NEET Practice Questions (MCQs) with Answers & Solutions

(c) Due to inert effect the stability of lower oxidation state gradually increase while stability of higher

      oxidation state gradually decreases down the group in elements of group 13th to 15th. So correct 

      orders are :

       (iii) ${\mathrm{Pb}}^{2+}>{\mathrm{Pb}}^{4+}, {\mathrm{Bi}}^{3+}>{\mathrm{Bi}}^{5+}$

       (v) ${\mathrm{Sn}}^{2+}<{\mathrm{Pb}}^{2+}, {\mathrm{Sn}}^{4+}>{\mathrm{Pb}}^{4+}$

Surface of Al forms ${\mathrm{Al}}_2{\mathrm{O}}_3$ on exposure to air and becomes passive.

Each combustion is exothermic.

${\mathrm{Al}}_2{\mathrm{O}}_3+3\mathrm{C}+{\mathrm{N}}_2 \stackrel{}{\to } 2\mathrm{AlN}+3\mathrm{CO}$

It is a method for refining of Al.

The cell used in this process consists of an iron tank lined with carbon at the bottom. A molten alloy of copper, crude aluminium and silicon is used as the anode. It forms the lowermost layer in the cell. The middle layer consists of molten mixture of fluorides of sodium, aluminium and barium (cryolite + BaF₂). The uppermost layer consists of molten aluminium. A set of graphite rods dipped in molten aluminium serve as the cathode. During electrolysis, Al³⁺ ions from the middle layer migrate to the upper layer, where they are reduced to aluminum by gaining 3 electrons. Equal numbers of Al³⁺ ions are produced in the lower layer. These ions migrate to the middle layer. Pure aluminium is tapped off from time to time. The Hoopes process gives about 99.99% pure aluminium.

Diamond is an allotropic form of carbon, carborundum is $\mathrm{SiC},$ corundum is ${\mathrm{Al}}_2{\mathrm{O}}_3,$ borazon is $\mathrm{BN}.$

Surface of $\mathrm{Al}$ forms ${\mathrm{Al}}_2{\mathrm{O}}_3$ in nitric acid and becomes passive.

$2\mathrm{Al}+{\mathrm{Fe}}_2{\mathrm{O}}_3 \stackrel{ }{\to } {\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{Fe} ; ∆\mathrm{H}=-\mathrm{ve} ;$

The heat given out is used in welding. This is also called Goldschmidt alumino thermic process.

thermite welding.

As we discussed thermite welding is a liquid state chemical welding process, in which joint formation takes place in molten state. Practically, it is combination of welding and casting process in which, the molten iron poured at the welding plates and allow solidifying to make a permanent strong joint. The molten state of iron creates without application of external heat or conventional furnace so this is taken as a welding process. In this type of welding, a mixture of aluminum and iron oxide is used in ratio 1:3 by weight. This mixture chemically reacts as follow.

8 Al + 3 Fe3O4 –> 9 Fe + 4 Al2O3 + Heat

This reaction gives aluminum oxide and iron and also liberate a huge amount of heat which coverts this mixture into molten state. Because the molten iron have higher density compare to aluminium oxide so it remains bottom of the crucible. There is an opening at bottom of the crucible from where, this molten iron poured at the required plates where we want to create joint.  This is basic principle of thermite welding.

It is an use of alum.

A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue).^[1] It may be used for dyeing fabrics or for intensifying stains in cell or tissue preparations. Although mordants are still used, especially by small batch dyers, it has been largely displaced in industry by directs.^[2]