12th Class Chemistry Short Questions Preparation for Pakistani Students

Question # 1

Why d- block and f-block elements are called transition elements?

Ans 1: d-block and f-block elements are called transition elements because they are located between the s and p block elements and their properties are in transition between the metallic elements of the s-block and nonmetallic of the p block.

Question # 2

Mention two major problems that may arise in Nelson's cell.

Ans 1: i) Chlorine produced can react with hydroxide ions in cold giving hypochlorite ions.
ii) Hydroxide ions may be attracted toward anode.where they can be discharged releasing oxygen gas.This oxygen gas may contaminate the chlorine and renders it impure.

Question # 3

What are fused rings aromatic compounds?

Ans 1: The aromatic compounds in which two or more than two benzene rings are fused together are called fused aromatic compounds.

Question # 4

How the entrapped gases are removed form the steel?

Ans 1: In order to remove entrapped bubbles of gases such,as O₂N₂and Co₂a little aluminum or ferro silicon is also added,Aluminium removes nitrogen as nitrate.

Question # 5

Explain acidic and basic character of amino acid?

Ans 1: On the basic of dipolar ion structure,the acidic and basic reaction of amino aids may be represented :
1.When an acid is added to an amino acid the carboxylic acid the carboxylate ion accepts the proton,therefore the baci character is due to this group,
2.When an alkali is added to an amino acid,-NH₂ group releases the proton and therefore the acidic character is due to the group.

Question # 6

Write the formula of Borax and Chile Saltpeter.

Ans 1: Borax: Na₂B₄. 10H₂O
Chile Saltpeter : NanO₃

Question # 7

Boron differs from its family members,Discuss?

Ans 1:
1. Boron is only element in group IIIA which is non metallic in behaviour.
2. Its is the only element with less tha four electron in the outermost shell which is not a metal.
3. One of the outstanding feature of the chemistry of boron is its ability to form molecular addition compounds.

Question # 8

Discuss use of PbCrO₄in paints?

Ans 1: 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 # 9

How lime and sand are used to make glass?

Ans 1: 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 # 10

Write two use of bleaching powder?

Ans 1: Bleaching powder is used :
1. for the laboratory preparation of chlorine and oxygen.
2. for making unshrinkable wool.

Question # 11

Which products are formed by the catalytic reduction of aldehydes?

Ans 1: Aldehyde and ketones on reduction with hydrogen in the presence of a metal catalyst like Pd,Pt or Ni form primary and secondary alcohols respectively.Hydrogen is added across the carbonyl group.

Question # 12

What is the difference between aldhydes an ketones?

Ans 1: Aldehydes: In aldehydes,the cabonyl group is bonded to at leat one hydrogen atom,and so occurs at the end or a chain,An aldehyde can be represented by the general formula.
Ans 2: Ketones: In ketones ,the carbonyl group is bonded to two carbons atoms and so it occurs within a chain,A ketone may be represented by the general formula.

Question # 13

Define substitutional alloy?Give an example.

Ans 1: Due to the similarity in the sizes of transition metals,some transition metals atom are able to replace one another in the metallic lattice and from substitutional alloy among themselves,Alloy of steels are an important example of this of material in which iron atoms are substituted by chromium,manganese and nickel atom.

Question # 14

Write down four major component of paper machine?

Ans 1:
1. Flow spreader
2. Head box
3. press section
4. dryer section.

Question # 15

Write the name of those weal oxidizing agents which can oxidize aldehyde but not the ketone?

Ans 1: Tollen's reagent ,Fehling solution and Bendict's solution.

Question # 16

Write two uses of halogen?

Ans 1:
1. Fluorine is used for the preparation of freons,which is being used in refrigerator and aerosol propellants.
2. Chlorine is used in the manufacturing of bleaching powder.

Question # 17

What is asbestos?Give its two uses.

Ans 1: Asbestos is hydrated calcium magnesium silicate CaMg₃(SiO₃)₄.It commonly used in making incombustible fabrics and hardboards,etc.

Question # 18

Define Nucleophile.

Ans 1: Nucleophile means nucleus loving ,It has an unshared electron paired available for bonding and in most cases it is basic in character,It may have negatively charged or neutral.

Question # 19

What are the commercial use of halogens and their compounds?

Ans 1:
1. Fluorine is used for the preparation of ferons,Which is being used in refrigerants and aerosol propellants.
2. Fluorine is used to prepare Teflon.
3. Iodine is used in pharmaceutical industries as disinfectants and germicides.

Question # 20

How PVC is prepared?

Ans 1: It is an addition polymer obtained by polymerizing vinyl chloride at 52 Degree and 9 atmospheric pressure.

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