CBSE Class 10 Chemistry Metals and Non-metals MCQs Set 34

Answer: (c) applying a coating of zinc

Explanation:
1. Grease and paint would burn off or contaminate food on a frying pan, which is heated and used for cooking.
2. A coating of zinc (galvanising) is safe and gives lasting protection, so (c) is the suitable method.

Teacher's Note:
1. Galvanising protects iron even if the coating is scratched, because zinc is more reactive.
2. Paint and grease are suitable only for objects that are not heated.

Answer: (c) fuming sulphuric acid

Explanation:
1. Oleum is concentrated sulphuric acid saturated with SO₃, which fumes in air; it is H₂S₂O₇.
2. Oil of vitriol is the old name of ordinary concentrated sulphuric acid.

Teacher's Note:
1. The contact process gives oleum first, which is then diluted.
2. Oleum is also called pyrosulphuric acid.

Answer: (b) Mg

Explanation:
1. In an aqueous solution water is reduced in preference to highly reactive metal ions such as Mg²⁺, so hydrogen is liberated instead of the metal.
2. Ag, Cu and Cr are less reactive and can be deposited.

Teacher's Note:
1. Reactive metals (K, Na, Ca, Mg, Al) are obtained by electrolysis of molten salts.
2. Aqueous electrolysis is used for plating and refining.

Answer: (b) aluminium

Explanation:
1. Aluminium makes up about 8% of the earth's crust by mass.
2. It is the most abundant metal and oxygen the most abundant element overall.

Teacher's Note:
1. Order of abundance of elements: O, Si, Al, Fe.
2. Aluminium is found as bauxite.

Answer: (d) Fe

Explanation:
1. In Haber's process N₂ + 3H₂ ⇌ 2NH₃, finely divided iron with molybdenum as promoter is the catalyst.
2. So the answer is Fe.

Teacher's Note:
1. Nickel is the catalyst for hydrogenation of oils.
2. Vanadium pentoxide is the catalyst of the contact process.

Answer: (c) hydrogenation

Explanation:
1. Hydrogen is added to the double bonds of an unsaturated vegetable oil with a nickel catalyst.
2. The addition of hydrogen is hydrogenation, and it converts the liquid oil into a solid fat.

Teacher's Note:
1. Unsaturated oils are liquids; saturated fats are solids.
2. Ni is the catalyst.

Answer: (a) solder

Explanation:
1. Solder is an alloy of lead and tin.
2. Bronze (Cu + Sn), brass (Cu + Zn) and bell metal (Cu + Sn) all contain copper.

Teacher's Note:
1. Solder has a low melting point and is used for joining wires.
2. Amalgam is an alloy with mercury.

Answer: (a) (ii), (iii) and (iv)

Explanation:
1. Magnesite is MgCO₃, asbestos is a magnesium silicate and carnallite is KCl·MgCl₂·6H₂O.
2. Magnetite is Fe₃O₄ and contains no magnesium.

Teacher's Note:
1. Magnetite is magnetic iron ore.
2. Dolomite CaCO₃·MgCO₃ also contains Mg.

Answer: (a) X is a metal, because it has one electron in its outer shell and loses it easily

Explanation:
1. The electronic configuration is 2, 1, so it has one valence electron.
2. It loses that electron easily to form X+, so X is a metal (lithium).

Teacher's Note:
1. Metals usually have 1 to 3 valence electrons.
2. Non-metals have 4 to 8.

Answer: (c) it decreases, because the vibrations of the metal ions hinder the flow of electrons

Explanation:
1. On heating, the metal ions vibrate more and collide with the moving electrons.
2. This resistance increases, so the conductivity decreases.

Teacher's Note:
1. Semiconductors behave oppositely.
2. Silver is the best conductor.

Answer: (a) does not change, because the number of protons stays the same

Explanation:
1. An ion forms by gain or loss of electrons only.
2. The number of protons does not change, so the atomic number is the same.

Teacher's Note:
1. Only the electron count and the charge change.
2. Nuclear changes alter the atomic number.

Answer: (d) froth floatation and gravity separation or magnetic separation

Explanation:
1. Sulphide ores are concentrated by froth floatation.
2. Heavy oxide ores are concentrated by hydraulic washing (gravity separation) or magnetic separation.

Teacher's Note:
1. Concentration is the removal of gangue.
2. Roasting and calcination are not concentration processes.

Answer: (d) silver reacts with sulphur compounds in the air to form black silver sulphide

Explanation:
1. Traces of hydrogen sulphide and sulphur compounds in the air form a black layer of Ag₂S.
2. This is the tarnish of silver.

Teacher's Note:
1. Cleaning with baking soda and aluminium foil removes it.
2. Copper turns green and iron turns brown.

Answer: (b) metals lose electrons easily and give them to H+ ions, but non-metals tend to gain electrons

Explanation:
1. Metals are electropositive and give up electrons to H+, so H₂ gas is formed.
2. Non-metals do not lose electrons to H+ because they tend to gain electrons.

Teacher's Note:
1. Metal + dilute acid → salt + hydrogen.
2. The reactivity series decides which metals do this.

Answer: (b) bromine; graphite; iodine

Explanation:
1. Bromine is the only liquid non-metal at room temperature.
2. Graphite conducts electricity, and iodine crystals have a shiny metallic lustre.

Teacher's Note:
1. The only liquid metal is mercury.
2. These are exceptions to the general properties of non-metals.

Answer: (c) Al₂O₃

Explanation:
1. An amphoteric oxide reacts with both acids and bases to form salt and water.
2. Al₂O₃ and ZnO are amphoteric, whereas Na₂O and CaO are basic and CO₂ is acidic.

Teacher's Note:
1. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O.
2. Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O.

Answer: (b) copper and tin; copper reacts with moist air to form basic copper carbonate

Explanation:
1. Bronze is an alloy of copper and tin.
2. Copper on the surface reacts with moisture and CO₂ of the air to form green basic copper carbonate.

Teacher's Note:
1. Brass is Cu and Zn.
2. The green layer is verdigris.

Answer: (b) (iv) only

Explanation:
1. Copper is below sodium, aluminium is below magnesium and silver is below iron in the reactivity series, so (i), (ii), (iii) do not react.
2. Copper is above silver, so copper displaces silver from AgNO₃, and only (iv) gives a reaction.

Teacher's Note:
1. The free metal must be more reactive than the metal in the salt.
2. Reactivity order: Mg > Al > Fe > Cu > Ag.

Answer: (a) the acids in them dissolve the basic copper carbonate or oxide layer

Explanation:
1. The black or green layer is basic in nature (copper oxide or basic carbonate).
2. The citric and tartaric acids in lemon and tamarind react with it and make the vessel shiny.

Teacher's Note:
1. Metal oxide + acid → salt + water.
2. This is neutralisation.

Answer: (b) moist blue litmus red but has no effect on dry litmus: S + O₂ → SO₂

Explanation:
1. Sulphur burns to give sulphur dioxide: S + O₂ → SO₂.
2. SO₂ is acidic only in water (it forms sulphurous acid), so dry litmus is unaffected and moist blue litmus turns red.

Teacher's Note:
1. Non-metal oxides are acidic.
2. SO₂ + H₂O → H₂SO₃.

Answer: (b) Fe₃O₄

Explanation:
1. Iron reacts with steam to form Fe₃O₄, which is FeO·Fe₂O₃, a mixed oxide of Fe(II) and Fe(III).
2. The other oxides have iron or aluminium in a single oxidation state.

Teacher's Note:
1. Fe₃O₄ is magnetic.
2. 3Fe + 4H₂O → Fe₃O₄ + 4H₂.