Chapter 8 – Metallurgy
1. Write names.
a. Alloy of sodium with mercury.
Ans: Sodium amalgam
b. Molecular formula of the common ore of aluminium.
Ans: Al₂O₃.nH₂O
c. The oxide that forms salt and water by reacting with both acid and base.
Ans: Aluminium oxide, Zinc oxide (All Amphoteric oxide)
d. The device used for grinding an ore.
Ans: Ball mill
e. The nonmetal having electrical conductivity.
Ans: Graphite
f. The reagent that dissolves noble metals.
Ans: Aqua regia
2. Make pairs of substances and their properties
| Substance | Property |
|---|---|
a. Potassium Bromide | 1. Combustible |
b. Gold | 2. Soluble in water |
c. Sulphur | 3. No chemical reaction |
d. Neon | 4. High ductility |
Ans:
| Substance | Property |
|---|---|
a. Potassium Bromide | 2. Soluble in water |
b. Gold | 4. High ductility |
c. Sulphur | 1. Combustible |
d. Neon | 3. No chemical reaction |
3. Identify the pairs of metals and their ores from the following.
| Group A | Group B |
|---|---|
a. Bauxite | i. Mercury |
b. Cassiterite | ii. Aluminium |
c. Cinnabar | iii. Tin |
Ans:
| Group A | Group B |
|---|---|
a. Bauxite | ii. Aluminium |
b. Cassiterite | iii. Tin |
c. Cinnabar | i. Mercury |
4. Explain the terms.
a. Metallurgy
Ans: The process used for the extraction of metals in their pure form from their ores is referred to as Metallurgy. The processes involved in Metallurgy are:
(i) Crushing and Grinding
(ii) Concentration
(iii) Roasting and calcination
(iv)Reduction
(v) Refining
b. Ores
Ans:
(i) An ore is a combination of minerals and impurities.
(ii) Ore is a mineral from which the metal can be extracted conveniently and economically.
(iii) All minerals are not ores, but all ores are minerals.
(iv) Examples: Bauxite (Al₂O₃.2H₂O) is the ore of Aluminium, and copper pyrite (CuFeS₂) is an ore of copper.
c. Minerals
Ans:
(i) Elements or compounds which occur naturally in earth’s crust are called Minerals.
(ii) These minerals may or may not contain metals.
(iii) Examples: Salt, Clay Marble.
d. Gangue
Ans:
(i) Gangue is an unwanted substance or impurity that surrounds the mineral in an ore deposit, such as sand, rock, or some other material.
(ii) Mineral dressing, mineral processing, or ore dressing are terms used to describe the method of separating gangue from mineral.
5. Write scientific reasons.
a. Lemon or tamarind is used for cleaning copper vessels turned greenish.
Ans:
(i) Copper on exposure to moist air combines with carbon dioxide and a green coloured copper carbonate salt is formed, tarnishing the copper.
(ii) Lime juice and tamarind contain citric acid and tartaric acid respectively.
(iii) These acids react with copper carbonate forming water-soluble salts, which get washed off easily.
(iv) Due to this, the tarnished copper becomes clean and regains its lustre.
(v) Hence, lemon or tamarind is used for cleaning copper vessels turned greenish.
b. Generally the ionic compounds have high melting points.
Ans:
(i) Ionic compounds are formed when an atom of a combining metal element transfers its electrons to the atom of another non-metallic element.
(ii) During this process, the atom of the combining metal after losing electrons, forms a positively charged ion called as cation and the atom of the combining non-metal after gaining electrons forms a negatively charged ion called as anion.
(iii) Thus ionic bonds are formed between the cations and anions during compound formation.
(iv) Due to the presence of cations and anions, there is a strong electrostatic attraction between the molecules.
(v) Hence to break these intermolecular bonds a lot of energy is needed.
(vi) Hence ionic compounds have high melting points.
c. Sodium is always kept in kerosene.
Ans:
(i) Sodium is a highly reactive metal.
(ii) It reacts with the oxygen and moisture present in the air to form sodium hydroxide and hydrogen gas.
(iii) Hydrogen which is released, catches fire in presence of oxygen due to liberation of heat during this reaction.
(iv) Sodium does not react with kerosene and hence, it sinks in it.
(v) Thus, to prevent sodium from coming in contact with air, it is immersed in kerosene.
d. Pine oil is used in froth flotation.
Ans:
(i) Froth floatation is a process to purify sulphide ores, in presence of pine oil.
(ii) The froth floatation method is based on the two opposite properties, hydrophilic and hydrophobic, of the particles. Here the particles of the metal sulphides, due to their hydrophobic property, get wetted mainly with pine oil, while due to the hydrophilic property the gangue particles get wetted with water.
(iii) Pressurized air is blown through the mixture and simultaneously mixture is agitated using an agitator, due to agitation particles of sulphide ore form a foam with pine oil. This foam rises to the surface of water and floats and can be easily separated.
e. Anodes need to be replaced from time to time during the electrolysis of alumina.
Ans:
(i) In extracting aluminium from aluminium oxide, the molten electrolyte is a liquid mixture of cryolite (Na₃AlF₆) Fluorspar (CaF₂) and aluminium oxide (alumina) (Al₂O₃).
(ii) The cathode and anodes are made of graphite.
(iii) During the electrolysis process, aluminium is deposited on the cathode on passing electric current.
(iv) Oxygen is liberated at the anode.
(v) Some of this oxygen reacts with the carbon in the graphite to form carbon-dioxide gas then slowly burns away the anodes.
(vi) Thus, the anodes need to be replaced from time to time during the electrolysis of alumina.
6. When a copper coin is dipped in silver nitrate solution, a glitter appears on the coin after some time. Why does this happen? Write the chemical equation.
Ans:
(i) Copper being more reactive than silver, displaces silver from silver nitrate to form element silver and copper nitrate.
Cu\(_{(s)}\) + 2AgNO₃\(_{(aq)}\) → Cu(NO₃)₂\(_{(aq)}\) + 2Ag\(_{(s)}\)
Copper Silver Copper Silver
nitrate nitrate
(ii) The silver displaced gets coated on to the coin, hence the coin gets the silver shine.
7. The electronic configuration of metal ‘A’ is 2,8,1 and that of metal ‘B’ is 2,8,2. Which of the two metals is more reactive? Write their reaction with dilute hydrochloric acid.
Ans:
Metal ‘A’ Electronic configuration – (2, 8, 1),
Metal ‘B’ Electronic configuration – (2, 8, 2),
Metal ‘A’ loses one electron and forms a positive ion. Metal ‘B’ also loses 2 electrons and forms positive ions; but metal ‘A’ is more reactive than metal ‘B’, as metal ‘B’ takes more energy to remove two electrons whereas metal ‘A’ requires less energy to lose one electron.
Metal ‘A’ is sodium and metal ‘B’ is magnesium. Hence, sodium is more reactive than magnesium.
2Na + 2HCl → 2NaCl + H₂↑
Sodium Hydrochloric Sodium Hydrogen
acid chloride gas
Mg + 2HCl → MgCl₂ + H₂↑
Magnesium Hydro- Magnesium Hydrogen
chloric acid chloride gas
8. Draw a neat labelled diagram.
a. Magnetic separation method.
Ans:
b. Froth floatation method.
Ans:
c. Electrolytic reduction of alumina.
Ans:
d. Hydraulic separation method.
Ans:
9. Write chemical equation for the following events.
a. Aluminium came in contact with air.
Ans: When aluminium is exposed to air, it develops a thin oxide layer of aluminium.
The chemical reaction for the above is as follows:
4Al + 3O₂ → 2 Al₂O₃
Aluminum Oxygen Aluminium
Oxide
b. Iron filings are dropped in aqueous solution of copper sulphate.
Ans: When iron filings are dropped in copper sulphate solution, more reactive iron displaces copper from copper sulphate solution. The iron filings get coated with reddish brown copper metal and the blue colour of copper sulphate fades gradually and ferrous sulphate is formed.
The chemical reaction for the above is as follows:
Fe + CuSO₄ → FeSO₄ + Cu
Iron Copper Iron Copper
Sulphate Sulphate
c. A reaction was brought about between ferric oxide and aluminium.
Ans: The reaction between ferric oxide and iron produces aluminium oxide and iron. It is a thermite reaction and is highly exothermic. It produces a large amount of heat, which is released to melt oxygen and aluminium.
The chemical reaction for the above is as follows:
Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat
Ferric Aluminium Iron Aluminium
Oxide Oxide
d. Electrolysis of alumina is done.
Ans: During electrolysis of alumina, aluminium is deposited at the cathode. Molten aluminium being heavier than the electrolyte, is collected at the bottom of the tank. Oxygen gas is liberated at the anode.
Electrode reaction:
Anode reaction – 2O₂⁻ → O₂ + 4e⁻ (Oxidation)
Cathode reaction – Al₃⁺ + 3e⁻ → Al (Reduction)
e. Zinc oxide is dissolved in dilute hydrochloric acid.
Ans: When zinc oxide dissolves in dilute hydrochloric acid, zinc chloride and water is formed.
The chemical reaction for the above is as follows:
ZnO + 2HCl → ZnCl₂ + H₂O
Zinc Hydrochloric Zinc Water
Oxide Acid Chloride
10. Complete the following statement using every given option.
During the extraction of aluminium ______
a. Ingredients and gangue in bauxite.
Ans:
(i) Aluminium is extracted from its ore Bauxite (Al₂O₃.nH₂O).
(ii) Bauxite contains 30% to 70% of Al₂
O₃ and the remaining part is gangue. It is made up of sand,silica,iron oxide etc. Silica (SiO₂), ferric oxide (Fe₂O₃) and titanium oxide (TiO₂) are the impurities present in bauxite.
b. Use of leaching during the concentration of ore.
Ans:
(i) Leaching is a method in which ore is soaked in a certain solution for a long time.
(ii) The ore dissolves in that solution due to a specific chemical reaction.
(iii) The gangue does not react and therefore does not dissolve in that solution.
(iv) During the concentration of ore, separation of impurities is done by the leaching process using either Bayer’s method or Hall’s method.
c. Chemical reaction of transformation of bauxite into alumina by Hall’s process.
Ans:
(i) In the Hall’s process, the ore is powdered and then leached by heating with aqueous sodium carbonate to form water soluble sodium aluminate.
(ii) Then the insoluble impurities are filtered out.
(iii) The filtrate is warmed and neutralised by passing carbon dioxide gas through it.
(iv) This results in the precipitation of aluminium hydroxide.
(v) Al₂O₃.2H₂O\(_{(s)}\) + Na₂CO₃\(_{(aq)}\) → 2NaAlO₂\(_{(aq)}\) + CO₂\(_{(g)}\) + 2H₂O\(_{(s)}\)
(vi) 2NaAlO₂\(_{(aq)}\) + 3H₂O\(_{(l)}\) + CO₂\(_{(g)}\)→ 2Al(OH)₃↓+ Na₂CO₃
(vii) The precipitate of Al(OH)₃ obtained in Hall’s process is filtered, washed, dried and then calcined by heating at 1000°C to obtain alumina.
(viii) 2Al(OH)₃ → Al₂O₃ + 3H₂O.
d. Heating the aluminium ore with concentrated caustic soda.
Ans: Aluminium oxide being amphoteric in nature, it reacts with the concentrated solution of sodium hydroxide (caustic soda) at 140°C to 150°C under high pressure for 2 to 8 hours in a digester to form water soluble sodium aluminate.
Al₂O₃.2H₂O\(_{(s)}\) + 2NaOH\(_{(aq)}\) → 2NaAlO₂\(_{(aq)}\) + 3H₂O\(_{(l)}\)
Aluminium Sodium Sodium Water
oxide hydroxide aluminate
11. Divide the metals Cu, Zn, Ca, Mg, Fe, Na, Li into three groups, namely reactive metals, moderately reactive metals and less reactive metals.
Ans:
| Reactive metals | Moderately reactive | Less reactive |
|---|---|---|
Na | Mg | Cu |
Li | Zn | |
Ca | Fe |