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What is the colour of the flame when alkenes are burnt?
1. Blue
2. Grey
3. White
4. Yellow

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Correct Answer - Option 4 : Yellow


Sooty flame:

  • The unsaturated hydrocarbons like alkenes, alkynes are aromatic hydrocarbons that partially burned with a yellow sooty flame.
  • It is due to the incomplete combustion of the hydrocarbon in air.
  • A large amount of carbon does not completely oxidize in the air and gives sooty flame.

Non-Sooty flame:

  • The saturated hydrocarbons burn with a Blue colour non-sooty flame.
  • Alkanes are saturated hydrocarbons and classified as a type of aliphatic hydrocarbon.
  • Hexane is a type of Alkanes that burns with a blue non-sooty flame. Hence option 2 is correct.
  • The blue flame emerges when the complete combustion of carbon takes place.

Hydrocarbons are the derivates of the carbon and hydrogen compounds.  In organic chemistry, Hydrocarbons are classified into three:- 

  1. Alkanes
  2. Alkenes
  3. Alkynes

1. Alkanes: These are represented by Single Bond between two carbons. 

  • The chemical formula CnH2n+2 
  • the first member of the alkane group  - Methane (CH4) 
  • Symbolic representation of alkanes is  —C—C—
  • Methane is colorless, odorless, flammable gas. it is used primarily as fuel to make heat and light. it is used in the manufacturing of organic chemicals.

2. Alkenes: These are represented by Double Bond between two carbons 

  • The chemical formula CnH2n 
  • The first member of the alkene group  - Ethene (C2H4
  • Symbolic Representation of alkenes is —C=C—
  • Ethene is used in the production of ethylene glycol ( 1,2-ethanediol ) an automotive anti-freezing agent, used in the production of polymers such as polythene, polyvinyl chloride, polystyrene.

3. Alkynes: These are represented by a Triple Bond between two carbons

  • The chemical formula CnH2n-2 
  • The first member of the alkyne group - Ethyne (C2H2) 
  • Symbolic representation of alkynes is —C=C—
  • Ethyne is also called Acetylene which is used in portable lighting, welding, and cutting, production of Chemicals.

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