CBSE Class 10 Science Chemistry Reactions Acids Metals and Carbon Assignment with Answers Set 02

Answer:

Teacher's Note: Oxidation is gain of oxygen or loss of hydrogen; reduction is the opposite.

Answer: Corrosion is the slow eating away of a metal by air and moisture, e.g. iron rusts (Fe₂O₃·xH₂O) and silver turns black. Rancidity is the oxidation of fats and oils in food kept for long, which gives a bad smell and taste. It can be prevented by adding antioxidants, flushing the pack with nitrogen and storing food in airtight containers in a cool place.

Teacher's Note: Give an example if the question allows.

Answer: (a) Magnesium ribbon is cleaned before burning because it is covered with a layer of magnesium oxide, which stops it from burning.
(b) Respiration is exothermic because glucose is oxidised in the cells and releases energy as heat.
(c) Fat and oil foods are flushed with nitrogen to stop oxidation, so that they do not turn rancid.
(d) Silver chloride is stored in dark bottles because it decomposes in sunlight into silver and chlorine.

Teacher's Note: Write a short, clear answer.

Answer: (a) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
(b) 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂
(c) Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
(d) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄

Teacher's Note: Check that every element has the same number of atoms on both sides.

Answer:

Teacher's Note: Learn this point and write it in your own words.

Answer: (a) Curd and sour things are not kept in brass or copper vessels because the acid reacts with the metal to form poisonous salts.
(b) Dry HCl gas does not change the colour of dry litmus because it has no H⁺ ions without water.
(c) Acid is added to water and not water to acid because the process gives out a lot of heat, and the mixture could splash out and burn.
(d) Distilled water has no ions, so it does not conduct electricity. Rain water has dissolved acids such as carbonic acid, which give ions.

Teacher's Note: Use the key term in your answer.

Answer: Neutralisation: the reaction of an acid with a base to form salt and water, e.g. HCl + NaOH → NaCl + H₂O.
Chlor-alkali process: electrolysis of brine (sodium chloride solution), which gives sodium hydroxide at the cathode, chlorine at the anode and hydrogen: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
Antacids: mild bases, such as magnesium hydroxide (milk of magnesia), that neutralise excess acid in the stomach.

Teacher's Note: Give an example if the question allows.

Answer: (a) Baking powder, a mixture of baking soda and tartaric acid. It gives out carbon dioxide, which makes the cake rise.
(b) Bacteria in the mouth break down sugar and leftover food into acids. When the pH in the mouth falls below 5.5, the tooth enamel is attacked and decay begins. It is prevented by brushing after meals with a basic toothpaste, avoiding sweets and rinsing the mouth.

Teacher's Note:

Answer:

Teacher's Note: Remember this for your exam.

Answer: Amphoteric oxide: an oxide that reacts with both acids and bases, e.g. Al₂O₃ and ZnO.
Aqua regia: a 3 : 1 mixture of concentrated HCl and HNO₃, which dissolves gold and platinum.
Thermite process: reduction of a metal oxide, such as iron(III) oxide, with aluminium, which gives molten metal and a lot of heat, used to join rail tracks.
Roasting: heating a sulphide ore strongly in excess air.
Calcination: heating a carbonate ore strongly in limited air.
Reactivity series: a list of metals in decreasing order of their reactivity.

Teacher's Note: Learn this point and write it in your own words.

Answer: (a) Sodium is kept in kerosene because it reacts vigorously with oxygen and moisture and can catch fire.
(b) Ionic compounds have high melting points because the electrostatic forces between ions are strong and need a lot of energy to break.
(c) Zinc does not give hydrogen with nitric acid because HNO₃ is a strong oxidising agent that oxidises the hydrogen to water.
(d) Silver turns black due to silver sulphide, and copper turns green due to basic copper carbonate.

Teacher's Note: Use the key term in your answer.

Answer: The impure copper is made the anode, and a thin strip of pure copper is the cathode. The electrolyte is acidified copper sulphate solution. When current is passed:
Anode: Cu → Cu²⁺ + 2e⁻ (copper dissolves)
Cathode: Cu²⁺ + 2e⁻ → Cu (pure copper deposits)
Insoluble impurities settle below the anode as anode mud.

Teacher's Note:

Answer: Metals high in the series (K, Na, Ca, Mg, Al) are very reactive, and carbon cannot reduce their oxides, so they are obtained by electrolysis of their molten chlorides or oxides. For example, sodium is obtained from molten NaCl. Metals in the middle (Zn, Fe, Pb, Cu) are less reactive: their sulphides or carbonates are first changed to oxides by roasting or calcination, and then reduced with carbon.

Teacher's Note: Write a short, clear answer.

Answer: (a) Sodium (2, 8, 1) has one dot in its outer shell: Na• . Oxygen (2, 6) has six dots around its symbol. Magnesium (2, 8, 2) has two dots: Mg••.
(b) Na₂O: each of two sodium atoms gives one electron to an oxygen atom, forming 2Na⁺ and O²⁻, which attract to form Na₂O.
MgO: magnesium gives its two outer electrons to oxygen, forming Mg²⁺ and O²⁻, which attract to form MgO.

Teacher's Note: Remember this for your exam.

Answer:

Teacher's Note: Soap molecules have a water-loving head and a water-hating tail. In water they form micelles, with the tails around the dirt and the heads facing outward, so that the oily dirt is washed away. In hard water, soap reacts with calcium and magnesium salts to form an insoluble, sticky scum.

Answer:

Teacher's Note: Use the key term in your answer.

Answer: Homologous series: a series of compounds with the same functional group, in which each member differs by CH₂, e.g. methane, ethane, propane.
Denatured alcohol: ethanol made unfit to drink by adding poisonous methanol and a dye.
Hydrogenation: adding hydrogen to unsaturated compounds with a nickel catalyst: vegetable oil → vanaspati ghee.
Addition reaction: an unsaturated compound adds on atoms: CH₂=CH₂ + H₂ → CH₃CH₃.
Substitution reaction: one atom is replaced by another: CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight).
Combustion: burning in oxygen to give CO₂, water, heat and light: CH₄ + 2O₂ → CO₂ + 2H₂O.
Saponification: an ester reacts with sodium hydroxide to give soap and glycerol.
Esterification: a carboxylic acid and an alcohol react with a little concentrated H₂SO₄ to give an ester: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O.
Oxidation: alcohols are oxidised to acids with alkaline KMnO₄: CH₃CH₂OH → CH₃COOH.

Teacher's Note: Give an example if the question allows.

Answer: Carbon has four electrons in its outer shell. To form ions it would have to lose four electrons, which needs a lot of energy, or gain four electrons, which is hard for a small nucleus with six protons to hold. So it shares electrons and forms covalent bonds.

Teacher's Note: Write a short, clear answer.

Answer: (a) CH₃CH₂CH=CH₂: But-1-ene
(b) CH₃CH₂CH₂–C(CH₃)₂–CH₃: 2,2-Dimethylpentane
(c) CH₃CH₂CHO: Propanal
(d) CH₃CH(OH)CH₃: Propan-2-ol

Teacher's Note: Remember this for your exam.

Answer: Cyclohexane, C₆H₁₂: six carbon atoms in a ring joined by single bonds, each carbon with two hydrogen atoms.

Teacher's Note: