The difference between whose and who’s
TLDR: Whose is reserved for the possessive meaning ‘belonging to which person’. Who’s is a contraction.
These two words, like with its and it’s, is an example of an exception to the apostrophe rule when using possessives. Just as its is the possessive form of it, so is whose the possessive form of who.
whose (possessive)—belonging to which person
‘Whose lunch box is this?’ the teacher asked.
The children scratched their heads, wondering whose it was.
Peter Johnson was a very important businessman whose particular attention Charlotte did not readily favour.
who’s (contraction)—who is, who has
‘Who’s left their toys lying all over the floor?’ Mother asked wearily.
‘Who’s that strange man?’
My boss, who’s never around, nevertheless makes his presence felt.
Unlike other examples of a possessive pronoun (e.g. John’s), whose does not require an apostrophe, just like his or hers. An easy way to distinguish which one is the best to use is to remember that who’s is a contraction. If expanding who’s back into who is or who has leaves your sentence sounding a bit like this, ‘Who is lunch box is this?’ or ‘The children scratched their heads, wondering who has it was’, then it becomes blindingly obvious that who’s was not the correct word to use!