Metallurgy

(a) The three classes in which elements are classified are –

1. metals,
2. non-metals and
3. metalloids.

Copper was the first metal used by man for making utensils, weapons and for other purposes.

(b) The most abundant metal present in the earth’s crust is Aluminum and the most abundant non-metal present in the earth’s crust is Oxygen.

(a) Iron is a component of blood pigment (haemoglobin).

(b) Magnesium is present in plant pigment (chlorophyll).

(a) Nitrogen — It is the most abundant element present in the atmosphere. It's presence in air reduces the rate of combustion. Due to it's inertness, it is used to preserve food.

(b) Hydrogen — It is the lightest element known and is used in the hydrogenation of vegetable oils to make ghee. As a fuel and in the manufacture of compounds. It is also the essential part of organic compounds.

(c) Carbon — Proteins, fats, carbohydrates, enzymes, vitamins, etc. are all compounds of carbon and are essential for the growth and development of living organisms.

(a) Alkali metals — Group I (IA), the first column on the left of the periodic table.

(b) Alkaline earth metals — Group 2 (IIA), the second column on the left of the periodic table

(c) Halogens — Group 17 (VII A)

(d) Aluminium — Group 13 (III A), present on the right of periodic table.

(a) a liquid non-metal — bromine

(b) two metalloids — boron and silicon

(c) metal which does not corrode easily — gold

(d) two metals which react with cold water — potassium and sodium

(e) a non-metal which can form a positive ion — hydrogen

(f) a non-metal which shows reducing property — carbon

D. Acidic oxides

C. Brittle

F. Discharged at anode

I. Covalent chlorides

L. 5,6,7 valence electrons

Alkali metals are very reactive, they react with atmospheric reagents like oxygen, carbon dioxide and water vapour and form compounds, so they are kept in inert solvent.

Hydrogen, though a non-metal is kept in the metal activity series because it can form a positive ion H⁺ similar to metals.

As gold is less reactive, and do not react under normal conditions with oxygen, water, carbon dioxide and other reagents. Hence, it looks new and does not lose it's glitter and shine even after years.

(a) does not react with dilute hydrochloric acid — Copper

(b) can form 2+ and 3+ ions — Iron

(c) arrange the above metals in the decreasing order of reactivity — Na > Mg > Zn > Fe > Cu

(a) a sulphide — Zn; Ore — Zinc Blende [ZnS]

(b) a halide — Sodium; Ore — Rock salt [NaCl]

(c) a carbonate — Calcium; Ore — Limestone [CaCO₃]

(d) an oxide — Aluminium; Ore — Bauxite [Al₂O₃.2H₂O]

(a) Bauxite - Aluminum [Al]

(b) Calamine - Zinc [Zn]

(c) Haematite - Iron [Fe]

(a) Ores are those minerals from which metals are extracted commercially at a comparatively lower cost with minimum efforts.

(b) The earthly impurities like soil, mud, sand, silica (SiO₂), limestone, rocks, etc. that are mixed with valuable minerals in an ore deposit are called gangue or matrix.

(a) Magnesium
Reason — Oxides of highly active metals like potassium, sodium, calcium, magnesium and aluminium have great affinity for oxygen

(b) Magnesium oxide
Reason — Oxides of highly active metals like potassium, sodium, calcium, magnesium and aluminium have great affinity for oxygen and so cannot be reduced by common reducing agents like carbon, carbon monoxide or hydrogen.
They are obtained by electrolytic reduction of fused metallic salts (halides and oxides) using inert electrodes.

(a) The processes involved in:

(i) Dressing of ores are:

1. Gravity separation /Hydraulic washing
2. Froth floatation
3. Magnetic separation
4. Chemical method/Leaching

(ii) Refining of ores are:

1. Distillation
2. Liquation
3. Electro refining

(b) sodium oxide and potassium oxide

(a) Hydrolytic method — The difference in the densities of the ore and the gangue is the main criterion.

(b) Froth floatation process — The process depends on the preferential wettability of the ore with oil (pine oil) and the gangue particles by water .

(c) Electromagnetic separation — Magnetic properties of the ores.

(a) Roasting and Calcination are the methods of converting concentrated ore to metallic oxide.

(b) During roasting:

1. Moisture is removed
2. Organic matter is oxidised and removed.
3. It makes the ore porous and so ore gets heated uniformly.

(a) Roasting — It is the process of heating the concentrated ore to a high temperature in the presence of excess air.

Example — Zinc blende (ZnS) is roasted to yield it's oxide, zinc oxide

$2\text{ZnS} + 3\text{O}_2 \xrightarrow{800 ^{\circ}\text{C}} \text{2ZnO} + \text{2SO}_{2}$

(b) Calcination — If an ore is carbonate or a hydrated oxide, it is heated in the absence of air to a temperature that is high but insufficient to melt the ore. The process is known as Calcination.

Example — carbonate and hydrated ores are calcinated like ZnCO₃

$\text{ZnCO}_3 \rightarrow \text{ZnO} + \text{CO}_{2} [\text{g}]$

Iron and zinc are moderately reactive metals. They tend to react with other elements, such as oxygen and sulphur forming iron oxides and zinc sulphides. These compounds are more stable and are found as minerals in nature.

When hydrogen is passed over heated copper oxide (CuO), the black CuO changes to pink/brown Cu and H₂O is released.

$\underset{\text{black}}{\text{CuO}} + \text{H}_2 \xrightarrow{\Delta} \underset{\text{pink/brown}}{\text{Cu}} + \text{H}_{2}\text{{O}}$

(a) Zinc blende, chemical name is Zinc sulphide and formula is ZnS.

(b) Froth floatation process is applied to concentrate ZnS.

(c) The concentrated ore is changed to oxide by heating Zinc blende (ZnS) in excess of air. The reaction is as follows:

$2\text{ZnS} + 3\text{O}_2 \xrightarrow{800 ^{\circ}\text{C}} \text{2ZnO} + \text{2SO}_{2}$

(a) Reduction of Iron (II) oxide

$\text{FeO} + \text{C} \xrightarrow{\Delta} \text{Fe} + \text{CO}$

$\text{FeO} + \text{CO} \xrightarrow{\Delta} \text{Fe} + \text{CO}_2$

${\text{FeO}} + \text{H}_2 \xrightarrow{\Delta} {\text{Fe}} + \text{H}_{2}\text{{O}}$

(b) Reduction of Iron (III) oxide

$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{2Fe} + \text{Al}_2\text{O}_3 + \text{heat}$

(c) Reduction of Lead (II) oxide

$\text{PbO} + \text{C} \xrightarrow{\Delta} \text{Pb} + \text{CO}$

$\text{PbO} + \text{CO} \xrightarrow{\Delta} \text{Pb} + \text{CO}_2$

$\underset{\text{yellow}}{\text{PbO}} + \text{H}_2 \xrightarrow{\Delta} \underset{\text{greenish white}}{\text{Pb}} + \text{H}_{2}\text{{O}}$

(d) Reduction of Zinc oxide

$\text{ZnO} + \text{C} \xrightarrow{1400^{\circ} \text{C}} \text{Zn} + \text{CO}$

(a) The metals in the middle of the activity series such as iron, lead, copper, etc., are moderately reactive and are found as sulphides or carbonates in nature. They are obtained by the reduction of their oxides with carbon, carbon monoxide and hydrogen.
While, oxides of highly active metals like sodium and potassium have great affinity for oxygen and so cannot be reduced by common reducing agents like carbon, carbon monoxide or hydrogen.

(b) $\underset{\text{black}}{\text{CuO}} + \text{H}_2 \xrightarrow{\Delta} \underset{\text{pink/brown}}{\text{Cu}} + \text{H}_{2}\text{{O}}$

Froth floatation process can be used for the concentration of this ore as formation of sulphurous anhydride suggests that it is a sulphide ore. Sulphide ores are lighter than the impurities present hence froth floatation process is used for the concentration of this ore.

1^st Incorrect Step: Leaching
Correction: Froth Floatation
Reason: Zinc blende is a sulphide ore. Sulphide ores are preferentially wetted by oil and are concentrated using the Froth Floatation process rather than leaching.

2^nd Incorrect Step: Calcination
Correction: Roasting
Reason: Calcination is used for carbonate ores (heating in the absence of air). Since Zinc blende is a sulphide ore, it must be heated strongly in the presence of excess air to convert it into zinc oxide (ZnO). This process is called Roasting.

Oxides of highly active metals like potassium, sodium, calcium, magnesium and aluminium have great affinity towards oxygen and so cannot be reduced by common reducing agents like carbon, carbon monoxide or hydrogen.

They are obtained by electrolytic reduction of fused metallic salts (halides and oxides) using inert electrodes.

The metals in the middle of the activity series such as iron, lead, copper, etc., are moderately reactive and are found as sulphides or carbonates in nature. They are obtained by the reduction of their oxides with carbon, carbon monoxide and hydrogen.

Mercury and silver are very less reactive, placed in the end of the reactivity series. The oxides of these metals are reduced to metals by heating alone.

Oxides of highly active metals like sodium, potassium and calcium are obtained by electrolytic reduction of fused metallic salts (halides and oxides) using inert electrodes.

Sodium

Electrolyte : Fused sodium chloride

Reaction : NaCl ⇌ Na⁺ + Cl^-

Reaction at cathode : Na⁺ + e^- ⟶ Na

Reaction at anode : Cl^- - e^- ⟶ Cl

Cl + Cl ⟶ Cl₂

Potassium

Electrolyte : Fused potassium bromide

Reaction : KBr ⇌ K⁺ + Br^-

Reaction at cathode : K⁺ + e^- ⟶ K

Reaction at anode : Br^- - e^- ⟶ Br

Br + Br ⟶ Br₂

Calcium

Electrolyte : Fused calcium chloride

Reaction : CaCl₂ ⇌ Ca²⁺ + 2Cl^-

Reaction at cathode : Ca²⁺ + 2e^- ⟶ Ca

Reaction at anode : Cl^- - e^- ⟶ Cl

Cl + Cl ⟶ Cl₂

(a) Purification of metal depends on:

1. nature of metal
2. nature of impurities present in the metal and
3. purpose for which the metal is to be used.

(b) Methods used for purification are :

1. Distillation
2. Liquation
3. Electro refining

(c) The below labelled diagram shows the electro-refining of a particular metal:

An electrolytic cell is used in electro-refining to refine impure metal using electrical energy to drive a chemical reaction. The cell consists of an electrolyte, two conducting electrodes (cathode and anode) in a non-conducting vessel. The impure metal is made the anode, and a thin sheet of pure metal is the cathode. The electrolyte is a salt solution of the metal to be refined, with a small amount of sulphuric acid added to increase conductivity. On passing electric current, metal ions are reduced to metal at the cathode. Equivalent mass of anode dissolves from the anode and goes into the solution as metal ions.

At anode: M - ne^- ⟶ Mⁿ⁺

At cathode: Mⁿ⁺ + ne^- ⟶ M [M is a metal]

Pure metal deposits at cathode while impurities settle down and form anode mud.

Aluminum — Period 3, Group 13 (III A).

(i) Ores of aluminium :

(ii) Ores of iron:

(iii) zinc

(b) Bauxite contains 60% aluminium oxide, the rest being sand, ferric oxide and titanium oxide.

(c) Finely grinded Bauxite is heated under pressure with caustic soda. Bauxite dissolves and forms sodium meta aluminate, leaving behind insoluble impurities called red mud. It consists of ferric oxide, sand etc. which are removed by filtration.

(a) Aluminum forms a thin, protective layer of aluminium oxide (Al₂O₃) on its surface that prevents further oxidation and corrosion. The oxide layer acts as a barrier that protects the metal underneath from being exposed to the elements, thus minimizing corrosion.

(b) Aluminium vessels should not be cleaned with powders containing alkalis because alkalis are water soluble and aluminum metal will react with alkalis to form sodium meta aluminate that can be toxic to our health.

2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂

In the electrolytic reduction of alumina, the graphite (anode) is oxidized by oxygen to CO and further forms CO₂, so it is consumed and has to be replaced from time to time.

Equation:

2C + O₂ ⟶ 2CO

2CO + O₂ ⟶ 2CO₂

Roasting is done in presence of air and provides oxygen to metal sulphides, which is required to convert them to metallic oxide and SO₂. Whereas, carbonate is changed to oxide by loss of CO₂, which occurs in the absence of air and when heated to a high temperature. Hence, roasting is not done on carbonates.

Oxides of highly active metals like sodium, potassium, aluminium have great affinity for oxygen and so cannot be reduced by common reducing agents like carbon, carbon monoxide or hydrogen whereas lead is in the middle of activity series and is moderately active. So carbon can reduce lead oxide easily.

Oxides of highly active metals like potassium, sodium, calcium, magnesium and aluminium have great affinity for oxygen and so cannot be reduced by common reducing agents like carbon, carbon monoxide or hydrogen.

They are obtained by electrolytic reduction of fused metallic salts (halides and oxides) using inert electrodes.

As aluminium is above iron in the metal reactivity series hence, it can displace iron from iron salts. Therefore, food containing iron salts should not be cooked in aluminium utensils.

The oxygen gas produced at anode reacts with carbon electrode, which further produces carbon monoxide, a neutral gas.

2C + O₂ ⟶ 2CO

The layer of powdered coke is sprinkled on top of the electrolyte as :

1. it prevents the burning of anode.
2. it reduces heat loss by radiation.

(a) Bauxite — Main ore from which aluminium is extracted. It contains 60% Al₂O₃

(b) Sodium hydroxide — Sodium hydroxide is used in the purification of bauxite ore by converting it into alumina. It acts as an effective solvent for the aluminum oxide in bauxite and helps to remove impurities like ferric oxide, sand, etc. which are collectively called as red mud.

(c) Cryolite — Lowers the fusion temperature from 2050°C to 950°C. and enhances conductivity.

(d) Graphite — Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.

(a) Aluminium oxide dissolves and forms sodium meta aluminate. Iron (III) oxide is left behind as red mud and is removed by filtration.

Al₂O₃.2H₂O + 2NaOH ⟶ 2NaAlO₂ + 3H₂O

(b) (i) Bayer's process

(ii) The equation for the action of heat on aluminium hydroxide is:

$2\text{Al(OH)}_3 \xrightarrow{1000 ^{\circ}\text{C}} \text{Al}_2\text{O}_3 + 3\text{H}_2\text{O}$

(c) (i) Na₃AlF₆

(ii) By dissolving aluminium oxide in cryolite, a conducting solution is produced.

(iii) Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode. The graphite (anode) is oxidized by oxygen to CO and further forms CO₂, so it is consumed and has to be replaced from time to time. Hence, large amount of graphite is required.
Equation:
2C + O₂ ⟶ 2CO
2CO + O₂ ⟶ 2CO₂

(iv) Reaction at the cathode:
4Al³⁺ + 12e^- ⟶ 4Al

(v) Inner carbon lining of the electrolytic cell

(a) The chemical used for dissolving aluminium oxide is cryolite. It is used in molten state.

(b) Reason for addition of caustic alkali:

During the Baeyer's Process, bauxite is treated with concentrated Caustic Soda (NaOH) to selectively dissolve the aluminium oxide.
Aluminium oxide is amphoteric and reacts with the alkali to form soluble sodium aluminate. The major impurities in the ore (like ferric oxide and silica) remain insoluble and can be filtered out as "red mud."

(c) The three balanced equations for the purification of bauxite are:

Al₂O₃.2H₂O + 2NaOH ⟶ 2NaAlO₂ + 3H₂O

NaAlO₂ + 2H₂O ⟶ NaOH + Al(OH)₃↓

2Al(OH)₃ $\xrightarrow[1000 ^{\circ}\text{C}]{\text{heat}}$ Al₂O₃ + 3H₂O

(d) At anode:

6O^2- - 12e- ⟶ 6[O]

3[O] + 3[O] ⟶ 3O₂

Anode is oxidised to carbon monoxide which further forms carbon dioxide

2C + O₂ ⟶ 2CO

2CO + O₂ ⟶ 2CO₂

1. F → Zinc blende, D → Froth flotation, B → Coke
2. C → Cryolite, A → Bauxite, E → Sodium hydroxide solution

(ii) Fill in the blanks using the most appropriate words from A to F.

1. The ore from which aluminium is extracted must first be treated with sodium hydroxide so that pure aluminium oxide can be obtained.

2. Pure aluminium oxide is dissolved in cryolite to make a conducting solution.

(iii) Cryolite [Sodium aluminium flouride] — Na₃AlF₆

(a) The layer of powdered coke is sprinkled over the electrolytic mixture as:

1. It prevents burning of carbon electrodes in air at the emergence point from the bath.
2. It minimizes or prevents heat loss by radiation.

(b) The graphite anodes are continuously replaced during the electrolysis because:

1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet.
   2C + O₂ ⟶ 2CO
   2CO + O₂ ⟶ 2CO₂

(c) Cryolite and fluorspar are added to the electrolytic mixture because their addition:

1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(d) Fluorspar (CaF₂) is the additional substance added because it acts as a solvent and increases the fluidity of the electrolytic bath so the molten aluminium can settle at the bottom more easily.

(a) As zinc is more reactive than iron, hence it forms a dense and impermeable layer of zinc oxide over iron which protects the iron beneath from rusting.

(b) As duralumin has strength up to six times greater than pure aluminium hence, alloy of aluminium–duralumin is used rather than pure aluminium in construction work.

An alloy is a homogeneous mixture of two or more metals or of one or more metals with certain non-metallic elements.

The properties of alloys are often greatly different from those of the components.

1. Gold is too soft to be used without a small percentage of copper.
2. The corrosion and oxidation resistance of steel is markedly increased by adding 15 to 18% of chromium and often a few percent of nickle (stainless steel).
3. The presence of carbon up to 1.5% greatly affects the properties of steel.
4. A low percentage of molybdenum improves the toughness and wear resistance of steel.

Alloys are made to change the property of their major constituent to achieve a specific objective.

1. The other element in Brass is Zinc
2. The other elements in Bronze are Tin and Zinc.

(a) Aircraft construction — Duralumin

(b) Electrical wiring or electrical work in joining metals — Solder or fuse metal

(c) Electrical appliances — Brass
     Household vessels — Bronze

(d) Naval ships — Brass

Amalgam is a mixture or an alloy of mercury with a number of metals or alloys such as sodium, zinc, gold, and silver, as well as some non-metals.

For example, Dental amalgam is a mixture of mercury and a silver-tin alloy. It is often used in dentistry as a filling material for cavities in teeth. The mercury acts as a binding agent that holds the other metals together to form a solid, durable material.

(a) Two properties of brass are:

1. Hardness and tensile strength — Brass is stronger than its components, Copper and Zinc.
2. Corrosion resistance — Brass has good corrosion resistance due to the presence of zinc in its composition.

(b) Sodium.

(a) Aluminium (95%), copper (4%), magnesium (0.5%) and manganese (0.5%).

(b) Lead (50%) and tin (50%).

(c) Copper (80%), tin (18%) and zinc (2%).

(d) Copper (60-70%) and zinc (40-30%)

(a) Mercury

(b) Mercury

(c) Roasting

(d) Slag i.e., Calcium silicate
CaO + SiO₂ ⟶ CaSiO₃ [Calcium silicate]

(e) Cryolite

(f) Graphite

(a) Making electric circuits — Brass

(b) Making medals — Bronze

(c) Making parts of watches — Brass

(d) Surgical instruments — Stainless steel

(e) Aircraft — Duralumin

No. Tin is used because it lowers the melting point of the lead–tin alloy, so the fuse melts quickly when excess current flows and breaks the circuit. Copper has a much higher melting point, so a lead–copper alloy would not melt easily and would not act as an effective fuse.

Duralumin is used rather than pure aluminium in construction work due to the following reasons:

1. Duralumin is light and strong, while aluminium is light and weak. It is also unaffected by moist air. Duralumin has a strength up to six times greater than pure aluminium.
2. It is corrosion-resistant and has high tensile strength.

(a) Y is the metallic element.

(b) Metal atoms tend to have a maximum of three electrons in the outermost energy level.

(c) Non-metallic elements tend to form acidic oxides while metals tend to form basic oxides.

(d) Non-metallic elements tend to be poor conductors of heat and electricity.

(e) Metals tend to lose electrons and act as reducing agents in their reactions with elements and compounds.

Haematite

Reason — Iron is mainly extracted from its chief ore haematite (Fe₂O₃)

Calcination

Reason — When an ore is a carbonate or a hydrated oxide, it is heated in the absence of air (or air might be supplied in a limited quantity) to a temperature that is high but insufficient to melt the ore. The process is known as Calcination.

Fluorspar

Reason — Fluorspar, also known as fluorite, is a mineral form of calcium fluoride which acts as a solvent along with cryolite, for the electrolytic mixture. Whereas, Bauxite, Corundum and cryolite are the ores of aluminium.

alumina is not reduced by reducing agents.

Reason — Electrolytic reduction is chosen as the method for reducing alumina. Since, aluminium oxide due to its great affinity for oxygen is a very stable compound. It is not reduced easily by common reducing agents like carbon, carbon monoxide or hydrogen.

hydrated aluminium oxide

Reason — Main ore of aluminium is bauxite (Al₂O₃.2H₂O) and its chemical name is hydrated aluminium oxide

Copper and zinc

Reason — Brass is an alloy of Copper and zinc. Brass is stronger than its components.

Carbon

Reason — Steel is an alloy of iron, chromium, nickle and carbon. it is made to increase the strength of iron.

Aluminium brings lightness.

Reason — Aluminium is used in duralumin along with copper because it imparts lightness.

Only Q

Reason — Duralumin ia an alloy with composition of Aluminium (95%), Copper (4%) and Magnesium (0.5%).

Both A and R are true but R is not the correct explanation of A.

Explanation — Hydrogen is a non-metal which also forms positive ions by losing its electron like metals. Hence, assertion (A) is true.
Metals are defined as the elements which form positive ions by the loss of electrons. Hence, reason (R) is true.

However, reason (R) does not explain how hydrogen form positive ion. Hence, reason (R) is not correct explanation of assertion (A).

A is true but R is false.

Explanation — Sodium and potassium are highly reactive in nature. Hence, assertion (A) is true. Sodium and potassium are so reactive. So, they do not occur in free states. They quickly combine with other elements. Hence reason (R) is false.

A is false but R is true.

Explanation — Minerals are the naturally occurring compounds of metals. But not every mineral can be used to extract metals profitably. Hence assertion (A) is false.
Ores are those minerals from which metals are extracted commercially at a comparatively lower cost and with minimum effort. Hence reason (R) is true.

A is true but R is false.

Explanation — Main ore of aluminium is bauxite (Al₂O₃.2H₂O). Hence, assertion (A) is true.
Bauxite is not the only ore of aluminium. Aluminium can be extracted from other ores like cryolite and corundum. Hence reason (R) is false.

Both A and R are true but R is not the correct explanation of A.

Explanation — Metallurgy deals with the production and purification of metals and manufacture of alloys. Hence, assertion (A) is true.
They are many different methods of concentration or purification of an ore and reduction is one of the methods of extraction of metals. Hence, reason (R) is true.

But reason (R) doesn't explain why it is important to purify and produce the pure metal from the process of metallurgy. Hence, reason (R) is not correct explanation of assertion (A).

A is false but R is true.

Explanation — The leaching method is used for the concentration of metals like aluminium, silver and gold. Hence, assertion (A) is false.
The ore is treated with a suitable reagent like acid, base or some other reagent, such that the ore is soluble in it but the impurities are not. The impurities are removed by filtration. Hence, reason (R) is true.

A is true but R is false.

Explanation — Bayer's process is used for the purification of bauxite ore. Where, bauxite is converted into alumina. Which is reduced to aluminium using electrolytic reduction. Hence, assertion (A) is true.
Refining of aluminium is done by hoope's electrolytic process not bayer's process. Hence, reason (R) is false.

Both A and R are true and R is the correct explanation of A.

Explanation — Highly reactive metals like calcium, magnesium have great affinity towards oxygen and so cannot be reduced by common reducing agents like coke (carbon), carbon monoxide or hydrogen. Such metals are obtained by electrolytic reduction. Hence, both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A is true but R is false.

Explanation — Since aluminium is very reactive, it exists in nature as stable oxide (Al₂O₃). To extract aluminium from this stable compound, simple heating or chemical reduction is not sufficient. Therefore, electrolysis of molten alumina is used in the Hall–Héroult process. Hence, assertion (A) is true.
Aluminium is actually most commonly found in nature in the form of aluminium oxide (Al₂O₃), primarily within its main ore, Bauxite. Hence, reason (R) is false.

Both A and R are true but R is not the correct explanation of A.

Explanation — Alloys are used for heating devices like electric iron and heater because of its high resistance to electricity. Hence, assertion (A) is true. Generally alloys have great resistance to electricity. Thus, they can produce more heat and alloys also have lower melting point than its individual component. Hence, reason (R) is true.

However, in case of electrical iron and heater, alloys have high melting point to withstand the heat, Hence, reason (R) is not correct explanation of assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — Bronze is composed of copper (80%), tin (18%) and zinc (2%). It is used in making medals, statues, utensils, etc. Hence, assertion (A) is true.
Bronze is used in making of medals, statues, utensils, bearing and coins because of its hardness and it can be casted easily. Hence, reason (R) is true and it is the correct explanation of assertion (A).

(a) During the concentration of bauxite ore, aluminium goes in soluble part because of it's amphoteric nature.

(b) In Hoope's process, pure aluminium is collected at the top of the electrolytic cell.

(c) Usually carbonate ores are subjected to calcination which is done in the absence of air

(d) Zinc blende is converted to oxide by roasting process.

(e) Froth floatation process is generally used to concentrate sulphides ores.

From the above table:

A ⟶ 3. Cathode
B ⟶ 2. Anode
C ⟶ 4. Electrolytic mixture

(a) Aluminium

(b) Froth floatation process

(c) Aluminium

(d) Gold

(e) Sodium

(f) Malleability

(g) Cryolite (Na₃AlF₆)

(h) Zinc Blende (ZnS).

(i) Sodium hydroxide

(j) Cryolite (Na₃AlF₆)

(a) Na₂O

(b) SO₂

(c) Al₂O₃

(d) SiO₂

(a) Brass — Cu [60-80%], Zn [40-20%]

(b) Duralumin — Al [95%], Mg [0.5%], Mn [0.5%], Cu [4%]

(c) Bronze — Cu [80%], Zn [1%], Sn [19%]

(a) Addition of Cryolite :

1. Lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. Enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. Enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(b) Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of NaOH under pressure for 2 hrs as a first step in obtaining Al₂O₃. The impurities present in bauxite mainly Fe₂O₃ and SiO₂ remain unaffected with conc. NaOH as impurities are not amphoteric. Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.

(c) Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.

Chemical formulae of main ores of iron and aluminium are:

It is a mixture of molten alumina (Al₂O₃) 20%, cryolite (Na₃AlF₆) 60% and fluorspar (CaF₂) 20%.

Conversion of Al(OH)₃ - to pure alumina - heat on Al(OH)₃

$\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100^{\circ}\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}$

2Al³⁺ + 6e^- ⟶ 2Al

The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O₂ ⟶ 2CO; 2CO + O₂ ⟶ 2CO₂] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.

(a) Sodium hydroxide

(b) The equation for the action of heat on aluminium hydroxide is:

$2\text{Al(OH)}_3 \xrightarrow{1000 ^{\circ}\text{C}} \text{Al}_2\text{O}_3 + 3\text{H}_2\text{O}$

(c) The element is Carbon. As carbon lining acts as cathode and graphite acts as anode, hence, we can say that element that acts both as cathode and anode is Carbon.

(d) Reaction at the cathode:
Al³⁺ + 3e^- ⟶ Al

(e) Reaction at the anode:
Al - 3e^- ⟶ Al³⁺

(i) A (cathode) is made of carbon and B (anode) is made of graphite.

(ii) Aluminium is formed at cathode i.e., electrode A.

(iii) Two aluminium compounds in the electrolyte C are alumina [Al₂O₃] and Cryolite [Na₃AlF₆]

(iv) In the electrolytic reduction of alumina, the graphite (anode) is oxidised by oxygen to CO and further forms CO₂, so it is consumed and has to be replaced from time to time.

Equation:

2C + O₂ ⟶ 2CO

2CO + O₂ ⟶ 2CO₂