"Chemistry" can mean mutual attraction between people, or when people interact well with each other. "We have good chemistry" would mean, we get along well, or we like each other a lot. "Bad chemistry" is the opposite, when people have trouble getting along. So "reverse the chemistry" would mean to change a relationship from good to bad or from bad to good. In the song, "Energy", Keri Helson sings, "How do we reverse the chemistry?" At first, the couple got along really well. But now things have turned sour, and she is asking what they can do to change things, so that they will be in love with each other again. The word chemistry usually refers to the science which studies the reactions that happen when different substances are mixed. If you think of people as being the substances, then you could think of how two people interact as being similar to how two substances react - either having a good reaction and creating something wonderful, or having a bad reaction and perhaps exploding. So then you can call the interaction between people their chemistry, and so you get the word chemistry also meaning when people get along well with each other (they have good chemistry).

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