Revision notes, a question bank, assertion and reason questions and answers to the NCERT questions. Tap a group to open it; opening one closes the others.
Metal + oxygen → metal oxide. Metal oxides are basic.
2Cu + O₂ → 2CuO (black)4Al + 3O₂ → 2Al₂O₃Sodium and potassium react so vigorously that they catch fire in air, so they are kept immersed in kerosene oil. Magnesium burns with a dazzling white flame. Surfaces of aluminium, zinc and lead are covered with a thin protective oxide layer. Iron does not burn on heating but iron filings burn vigorously. Silver, gold and platinum do not react with oxygen. Anodising thickens the oxide layer on aluminium.
Oxides of aluminium and zinc show the properties of both bases and acids. They are amphoteric oxides.
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂OAl₂O₃ + 2NaOH → 2NaAlO₂ + H₂OMetal + dilute acid → salt + hydrogen. Reactivity is judged by the rate of bubbles. Magnesium is the fastest, then aluminium, zinc and iron; copper does not react. Nitric acid is a strong oxidising agent and generally does not give hydrogen, except with very dilute Mg and Mn.
Mg + 2HCl → MgCl₂ + H₂Zn + 2HCl → ZnCl₂ + H₂Aqua regia (3 parts concentrated HCl and 1 part concentrated HNO₃) can dissolve gold and platinum, which no single acid can.
A more reactive metal displaces a less reactive metal from its salt solution.
Fe + CuSO₄ → FeSO₄ + CuZn + CuSO₄ → ZnSO₄ + CuCu + 2AgNO₃ → Cu(NO₃)₂ + 2AgThe list of metals in order of decreasing reactivity is the activity series.
Metals above hydrogen displace it from dilute acids. Metals below hydrogen (Cu, Hg, Ag, Au) do not.
Atoms try to attain a full outer shell (the noble-gas arrangement). A metal loses electrons to form a positive ion (cation); a non-metal gains electrons to form a negative ion (anion). The electrostatic attraction between them is the ionic bond.
The method depends on the position of the metal in the activity series.
Thermit reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat. The iron formed is molten, and the reaction is used to weld railway tracks and cracked machine parts.
Sodium is obtained by the electrolysis of molten sodium chloride. At the cathode: Na⁺ + e⁻ → Na. At the anode: 2Cl⁻ → Cl₂ + 2e⁻. Aluminium is obtained by the electrolysis of molten alumina.
The most widely used method for refining impure metals is electrolytic refining. In the refining of copper:
Iron rusts in the presence of both air (oxygen) and moisture, forming hydrated iron(III) oxide, Fe₂O₃·xH₂O. Silver turns black, and copper turns green, in air.
An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. It is made by melting the main metal and dissolving the others in it.
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(i) Mercury. (ii) Sodium (or lithium, potassium). (iii) Silver. (iv) Lead (or mercury).
Sodium is very reactive; it reacts vigorously with oxygen and moisture of the air and may catch fire. Kerosene oil keeps it away from air and water.
(i) 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
(ii) Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) + heat
(i) B is the most reactive: it displaces iron, while A cannot.
(ii) B is more reactive than copper, so the blue colour of the solution fades and reddish-brown copper is deposited.
(iii) B > A > C > D. (B displaces iron; A displaces copper but not iron; C displaces only silver; D displaces nothing.)
Hydrogen gas.
Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)
Zinc is more reactive than iron, so it displaces it: the green colour of the solution fades and a grey/black deposit of iron forms on zinc.
Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s)
(i) Na has 1 valence electron (Na•); O has 6 (:Ö:, with two unpaired); Mg has 2 (•Mg•).
(ii) Na₂O: each of two Na atoms (2, 8, 1) gives one electron; the oxygen atom (2, 6) takes both to become O²⁻ (2, 8). So 2Na → 2Na⁺ + 2e⁻ and O + 2e⁻ → O²⁻. MgO: Mg (2, 8, 2) gives two electrons to O (2, 6): Mg → Mg²⁺ + 2e⁻, O + 2e⁻ → O²⁻.
(iii) Na₂O has Na⁺ and O²⁻ ions; MgO has Mg²⁺ and O²⁻ ions.
The ions in an ionic compound are held by strong electrostatic forces. A large amount of energy is needed to break this attraction, so the melting points are high.
(i) Mineral: an element or compound occurring naturally in the Earth's crust.
(ii) Ore: a mineral that contains a high percentage of a metal and from which it can be extracted profitably.
(iii) Gangue: the impurities (soil, sand, rocks) present in an ore.
Gold and platinum (silver and copper can also occur free).
A more reactive metal displaces a less reactive one from its oxide. Reactivity order: Mg > Zn > Cu.
Zinc oxide + Mg → displacement occurs (Mg displaces zinc). Magnesium oxide + Zn or Cu → no reaction. Copper oxide + Zn or Mg → displacement occurs.
Zinc oxide + Cu: no reaction; copper oxide + Cu, zinc oxide + Zn and magnesium oxide + Mg: same metal, no reaction.
Gold, platinum and silver (the least reactive metals) do not corrode easily.
An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal, for example brass (copper and zinc), steel (iron and carbon).
Copper is more reactive than silver and displaces it. The others involve a less reactive metal.
Answer: (d)
Grease and paint are not suitable for a frying pan, as they would burn or contaminate food on heating, while a coating of zinc (galvanising) protects it.
Answer: (c) Applying a coating of zinc
A high-melting, water-soluble oxide is ionic: calcium oxide.
Answer: (a) calcium
Zinc is more reactive than tin and would react with the acids in the food.
Answer: (c)
(a) Hammer test: beat the sample; metals flatten into sheets (malleable) while non-metals break or crumble (brittle). Conductivity test: connect the sample in a circuit with the battery, bulb and switch; the bulb glows with a metal and not with a non-metal.
(b) These tests are useful but not perfect. Graphite, a non-metal, conducts electricity, and sodium and potassium are metals that are soft, and so the results should be combined with other properties.
Oxides that react with both acids and bases to give salts and water are amphoteric oxides. Examples: aluminium oxide (Al₂O₃) and zinc oxide (ZnO).
Displace hydrogen: magnesium and zinc (also Al, Fe). Do not displace: copper and silver (also Hg, Au).
Anode: a block of the impure metal M. Cathode: a thin strip of pure M. Electrolyte: a solution of a salt of M (for example, acidified copper sulphate for copper).
(a) (i) No action on dry litmus. (ii) The moist blue litmus turns red, because sulphur dioxide forms sulphurous acid with water.
(b) S(s) + O₂(g) → SO₂(g)
(i) Painting, oiling or greasing, which keeps air and moisture away. (ii) Galvanisation: coating with zinc. (Others: alloying into stainless steel, chrome or tin plating.)
Acidic oxides (for example CO₂, SO₂), or neutral oxides such as CO and H₂O.
(a) They are very lustrous, malleable and ductile, and do not corrode, being unreactive.
(b) They react violently with oxygen and water, and may catch fire, so oil keeps air and moisture away.
(c) Aluminium forms a thin, tough layer of aluminium oxide that protects it from further reaction. It is also a good conductor of heat.
(d) It is easier to obtain a metal from its oxide than from its carbonate or sulphide, by reduction. So carbonates are calcined and sulphides roasted.
The tarnish is a layer of basic copper carbonate (and copper oxide). The acids in lemon and tamarind juice neutralise it and dissolve it, leaving the surface shiny.
The solution was aqua regia, a 3 : 1 mixture of concentrated hydrochloric and nitric acids. It dissolves gold, so the outer layer of the bangles dissolved, giving a bright surface and losing weight.
Copper does not react with water or steam, while iron reacts with steam and rusts. Copper is also a good conductor of heat, so it is better for hot-water tanks.