Question

ethene decolourise alk.KMnO4 or ethane?
give reasons

Answer 1

only double or triple bond containing compound decolouries the alkaline KMnO4 so  only ethene can decolouries the KMnO4.
CH2=CH2 + KMnO4 ---------------->  CH2OH-CH2OH  (glycol)
CH3-CH3 + KMnO4 -------------------> no reaction
 

Answer 2

only a double or triple bond containing compound decolourise the alk. KMnO4 so  it means ethene can decolourise the KMnO4.