Gem dihalide When the two halogen atoms are attached to the same carbon atom. Vicinal dihalide When the two halogen atoms are on adjacent carbon atoms.
Gem dihalide or vicinal dihalide gives alkyne on treating with alcoholic KOH
Gem dihalide on hydrolysis with (aq.) KOH can produce aldehydes and ketones.
While vinical dihalide gives alcohol with (aq.) KOH
Therefore, gem and vicinal dihalide can be distinguished by(aq.) KOH