CBSE Class 10 Science Metals and Non-metals, Carbon and its Compounds Assignment Set 10

Answer: Physical: metals are generally hard, lustrous, malleable, ductile and good conductors of heat and electricity (except mercury is liquid); non-metals are generally dull, brittle, non-ductile and poor conductors (graphite and iodine are exceptions).
Chemical: metals form basic oxides and lose electrons to become positive ions; non-metals form acidic or neutral oxides and gain electrons to form negative ions. Metals displace hydrogen from dilute acids, but non-metals do not.

Teacher's Note:
1. Exceptions such as graphite, iodine, sodium and mercury should be remembered.
2. Metals are electropositive and non-metals are electronegative.

Answer: An oxide that reacts with both acids and bases to give salt and water is amphoteric. Examples are aluminium oxide (Al₂O₃) and zinc oxide (ZnO). For instance, Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O.

Teacher's Note:
1. Amphoteric oxides show both acidic and basic behaviour.
2. Most metal oxides are only basic.

Answer: Aqua regia is a freshly prepared mixture of concentrated HCl and concentrated HNO₃ in the ratio 3 : 1. It can dissolve gold and platinum. The thermite reaction is the reaction of iron(III) oxide with aluminium, Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat, which gives molten iron and is used to weld railway tracks.

Teacher's Note:
1. Aqua regia means royal water because it dissolves the king of metals, gold.
2. The thermite reaction is a displacement reaction that gives off a large amount of heat.

Answer: Roasting is heating a sulphide ore strongly in excess air to convert it to an oxide, e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂. Calcination is heating a carbonate ore in limited air to give the oxide, e.g. ZnCO₃ → ZnO + CO₂. The reactivity series is a list of metals arranged in decreasing order of reactivity, from potassium at the top to gold at the bottom.

Teacher's Note:
1. Both processes produce metal oxides, which are then reduced to the metal.
2. The reactivity series decides how a metal is extracted.

Answer: Sodium is highly reactive and catches fire when exposed to oxygen and moisture of the air. Kerosene cuts off air and water and prevents accidents.

Teacher's Note:
1. Potassium and lithium are stored in the same way.
2. Sodium is soft and can be cut with a knife.

Answer: Ionic compounds consist of oppositely charged ions held together by strong electrostatic forces in a crystal lattice. A large amount of energy is needed to overcome these forces, so melting and boiling points are high.

Teacher's Note:
1. Ionic solids are also hard and brittle.
2. They conduct electricity only in the molten state or in solution.

Answer: Nitric acid is a strong oxidising agent. It oxidises the hydrogen produced to water and is itself reduced to oxides of nitrogen, so no hydrogen gas is evolved.

Teacher's Note:
1. Only magnesium and manganese give hydrogen with very dilute HNO₃.
2. Zinc nitrate is the salt formed.

Answer: Silver reacts with sulphur compounds in the air to form black silver sulphide. Copper reacts slowly with moist air, carbon dioxide and oxygen to form a green coating of basic copper carbonate.

Teacher's Note:
1. These changes are examples of corrosion.
2. The coating can be cleaned with a mild acid such as lemon juice.

Answer: The impure copper is made the anode and a thin strip of pure copper the cathode. The electrolyte is an acidified solution of copper sulphate. On passing current, copper from the anode dissolves as Cu²⁺ ions, and an equal amount of pure copper from the solution is deposited at the cathode. Soluble impurities remain in the solution, and insoluble ones settle below the anode as anode mud.

Teacher's Note:
1. At the anode Cu → Cu²⁺ + 2e⁻, and at the cathode Cu²⁺ + 2e⁻ → Cu.
2. Gold and silver are recovered from the anode mud.

Answer: Metals high in the series (K, Na, Ca, Mg, Al) form very stable compounds, so carbon cannot reduce their oxides. They are obtained by electrolysis of their molten chlorides or oxides. Metals in the middle (Zn, Fe, Pb, Cu) are first converted to oxides by roasting or calcination and then reduced by heating with carbon.

Teacher's Note:
1. The more reactive the metal, the more strongly it holds on to oxygen.
2. For example sodium is obtained by electrolysis of molten NaCl.

Answer: Sodium (2, 8, 1) has 1 valence electron: Na•. Oxygen (2, 6) has 6 valence electrons, written with 2 paired dots and 2 single dots around the symbol. Magnesium (2, 8, 2) has 2 valence electrons: •Mg•.

Teacher's Note:
1. Only the valence electrons are shown in the dot structure.
2. The number of valence electrons equals the number of dots.

Answer: Na₂O: each of two sodium atoms (2, 8, 1) loses one electron to give two Na⁺ (2, 8). One oxygen atom (2, 6) gains these two electrons to give O²⁻ (2, 8). The ions are held together by electrostatic attraction as Na₂O.
MgO: one magnesium atom (2, 8, 2) loses two electrons to give Mg²⁺ (2, 8), and one oxygen atom gains them to give O²⁻, forming MgO.

Teacher's Note:
1. Each ion reaches the stable octet of the nearest noble gas.
2. The formula follows from balancing positive and negative charges.

Answer: Soaps are sodium or potassium salts of long-chain fatty acids; detergents are usually sodium salts of sulphonic acids or ammonium salts. Soaps form scum with hard water, whereas detergents do not.
A soap molecule has a water-loving ionic head and a water-hating hydrocarbon tail. In water the tails surround oil and dirt while the heads face the water, forming a micelle. Rinsing carries the micelles away. Scum is the insoluble calcium and magnesium salt of the soap formed in hard water.

Teacher's Note:
1. Detergents work in both hard and soft water.
2. Micelles are clusters of soap molecules.

Answer: Physical: ethanol has a pleasant smell and does not freeze easily (melting point 156 K), whereas ethanoic acid has a pungent, vinegar smell and freezes at 290 K (glacial acetic acid).
Chemical: ethanoic acid reacts with sodium hydrogencarbonate to give CO₂ but ethanol does not. Ethanol reacts with sodium to give hydrogen and sodium ethoxide, and the acid does too.
Tests: (1) add NaHCO₃, and brisk effervescence indicates the acid; (2) add blue litmus, and it turns red in the acid but not in ethanol.

Teacher's Note:
1. Ethanoic acid is a weak acid, but still stronger than carbonic acid.
2. Both compounds react with sodium, so this test cannot tell them apart.

Answer: Concentrated H₂SO₄ acts as a dehydrating agent and removes a water molecule from ethanol. Ethene is formed: CH₃CH₂OH → CH₂=CH₂ + H₂O.

Teacher's Note:
1. A temperature near 443 K is needed for this dehydration.
2. The product is an unsaturated hydrocarbon.

Answer: A homologous series is a family of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a CH₂ unit, e.g. methane, ethane, propane.
Denatured alcohol is ethanol mixed with poisonous substances like methanol and a dye to make it unfit for drinking.
Hydrogenation is the addition of hydrogen to unsaturated hydrocarbons in the presence of nickel as catalyst, e.g. vegetable oil → vanaspati ghee, or CH₂=CH₂ + H₂ → CH₃CH₃.

Teacher's Note:
1. Members of a homologous series differ in mass by 14 u.
2. Denaturing is done to prevent misuse of industrial alcohol.

Answer: Addition: unsaturated hydrocarbons add a reagent across a double or triple bond, e.g. CH₂=CH₂ + H₂ → CH₃CH₃.
Substitution: one atom or group is replaced by another, e.g. CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight).
Combustion: burning in oxygen to give carbon dioxide, water, heat and light, e.g. CH₄ + 2O₂ → CO₂ + 2H₂O.

Teacher's Note:
1. Saturated hydrocarbons undergo substitution, and unsaturated ones undergo addition.
2. Combustion is always exothermic.

Answer: Saponification: the alkaline hydrolysis of an ester (fat or oil) with NaOH to form soap and glycerol.
Esterification: the reaction of a carboxylic acid with an alcohol in the presence of concentrated H₂SO₄ to form a sweet-smelling ester: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O.
Oxidation: ethanol is oxidised to ethanoic acid by alkaline potassium permanganate or acidified potassium dichromate: CH₃CH₂OH → CH₃COOH.

Teacher's Note:
1. Esters are used in perfumes and flavourings.
2. Saponification is the base of soap manufacture.

Answer: Carbon has four valence electrons. To get an octet it would have to gain four electrons or lose four electrons, which needs too much energy. It therefore shares electrons with other atoms, including other carbon atoms, to form covalent bonds.

Teacher's Note:
1. Carbon forms C⁴⁻ or C⁴⁺ ions with great difficulty.
2. This is why carbon compounds are poor conductors of electricity.

Answer: (a) But-1-ene (butene)
(b) Propanal
(c) Propan-2-ol (isopropyl alcohol)

Teacher's Note:
1. The suffix -ene shows a double bond, -al an aldehyde group and -ol an alcohol group.
2. Count the longest carbon chain to find the root name.

Answer: Cyclohexane, C₆H₁₂. Six carbon atoms are joined in a ring by single bonds, and each carbon carries two hydrogen atoms.

Teacher's Note:
1. Cyclohexane is a saturated hydrocarbon because it has only single bonds.
2. The formula C_nH_2n is the general formula for cycloalkanes.