12th Class Chemistry Chapter 3 Group III-A and Group IV-A Elements Short Question Answers

Chemistry short QA

12th Class Chemistry Chapter 3 Group III-A and Group IV-A Elements Short Question Answers Given Below

1.How the elements of group III-A show the valency of three?
The elements of group III-A have three electrons in the outermost principal quantum number.One of the electrons is promoted from s-orbital to one of the p-orbitals and three unpaired unpaired electrons make the valency three.
2.Why the heat of sublimation in III-A group elements decrease down the group?
It is due to the reason that the atoms of elements with greater atomic number are less tightly packed due to their bigger sizes.
3.B+3 ion does not exist but AI+3 ion does.Why?
The size of B+3 ion is very small.It has high charged density and immediately forms the covalent bonding due to greater polarizing power.AI+3 ion gives the ionic bonding and its compounds are dissociated in water to give AI+3 ions.
4.How the nature of the oxides of the group III-A change from upper to the downward direction?
B2O3 is acidic in nature.AI2O3  and Ga2O3 are amphoteric.In2O3 and TI2O3 are basic in character.
5.The hydrides of boron have different structures from the hydrides of other family members.How?
Boron gives polymeric hydrides having bridge structures.B2H6 and B4H10 have bridge structures and they are electron deficient molecules.The hydrides of other family members are not bridged molecules.
6.Give the names and the formula of different acids of boron?
Boron gives four importand acids.These acids are  as follows:
i) Orthoboric Acid,H3Bo3 or B2O3,3H2O also called boric acid.
ii)Metaboric Acid,HBO2 or B2O3,H2O.
iii)Tetraboric Acid,H2B4O7 or 2B2O3,H2O.
iv) Pyroboric Acid, H6B4O9 or B2O3,3H2O.
Orthoboric acid is the most important  and is also called boric acid.It is a stable compound while the other acids are stale in solid state and change to orthoboric acid in solution.
7.How will you convert boric acid into borax and vice versa?
When orthoboric  acid H3BO3 is neutralized by caustic soda we get borax.
4H3BO3 + 2NaOH………………. Na2B4O7+7H2O
When borax is treated with concentrated H2SO4 then boric acid is produced.On cooling the crystals of boric  acid are separated out.
Na2B4O7+H2SO4+5H2O…………….. Na2SO4+4H3BO3.
8.How the solubility of borax changes with the change of temperature?
Borax is sparingly soluble in cold water.3 grams is dissolved in 100 grams of water at 10ᵒC.At 100ᵒC,99.3 grams of H3BO3 becomes soluble in 100 grams of water.
9.How does the temperature influence the water of crystallization of borax?
Below 62ᵒC decahydrated crystals are formed.Above 62ᵒC octahedral crystals of pentahydrated borax are obtained.
10.Justify that the aqueous solution of borax turns red litmus blue?
Na2B4O7 is hydrolyzed by water to give H3BO3 and NaOH.Since the base is strong so the solution turns red litmus blue.
11.How borax can be converted to orthoboric acid?
When borax is treated with HCI or H2SO4  in the presence of water then orthoboric acid is produced.
Na2B4O7+2HCI+5H2O………………… 2NaCI+4H3BO3
12.Outline the four use of borax?
Borax is used: i) To prepare borate glass which is heat resistant. ii) In the softening of water. iii) In borax bead test for the detection of metallic cations. iv) In metallurgical operations. v) As a flux in welding. vi) In making washing powders. vii) In leather industry for tanning and dyeing. Viii) In cosmetics soaps textiles paints medicines match industry and as a preservative.
13.How the glassy mass is produced from borax?
When borax is heated it swell up.When it is heated further it melts into a clear transparent glass.
2Na2B4O7.10H2O………………. Na2B4O7.2NaBO2+B2O3.
14.Which basic radicals give the response to borax bead test?
There are seven basic radicals in salt analysis scheme which  give colored beads when  borax bead test is performed.These radicals are CU+2,Fe+2,Fe+3,Mn+2,Co+2,Ni+2,Cr+3.They have different colors  in the cold and hot states in the oxidizing and reducing flames.
15.What is the nature of compounds which give colored beads in borax bead test?
Those metals which give the borax bead test are converted into their  metaborates.These metaborates appear in the form of beads.They show different colors in the cold and hot state  and in oxidizing  and reducing flame.Fe+2, Fe+3,Cr+3,Cu+2, Co+2,Ni+2 and Mn+2 give borax lead tests.
16.Give the names and formulas for oxyacids of boron?
Orthoboric acid H3BO3
Metaboric acid HBO2.
Tetraboric acid H2B4O7.
Pyroboric acid H6B4O9.
17.How H3BO3 can be from colemanite?
Colemanite is suspended in boling water and SO2 gas is passed through it.H3Bo3 crystallizes out from the solution.
Ca2B6O11 + 2SO2+9H2O…………………. 2CaSO3 + 6H3BO3
18.How H3BO3 can be prepared from Na2B4O7?
Borax is treated with calculated quantity of conc.H2SO4.When the solution is cooled H3BO3 settles down in the form of precipitate.
Na2B4O7+H2SO4+5H2O…………………. NaSO4 + 4H3BO3
19.How does H3BO3 act as an acid?
H3BO3 reacts with water  and releases H+ ions from water.It accepts OH ion and acts  as a monobasic acid.
20.What is the effect of heat on H3BO3?
When H3BO3 is heated it releases water molecules in three stages and B2O3 is produced in the long run.
H3BO3……….100ᵒC……..HBO2+H2O        4HBO2…….140ᵒC……H2B4O7+H2O
21.How H3BO3 with C2H5OH and Na2CO3?
3C2H5OH + H3BO3…..H2SO4….( C2H5)3BO3+3H2O
4H3BO3.+Na2CO3……………… Na2B4O&+6H2O+CO
22.Give uses of boric acid?
Orthoboric acid is used: i) In medicines as antiseptic as dusting powder in boric ointment and as eye-wash. ii) In pottery as a glaze.The reason is that borate glazes are more fusible than silicate glazes.They have high co-efficient of expansion. iii)In candle industry for stiffening of wicks.
23.What are the important ores and minerals of AI?
Some important minerals of aluminium are as follows:
Feldspar                             KALSi3O8
Mica(muscovite)              KH2AI3(SiO4)3
Kaolin(china clay)             H2AI2(SiO4)2.H2O
Bauxite                               AI2O3.2H2O
Cryolite                               Na3AIF6
24.What is anodizing?
The layer of AI2O3 on the surface of aluminium is very useful.Its thickness is increased purposely in the industry by electrolytic process  which is called anodizing.
25.Why AI is not found free in nature?
Aluminium reacts with oxygen vigorously and this reaction is highly exothemic.This heat can be used for the melting of those metals whose oxides are reduced by aluminium powder.This strong affinity of AI for O2 does not allow AI to be free in nature.
26.How does AI react with dil.and conc.H2SO4?
Aluminium reacts with dilute and conc. H2SO4 as follows:
2AI+3 H2SO4………..dil…………AI2(SO4)3+3H2
2AI+6 H2SO4………..conc…………. AI2(SO4)3+3SO2+6H2O
27.How does AI react with conc.HNO3?
Conc.HNO3 has no action on aluminium.A protective layer of AI2O3 is developed which does not allow  aluminium to react with HNO3 further.
28.How does AI become water soluble by using NaOH?
Aluminium dissolves in alkalies with the evolution of hydrogen gas  and soluble  aluminates are produced.The reaction is as follows:
2AI + 2NaOH………..2Na2AIO2+3H2
29.How inert pair effect controls the oxidation state of Sn and Pb?
Sn and Pb can hardly promote their electrons from s-orbitals to vacant p-orbtials.So they show the valency of two rather than four.Well C,Si Ge do not show the inert pair effect.
30.How carbon on behaves differently from other members of its own group?
i) It is the hardest substance in its family. ii) It shows allotropy. iii) It shows catenation. iv) It is fundamental element in organic chemistry.
31.Mention various allotropic modifications of elements of group IV-A especially carbon?
The property of allotropy is associated with all the elements of group IV-A except Pb.Carbon shows two.SI two Ge two and Sn has three allotropic forms.Carbon shows as diamond and graphite.This is due to parallel layers in graphite ad loosely held electrons that it is a good conductor of electricity.
32.What are the important ores of Sio2?
Important ores of silicon are as follows:
Analcite (a zeolite)                    NaAI(Sio3)2.H2O
Asbestos                                     Ca Mg3(SiO3)4
Kaolin(china clay)                      H2AI2(SiO4)2.H2O
Zircon                                          ZrSiO4
Talc(soap stone)                       H2Mg3 (SiO3)4
33.What are the different forms in which SiO2 exists in the earth’s crust?
i) Rock crystal ii) Amethyst quartz iii) Smoky quartz iv) Rose quartz V) Milky quartz.
34.What is quartz and what are its properties?
Quartz is one of the common crystalline form of SiO2.It is hard brittle refactor and a colorless soild.
35.Why diamond is non-conductor of electricity and graphite is good conductor?
Graphite has layered structure.Free electrons are present in them which are responsible for conduction of current.In diamond there are no free electrons.All the bonds in diamond C-Co bonds.
36.Why is Co2 a gas at room temperature while SiO2 is a soild?
Co2 is a gas at room temperature while Sio2 is a solid.The reason is that CO2exists in the form of discrete molecules which exist independently.In case  of SiO2 there is a network structure.It is three dimensional structure giving a giant molecule.
37.What is silica glass?
It is also called fused quartz.It is obtained by heating SiO2.It has  random structure.When it is cooled it is not  crystallized readily.
38.How sodium silicate is prepared?
When Na2CO3 is reacted with pure sand and heated in furnace Na2SiO3 is obtained.It is called water  glass or soluble glass.
39.What are the properties of water glass?
It is water soluble and its aqueous solution is strongly alkaline.It is used in the preparation of chemical gardens and is used in soap manufacture.
40.What are the uses of sodium silicate?
It is used as a filler in the manufacture of soap as a fire proof in textile in fumiture polish in calicoprinting and in preservation of eggs.
41.What is chemical garden?
When the crystals of various soluble colored salts like NiCI2 FeSO4 and CuSO4 etc. Are placed in a solution of water glass there happens a beautiful growth of crystals.This is called silica garden or chemical garden.
42.What are the uses of aluminium silicates or clay?
It is used to make porcelin and china clay.Impure clay is used to make make bricks tiles and stone wares.
43.How the clay is hardened?
Clay is soaked in water and it is hydrated with the passage of time.It becomes more and more plastic in nature.On heating water of hydration is lost and hard rock like mass is obtained.
44.Give the formula of talc or soap stone.Gives its properties and uses?
Mg3H2 (SiO3) is the formula of soap stone.It is greasy to touch and is used in making cosmetics and household articles.
45.What is asbestos?Give its uses?
Asbestos is mixed silicate of Ca and Mg with formula CaMg3(SiO3)4.
It is used to  make incombustible fabrics and hard board.
46.Why are liquid silicones preffered over ordinary organic lubricants?
There happens a very small change in viscosity with the change in temperature for silicones.When the temperature drops from 100ᵒC to ᵒC then the viscosities of petrolem oils which are used as lubricants increase their viscosity 100 times.Anyhow viscosities of silicon oils increase less than four times for this change of temperature.Hence silicones are preferred over ordinary organic lubricants.
47.Which elements and compounds act as semiconductor?
Silicon germanium and selenium are good semiconductors.The important semiconductors among the compounds are PbS,SiC,CdS,etc.
48.What is the effect of temperature on semiconductor?
When a semiconductor is heated its resistance decreases and conductivity increases.Similarly when a semiconductor absorbs light the electrons become more mobilized and conductivity increases.
49.Define semiconductor?
A semiconductor is a substance which has different resistances for the passage of electrical current under different circumstances.
50.How litharge changes its colour?
Litharge varies in colors from pale yellow to reddish yellow.Actually it has two forms: i) Rhombic (yellow) ii) Tetragonal(red).

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