‘Rare’ means unique. It indicates that the frequency of something happening or existing is less.
Moreover, all scarce items are not rare! At times, non-rare or common items can be scarce too.
Words like ‘scarce’ and ‘rare’ are used quite interchangeably in general communication. While saying something like ‘Time is a ______ commodity for some’, we usually do not worry whether to choose ‘rare’ or ‘scarce’. We just pick one and write the sentence without putting much thought into it. In most cases, the listener gets the meaning without any trouble. However, can we take the same liberty in academic writing?
Technically, no!
Both ‘rare’ and ‘scarce’ are adjectives that mean ‘something is less or hard to come by’. But they have a subtle difference in their meanings.