12th Class Chemistry Short Questions Preparation for Pakistani Students

Question # 1

Define two reaction which confirm presence of three double bonds in benzene ring.

Ans 1: 1.Benzene is reduced to cyclohexane on heating at high temperature which hydrogen in the presence of Pt in an acidic solvent or Ni at 200 degree as a catalyst.
2.Benzene reacts with chlorine and bromine in the presence of sunlight to give addition products ,hexachlorobenzene or hexabromobenzene.

Question # 2

What is effect of aeration on quality of raw water?

Ans 1: The quality of raw water is improved by aeration.In this process air is passed through water to remove the dissolved gasses such as foul smelling,organo-sulphur compound and volatile organic compound,Some of the organic material in the raw water which could be easily oxidized with air produce in the aeration process,.

Question # 3

Picric acid is phenol which behaves like an acid .Justify.

Ans 1: Picric acid is 2,4,6 trintophenol,It is actually phenol but has three nitro groups attached to the benzene ring of phenol,Nitro groups are electrons with drawing in nature,Nitro groups attracts electron to themselves and makes easy for phenolic part to donate easily,so picric acid can donate a proton and behave like an acid .

Question # 4

How bleaching powder is prepared by Hasenclever method?

Ans 1: The apparatus used in this method consists of 4 to 8 iron cylinders placed one above the other horizontal ,They are interconnected and provided with stirrers,The slaked lime is added in through a hopper in the upper cylinder and transported from one cylinder rises p and reacts with slacked lime form bleaching powder ,which is collected through the outlet in the lower cylinder.

Question # 5

What is smog?What are the content of photochemical smog?

Ans 1: The word smog is combination of smoke and fog,Photochemical smog consists of higher concentration of oxidants like ozone and is also termed as oxidizing smog,The main reactants of photochemical smog are nitric oxide NO and unburnt hydrocarbons,Nitric acid is oxidizing to nitrogen dioxide within minutes to hours depending upon the concentration of pollutants gas.

Question # 6

Define cement.Give its essential components.

Ans 1: Cement is the material obtained by burning an intimate mixture of calcarious and argillaceous material at sufficiently high temperature to produce clinkers.These clinker are the ground to a fine powder.'
Essential components of cements:
1.Calcarious material as source of CaO.
2.Argillaceous material they provide acidic components such as aluminates and silicates.
3.Gypsum

Question # 7

What is Halothane ?Write its formula.

Ans 1: Definition: Chemical compounds of halogens with ethylene is called halothane.
Formula: Fluorine is used to firm Teflon(-CF₂-CF₂-)_n.

Question # 8

Write down the main factors on which reactivity of Alkyl halides depends.

Ans 1: There are two main factors which govern the reactivity of R-X bond.
C-X bonds: Strength of bond in compound would be the most reactive one while fluro compound will be the least reactive.
C-X bond polarity:The greatest electronegativity difference between carbon and fluorine atoms in alkyl fluorides,If an electrophile is the attaching reagent then this difference suggest that alkyl fluorides would be the most reactive alkyl halides.

Question # 9

How Portland cement is made?Why Gypsum is added in the cements?

Ans 1: Portland cement:Portland cement is made by strongly heating a finely powdered mixture of clay and limestone.The final product is known as clinker,is cooled and then ground into a very fine powder.
Ans 2: Addition of gypsum:During the grinding there is added about 2% of gypsum which prevents the cement from hardening too rapidly.The addition of gypsum increases the setting time of cement.

Question # 10

1-Butyne does not show geometrical isomerism but 2-Butene does .Give reasons?

Ans 1: The necessary and sufficient conditions for a compound to exhinit geomatric isomersim is that the two groups attached to the same carbon must be different,
In 1-Butene similar hydrogen atom are attached to the same carbon atom ,so it does not exhibit geometric isomerism.
But 2-Butene can exist in the form of cls and trans isomers.

Question # 11

Water has higher boiling point that ethanol.Explain.

Ans 1: Melting and boiling point of alcohol are higher than corresponding alkanes,Methyl alcohol and ethyl alcohol are liquids while methane and ethane are gases,This is also due to hydrogen bonding which is present in alcohols but absent in alkanes.

Question # 12

What is difference between clinker and cement?

Ans 1: Clinker : The resulting product obtained from the kiln is known as cement clinker,This has the appearance of greenish black or grey coloured balls varying in size from small nuts to paes.
Cement: Cement is the material obtained by burning an intimate mixture of calcareous and argillaceous material at sufficiently high temperature to produce clinker.

Question # 13

What id Ninhydrin Test?

Ans 1: Ninhydrin reacts with amino acids to form an intensely coloured bluish violet product,The ninhydrin reaction is also widely used to visualize amino acids separately by paper chromatography.

Question # 14

What happens when the hydrolysis of triglyceride is carried out?

Ans 1: Triesters are easily hydrolysed by enzymes called lipases to fatty acids and glycerol.

Question # 15

How Nylon 6,6 can be obtained?

Ans 1: Nylon 6,6 is obtained by heating adipic acid with hexamethylenediamine.Nylon 6,6 derives its name from its starting materials adipic acid and hexamethylenediamine,both of which have six carbons atoms.

Question # 16

What are elimination reaction?

Ans 1: When nucleophile attacks on hydrogen atom attached to the b carbon of the alkyl halide,we get an alkene such type of reaction are called elimination reaction.

Question # 17

Why the ionization energies decreases down the groups?

Ans 1: Going down in the group,the nuclear charge increases but as the size of the atom and the number of electrons causing the shielding effect also increases therefore ionization energy decreases from top to bottom.

Question # 18

What are Chelate and what they contain?

Ans 1: Chelate: When all the donor atoms of a polydentate ligand get coordinated with the same metal ion,a complex is formed which contain one or more rings in its structure and hence is called Chelate.

Question # 19

What is the effect of Acid rain on earth?

Ans 1: Acidification of the soil and rocks can leach metals like aluminium ,mercury ,lead and calcium and discharges them into water bodies.These heavy metals are accumulated in the fished and are health hazards for human and birds they eat these fishes,The elevated concentration of aluminium is harmful for fished as it clogs the gills this causing suffocation,Acidification of the soil can also each nutrient thus damaging leaves and plants and growth of forest.

Question # 20

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.