easier and cheaper.

Teacher's Note:
1. Methods: hydraulic washing, magnetic separation, froth floatation, leaching.
2. Concentration comes before roasting or calcination.

Answer: (a) 3Fe + 4H₂O(steam) → Fe₃O₄ + 4H₂; Ca + 2H₂O → Ca(OH)₂ + H₂

Explanation:
1. Iron reacts with steam to form the magnetic oxide Fe₃O₄ and hydrogen: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
2. Calcium reacts with cold water giving the hydroxide and hydrogen: Ca + 2H₂O → Ca(OH)₂ + H₂.
3. Both equations are balanced.

Teacher's Note:
1. Metals like Mg, Al, Zn, Fe react with steam forming oxides.
2. Metals like Na, K, Ca react with water forming hydroxides.

Answer: (b) aluminium is more reactive than carbon and has a greater affinity for oxygen

Explanation:
1. Aluminium holds oxygen more strongly than carbon does, so carbon cannot remove oxygen from Al₂O₃.
2. Aluminium is obtained by electrolysis of molten alumina instead.

Teacher's Note:
1. Metals above carbon in the series need electrolysis.
2. Al₂O₃ is mixed with cryolite to lower the melting point.

Answer: (d) Na+ and O^2– in the ratio 2 : 1

Explanation:
1. Each Na atom (2, 8, 1) transfers one electron to oxygen (2, 6), which accepts two electrons.
2. Two Na+ ions and one O^2– ion form Na₂O (and Mg²⁺ with O^2– form MgO).

Teacher's Note:
1. Ionic compounds form by electron transfer.
2. The ions attain the neon configuration.

Answer: (b) Mg > Al > Zn > Fe > Cu

Explanation:
1. The activity series reads K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au.
2. So the order of the five is Mg > Al > Zn > Fe > Cu.

Teacher's Note:
1. The series is found by displacement experiments.
2. Zn is more reactive than Fe, but less than Al.