12th Class Chemistry Chapter 3 Short Questions with Answers Solved

Question: 1

What is the action of an aqueous solution of Borax on litmus.

Answer: 1

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Aqueous solution of borax is alkaline in nature due to hydrolysis.It will turn red litmus to blue.
Na₂B₄O₇+ 7H₂O------>2NaOH + 4H₃BO₃
Borax Strong alkali Weak acid

Question: 2

Write the formula of Borax and Chile Saltpeter.

Answer: 2

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Borax: Na₂B₄. 10H₂O

Chile Saltpeter : NanO₃

Question: 3

Justify that solubility of borax changes with change in temperature.

Answer: 3

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It is sparingly soluble in cold water but is more soluble in hot water:100 grams of water dissolve 3 gram of decahydrate at 10^oC and 99.3 grams at 100^oC.

Question: 4

Show that H₃BO₃is monobasic acid?

Answer: 4

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H₃BO₃is a very weak acid and inoizes to a very limited extent mainly as a monobasic acid.
H₃BO₃ + H₂O ----->{B(OH)₄} + H⁺

Question: 5

How is borax converted into boric acid vice versa?

Answer: 5

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A hot concentrated solution of borax is treated with a calculated quantity of concentrated H₂SO₄.On cooling,crystals of boric acid formed separated out.
Na₂B₄O_{7 +}H₂SO₄+5H₂O------>Na₂SO₄+ 4H₃BO₃
Borax Boric acid

Question: 6

Aluminium is not found free in nature.Comment the statement?

Answer: 6

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It occurs primarily as alumino-silicate minirals found in the rocks of the outer portion of the earth.So,Aluminium is not found free in nature.

Question: 7

Discuss chemistry of borax bead test?

Answer: 7

7-55

Borax,when fused,is decomposed into sodium metaborate and boric anhydride.
Na₂B₄O₇----->2NaBO₂+B₂O₃

Question: 8

Aqueous solution of borax is alkaline in nature.Justify the statement.

Answer: 8

8-55

Aqueous solution is alkaline in nature due to hydrolysis.
Na₂B₄O₇+ 7H₂O------->2NaOH +4H₃BO₃
Borax Strong alkali Weak acid

Question: 9

Write the names and chemical formulas of four boric acids?

Answer: 9

9-55

Name Formulae
i) Orthoboric acid H₃BO₃
ii) Metaboric acid HBO₂
iii) Tetraboric acid H₂B₄O₇
iv) Pyroboric acid H₆B₄O₉

Question: 10

What happens when borax is treated with HCl and H₂SO₄separately?

Answer: 10

10-55

Aqueous solution of borax reacts with HCl or H₂SO₄to form boric acid.
Na₂B₄O₇+ 2HCI + 5H₂O------->4H₃BO₃+ 2NaCI
Na₂B₄O₇ + H₂SO₄+ 5H₂O------>4H₃BO₃+ Na₂SO₄

Question: 11

What is reaction of heat on orthoboricacid,H₃BO₃?

Answer: 11

11-55

When orthoboric acid is heated strongly,it swells to frothy mass losing water molecules.It is first converted into metaboric acid then to tetra boric acid and finally to boric anhydride.

Question: 12

Boric acid can be converted to borax and vice versa.Give reaction?

Answer: 12

12-55

Conversion of boric acid into borax:Borax is prepared by treating a hot solution of boric acid with proper amount of soda ash:
4H₃BO₃+ Na₂CO₃-------> Na₂B₄O₇+ 6H₂O +CO₂

Question: 13

Write any four uses of borax.

Answer: 13

13-55

i) It is used to prepare borate glass,which is heat resistant.

Question: 14

How borax is commercially prepared?Give two methods of preparation.

Answer: 14

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i) Borax is prepared by treating a hot solution of boric acid with proper amount of soda ash:
4H₃BO₃+ Na₂CO₃--------> Na₂B₄O₇+ 6H₂O + CO₂

Question: 15

Under what conditions aluminium corrode.

Answer: 15

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When aluminium sheet is exposed to moist air it acquires a thin,continuous coating of aluminium oxide,which is product of aluminium corrosion.

Question: 16

Write the behavior of Al with conc. HNO₃

Answer: 16

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Aluminium does not react with nitric acid at any concentration,probably because of the formation of protective layer of aluminium oxide.

Question: 17

Under what conditions aluminium corrodes?

Answer: 17

17-55

When aluminium sheet is exposed to moist air it acquires a thin,continuous coating of aluminium oxide,which is product of aluminium corrosion.

Question: 18

Give two similarities between carbon and silicon.

Answer: 18

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Carbon and silicon both form acidic oxides where as other oxides areamphoteric in nature.

Question: 19

How does Aluminium react with a) NaOH b) H₂SO₄.

Answer: 19

19-55

Alluminiumis dissolved in sodium hydroxide to give solube aluminate,with the evolution of hydrogen.
Al + 2NaOH-----> + 6H₂O----->NaAl(OH)₄+ 3H₂

Question: 20

How does H₃BO₃act as an acid?

Answer: 20

20-55

i) Boric acid turns blue litmus red.
ii) Boric acid partially neutralized by caustic soda to give borax.
4H₃BO₃+ 2NaOH------>Na₂B₄O₇+ 7H₂O

Question: 21

Outline any four uses of aluminium?

Answer: 21

21-55

i) It is non-magnetic and is thus used in navigational equipment.
ii) It is non toxic and can be used for making food and brewing equipments and is packaging.
iii) Aluminium readily forms alloys with other metals like copper,magnesium,nickel and zinc.
At home,aluminium is found in the form of cooking utensils,window frames and kitchen foil.

Question: 22

Why Boric acid cannot be titrated by NaOH?

Answer: 22

22-55

Boric acid is partially neutralized by caustic soda to give borax so,boric acid cannot be titrated by NaOH.
4H₃BO₃+ 2HaOH------->Na₂B₄O₇+ 7H₂O

Question: 23

Aluminium sheets are said to be corrosion free.Comment.

Answer: 23

23-55

When a sheet of aluminium is exposed to moist air it acquires a thin,continuous coating of aluminium oxide,which prevents further attack on the metal by atmospheric oxygen and water under normal conditions.Because of this aluminium sheets are said to be corrosion-free.

Question: 24

Give reaction of Aluminium with dilute and concentrated H₂SO₄?

Answer: 24

24-55

Reaction with dilute H₂SO₄:Aluminium does not react with dilute sulphuric acid.

Question: 25

Write down two uses of boric acid.

Answer: 25

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i) Boric acid are used in medicines as an antiseptic,e.g dusting powder,boric ointment and boric solution is used as an eye wash.

Question: 26

Why liquid silicones are preferred over organic lubricant?

Answer: 26

26-55

The outstanding physical attribute of silicone oil is its very small change in viscosity with changing temperature,compared with the behavior of other oils of similar viscosity.If the temperature is dropped from 100^oC to 0^oC the viscosity of petroleum oil may increase about one hundred folds,whereas,that of silicons oils will increase less than four folds,in the presence of air or oxygen at temperature as high as 300^oC silicon oils remain free from acid formation,oxidation and similar phenomenon,which frequently limit the usefulness of petroleum products and other synthetic organic liquids.

Question: 27

In which way Al becomes water soluble by using NaOH?

Answer: 27

27-55

Aluminium dissolves in sodium hydroxide to form a soluble aluminate,with the evolution of hydrogen.
2Al_(s)+ 2NaOH_(aq)+ 6H₂O(I)---->2NaAI(OH)_4(aq)+ 3H_2(g)

Question: 28

In what respects,carbon behaves differently from other members of group IV-A?

Answer: 28

28-55

Carbon differs from the remaining members of Group IVA in following respects;Carbon and silicon are non-metals while the other members of the family are metalloids or metals.

Question: 29

CO₂is a gas while SiO₂is solid at room temperature.Justify?

Answer: 29

29-55

Silicon atoms are much larger than carbon atoms and thus tend to surround themselves with more oxygen neighbours. Silicon forms only single bonds to oxygen atoms whereas carbon may form double bonds.Carbon,in fact forms double bond to each of the two oxygen atoms to produce a small,symmetrical,linear molecules CO₂,which is volatile and reasonably reactive.

Question: 30

What is asbestos?Give its two uses.

Answer: 30

30-55

Asbestos is hydrated calcium magnesium silicate CaMg₃(SiO₃)₄.It commonly used in making incombustible fabrics and hardboards,etc.

Question: 31

State procedure by which surface of stoneware is made less porous?

Answer: 31

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Stoneware are usually glazed to give it a less porous surface by throwing salt upon the articles while they are hot.This treatment produces sodium aluminate and sodium aluminium silicate,which melt readily and cover the entire surface.When the article cools,the covering solidifies,producing a compact,smooth,waterproof surface.

Question: 32

Write two points to show peculiar behavior of carbon.

Answer: 32

32-55

Carbon differs from remaining members of group IV-A the following respects;

Question: 33

Write four uses of Sodium Silicate?

Answer: 33

33-55

i) It is used as filler for soap in soap industry.

Question: 34

How Aluminium reacts with aqueous sodium hydroxide?

Answer: 34

34-55

Aluminium dissolves in sodium hydroxide to form a soluble aluminate,with the evolution of hydrogen.
2AI_(s)+ 2NaOH_aq)+ 6H₂O_(I)------->2NaAI(OH)_4(aq)+3H₂(g)

Question: 35

Discuss that CO₂is acidic in character.

Answer: 35

35-55

CO₂gas when dissolved in water gives H₂CO₃an acid.The reaction is given below as: CO₂+ H₂O------->H₂CO₃

Question: 36

What is vitreous silica?Give its two uses?

Answer: 36

36-55

When crystalline silica is heated sufficiently it melts to give a viscous liquid having a random structure,presumably with the silicon atoms still on the average closed to four oxygen atoms and the oxygen atoms close to two usually it undercools tremendously and eventually becomes rigid without having undergone orientation into a regular crystal pattern.

Question: 37

Give four common properties of group IVA elements of periodic table.

Answer: 37

37-55

i) All the elements of this group show a valency of four.

Question: 38

How Clay Articles are glazed?

Answer: 38

38-55

Clay articles are glazed by boric acid because borate glazes possesses a higher coefficient of expansion.

Question: 39

How lime and sand are used to make glass?

Answer: 39

39-55

Water glass or soluble glass (Sodium Silicate) is prepared by fusing sodium carbonate(Lime) with pure sand (silica).This process is carried out in a furnace called reverberatory furnace.
Na₂CO₃+ SiO₂------>Na₂SiO₃+ CO₂

Question: 40

Describe composition and used of chemical garden?

Answer: 40

40-55

When crystals of soluble coloured salts like nickle chloride,ferrous sulphate,cooper sulphate or cobalt nitrate,etc,are placed in a solution of sodium silicate,they produced a very beautiful growth,like plant,which is called chemical garden.

Question: 41

What are silicates?Give example

Answer: 41

41-55

Compounds derived from silicic acid (H₂SiO₃), are calledilicates. For sodium silicate (Na₂SiO₃),it is a sodium salt of metasilicic acid.
Na₂CO₃+ SiO₂------->Na₂SiO₃+ CO₂

Question: 42

Write two uses of silicons.

Answer: 42

42-55

i) They are used in hydraulic brakes and other hydraulic system.

Question: 43

Borate glazes are better than silicate glazes.Explain?

Answer: 43

43-55

Boric acid is used in pottery as a glaze because glazes are more fusible than silicate glazes and possess a higher coefficient of expansion.

Question: 44

Which elements and compounds can act as semiconductors?

Answer: 44

44-55

Elements:Semiconductor include the elements germanium,selenium and silicons.

Question: 45

What is meant by chemical garden?

Answer: 45

45-55

When crystals of soluble coloured salts like nickle chloride ferrous sulphate,copper sulphate or cobalt nitrate,etc,are placed in a solution of sodium silicate,they produced a very beautiful growth,like plant,which called chemical garden.

Question: 46

How weathering of potassium feldspar takes place?Give chemical equation also.

Answer: 46

46-55

Many important silicate rocks contain aluminium.The weathering of these rocks results in the disintegration of the complex silicates which they contain.The boiling and freezing of water in the rocks,and the chemical action of water and carbon dioxide convert these compounds into potassium carbonate,sand and clay.The following reaction explain the weathering of potassium feldspar:
K₂O.Al₂O₃.6SiO₂+ H₂CO₃+H₂O------>K₂CO₃+ 4SiO₂+Al₂O₃.(SiO₂)₂.2H₂O

Question: 47

What is effect of temperature on semiconductor?

Answer: 47

47-55

The electrical conductivity of semiconductor depends upon their temperautre. When a metal is heated,its resistance increases,when a semiconductor is heated its resistance decreases and vise versa.

Question: 48

What happens when borax is heated with NH₄CI?

Answer: 48

48-55

WHen borax is heated when ammonium chloride boron nitridride is produced,

Question: 49

Give the formula and use of Talc soap stone?

Answer: 49

49-55

Formula of soap stone: Mg₃H₂(SiO₃)₄

Question: 50

How semiconductors are used in transistors?

Answer: 50

50-55

Semiconductors may joined to other material,which may be a metal or a different semiconductor.The junction between the different materials form a boundary.It allows electricity to pass more properly and is used in transistors.

Question: 51

Write any two properties of boron which show peculiar behavior?

Answer: 51

51-55

1.Boron is only element in Group IIIA which is non metallic in behaviour.

2.It is the only element with less that four electrons in the outermost shell which is not a metal.

Question: 52

Discuss use of PbCrO₄in paints?

Answer: 52

52-55

It is used as a pigment under the name of chrome yellow.Orange or red basic lead chromates are formed when lead chromate is boiled with dilute alkali and are used as pigments.The stable yellow modification of lead chromate is monoclinic Mixture of lead chromate with lead sulphate or barium sulphate are also used as yellow pigments.

Question: 53

White lead is not a good pigment.Give reason?

Answer: 53

53-55

Basic lead carbonate 2PbCO₃.Pb(OH)₂is an amorphous white pigment.White lead is not suitable for use as good pigment since it is darkened by the hydrogen sulphide which is frequently present in the atmosphere.

Question: 54

Boron differs from its family members,Discuss?

Answer: 54

54-55

Boron is only element in group IIIA which is non metallic in behaviour.
Its is the only element with less tha four electron in the outermost shell which is not a metal.
One of the outstanding feature of the chemistry of boron is its ability to form molecular addition compounds.

Question: 55

Give uses of Lead Suboxide?

Answer: 55

55-55

It is back powder,obtained on heating plumbous oxalate in the absence of air.Other than pigments,it is used in the manufacture of lead storage batteries.

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12th Class Chapter 3 Chemistry Short Questions

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