CBSE Class 10 Science Chemical Reactions and Equations, Acids, Bases and Salts Assignment Set 05

Answer: 1. Combination: two or more reactants form one product, e.g. CaO + H₂O → Ca(OH)₂.
2. Decomposition: one compound breaks into simpler substances, e.g. CaCO₃ → CaO + CO₂ (on heating).
3. Displacement: a more reactive element displaces a less reactive one, e.g. Fe + CuSO₄ → FeSO₄ + Cu.
4. Double displacement: two compounds exchange ions, e.g. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
5. Redox: oxidation and reduction occur together, e.g. CuO + H₂ → Cu + H₂O.

Teacher's Note:
1. Reactions are classified by how reactants rearrange into products.
2. Decomposition can be caused by heat, light or electricity.

Answer: Corrosion is the slow eating away of a metal by air, moisture or chemicals in its surroundings. Rusting of iron, in which iron forms hydrated iron(III) oxide (Fe₂O₃·xH₂O) in the presence of air and water, is an example. It can be prevented by painting, oiling, galvanising or alloying.

Teacher's Note:
1. Corrosion needs both oxygen and moisture, so cutting off either one protects the metal.
2. Black silver and green copper articles are other examples.

Answer: Rancidity is the oxidation of fats and oils in food, which gives them a bad smell and taste. It can be slowed by adding antioxidants, storing food in air-tight containers, refrigerating it, or flushing packets with nitrogen.

Teacher's Note:
1. Oxidation of the fat molecules produces foul-smelling compounds.
2. Chips packets are filled with nitrogen to keep oxygen out.

Answer: Magnesium is covered by a thin, dull layer of magnesium oxide formed by slow reaction with air. This layer stops the metal from burning properly, so it is removed with sandpaper to expose fresh magnesium.

Teacher's Note:
1. The oxide coating is a barrier between the metal and oxygen.
2. Cleaning allows the ribbon to burn with a dazzling white flame.

Answer: During respiration glucose is oxidised in the cells to carbon dioxide and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Since energy is released, the process is exothermic.

Teacher's Note:
1. Exothermic reactions give out heat or energy to the surroundings.
2. This energy keeps the body warm and powers its activities.

Answer: Nitrogen is an unreactive gas. It removes oxygen from the packet, so the fats and oils are not oxidised and the food does not turn rancid.

Teacher's Note:
1. Nitrogen acts as a protective atmosphere.
2. This keeps the food fresh for longer.

Answer: Silver chloride decomposes in sunlight: 2AgCl → 2Ag + Cl₂. Dark bottles cut off light and prevent this photochemical decomposition.

Teacher's Note:
1. The grey colour seen on exposed AgCl is silver metal.
2. This property is used in black-and-white photography.

Answer: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (heat)

Teacher's Note:
1. Check that Fe, S and O are equal on both sides: 2 Fe, 2 S and 8 O.
2. The green crystals turn brown on heating.

Answer: 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂ (heat)

Teacher's Note:
1. Reddish-brown fumes of nitrogen dioxide are seen.
2. Both sides have 2 Cu, 4 N and 12 O.

Answer: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Teacher's Note:
1. This is a double displacement reaction that forms a yellow precipitate of lead iodide.
2. Two potassium atoms are needed to balance the lead compound.

Answer: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄

Teacher's Note:
1. A white precipitate of barium sulphate is formed, so this is a precipitation reaction.
2. Balance Ba and SO₄ first, then Al and Cl.

Answer: Chemical name: calcium oxychloride (CaOCl₂). Preparation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Uses: bleaching cotton and linen in the textile industry, and disinfecting drinking water.

Teacher's Note:
1. It is made by passing chlorine gas over dry slaked lime.
2. It also acts as an oxidising agent in chemical industries.

Answer: Chemical name: sodium carbonate decahydrate (Na₂CO₃·10H₂O). Preparation: heating baking soda gives sodium carbonate, which is recrystallised with water: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. Uses: removing permanent hardness of water and cleaning clothes, and in glass, soap and paper industries.

Teacher's Note:
1. It is a basic salt, so its solution turns red litmus blue.
2. The ten water molecules are its water of crystallisation.

Answer: Chemical name: sodium hydrogencarbonate (NaHCO₃). Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. Uses: as an ingredient of baking powder for cakes, and as an antacid.

Teacher's Note:
1. On heating it gives sodium carbonate, water and carbon dioxide.
2. It is mildly basic, so it neutralises excess stomach acid.

Answer: Chemical name: calcium sulphate hemihydrate (CaSO₄·½H₂O). Preparation: CaSO₄·2H₂O (gypsum) heated at 373 K loses 1½ molecules of water. Uses: plaster for fractured bones and making toys, statues and smooth wall surfaces.

Teacher's Note:
1. It sets into hard gypsum when mixed with water.
2. It must be stored in a moisture-proof container.

Answer: Curd and sour foods contain acids. These acids react with copper and brass to form harmful copper salts, which can spoil the food and make it poisonous.

Teacher's Note:
1. The vessel also gets corroded.
2. Steel or glass containers are safer for such foods.

Answer: HCl releases H⁺ ions only in the presence of water. Without water there are no free ions, so the gas shows no acidic behaviour on dry litmus.

Teacher's Note:
1. Acids show their acidic character only in aqueous solution.
2. Moist litmus paper would turn red.

Answer: Distilled water has no dissolved ions, so it cannot carry current. Rain water dissolves gases like carbon dioxide from the air, which forms carbonic acid and gives ions that carry current.

Teacher's Note:
1. Electric current in solutions is carried by ions.
2. Pure water is a very poor conductor.

Answer: A reaction in which an acid and a base react to form a salt and water is neutralisation. For example, NaOH + HCl → NaCl + H₂O.

Teacher's Note:
1. In ionic terms, H⁺ combines with OH⁻ to form water.
2. Heat is released in the process.

Answer: It is the electrolysis of aqueous sodium chloride (brine): 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chlorine is released at the anode, hydrogen at the cathode and sodium hydroxide stays in solution.

Teacher's Note:
1. The name comes from chlorine and alkali (NaOH).
2. The products are used to make bleach, soap, fertilisers and plastics.

Answer: Antacids are mild bases that neutralise excess hydrochloric acid in the stomach and relieve acidity. Examples are magnesium hydroxide (milk of magnesia) and sodium hydrogencarbonate. They raise the pH of the stomach contents toward neutral.

Teacher's Note:
1. A weak base is used so that the stomach lining is not harmed.
2. Too much acid in the stomach causes pain and irritation.

Answer: Baking powder, a mixture of baking soda and a mild edible acid such as tartaric acid, is added. On heating, CO₂ is released: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, and the bubbles make the cake rise.

Teacher's Note:
1. The acid in baking powder neutralises the bitter sodium carbonate formed.
2. Baking soda alone would leave a bitter taste.

Answer: Bacteria in the mouth break down leftover sugar and food particles and produce acids. When the pH in the mouth falls below 5.5, tooth enamel (calcium phosphate) starts corroding and decay begins. Brushing after meals with a basic toothpaste neutralises the acid, and cutting down sugary food and rinsing the mouth also help.

Teacher's Note:
1. Enamel is the hardest substance in the body but dissolves in acid below pH 5.5.
2. Toothpastes are generally basic.