Class is related to Chemistry. Chemistry is a different concept from Physics. 
Grocery is etymologically related to Gross. Grocery is related to Market. Market is a synonym of Fair. Fair is the opposite of Cheat. 
Class <--RelatedTo--> Chemistry <--DistinctFrom--> Physics 
Gross <--EtymologicallyRelatedTo-- Grocery <--RelatedTo--> Market <--Synonym--> Fair <--Antonym--> Cheat 