I find that people generally distinguish between love and infatuation. Is there a physiological difference between the two? Or are they biologically identical, and the difference is circumstantial?
Assuming that our emotions are the psychological manifestations of chemical changes in our bodies (an assumption I have no trouble with but one that others may take umbrage at), any emotion will be different.
If your brain can distinguish between two emotions, then the underlying chemistry through which these emotions are expressed will be different.
This is not to say that we, as scientists, can quantify this. Definitely not today at any rate. Simply that there will almost certainly be a physiological difference between any emotional state that can be differentiated by your brain. Otherwise the brain would not register different emotions.
Human language has different words for different feelings. There is often a type of feeling "continuum" with different words representing different ranges on this scale. For example:
How you decide to qualify an emotion you are feeling is more a question of semantics than biology. The point I am making here is that each point on that scale (and every point in between that we haven't explicitly assigned a word to) will represent a slightly different mix of various chemical signals in your body.
If your question is more about the English language words love and infatuation, their definition and distinguishing between them, try posting on English.SE.
Using the Triarchic Theory of Love proposed by Robert Sternberg, love is divided into three components:
Using the theory's particular Infatuation is just the passion portion of love which is lacking in intimacy and commitment required for Consummate Love or True Love.
Physiologically speaking, this is when a certain sense of dependency comes in and as we know from years of addiction research, there is a physiological difference between liking sex physically and liking sex with a particular person for non-physical reasons.