"Physical Attraction... Chemical Reaction" is the seventh episode of the fourth season of Grey's Anatomy and the 68th produced episode of the series. It is also the 74th produced episode of the Grey's Anatomy franchise. It was broadcast on November 8, 2007.
George and Izzie's romantic chemistry is put to the test as they try anything to recreate the magic of their first time together while Callie turns to Bailey when the pressures of being Chief Resident begin to build up. In the aftermath of her night with Alex, Meredith and Lexie bond over a patient who struggles to connect with his new adopted daughter resulting in Lexie telling Meredith her "five things" as Mark develops an interest in a reluctant Erica.
- "Before we were doctors we were med students which meant we spent a lot of time studying chemistry. Organic chemistry, biochemistry; we learned it all. But when you're talking about human chemistry only one thing matters. Either you got it or you don't."
- "Chemistry, either you've got it. Or you don't."
- "Name" - Derek Webb
- "Iluvitwhenya" - Northern State
- "San Jose" - Joe Purdy
- "Pouring On The Gasoline" - Calahan
- "Rain" - Priscilla Ahn
This episodes' title references the song Physical Attraction, originally sung by Madonna