Denotation refers to the definition of a word, and connotation refers to the way a word is interpreted, or a different and emotional meaning.
The literal meaning of the word “experiment” is the chemistry project—a scientific meaning. They literally mixed chemicals together.
Mixed the perfect chemical combinations
In the proper amount and order.
We thought we were a complete success.
And then the flash of light,
The loud, perplexing explosion…
However, the lab partners are also conducting an experiment. One connotation of the word “experiment” is the relationship the two of them are embarking on. They felt good about the relationship at first, and thought it was going well. They were shocked by the explosion and “slowly retreated from each other” after that. They did not just drop the course, they dropped each other.
The poem plays carefully with multiple meanings and layers of meaning. Although the poem describes an actual chemistry experiment, it also describes an experiment in personal chemistry between two people. When one does not go well, it is seen as symbolic that the other won't. Dropping the class involved dropping each other, and the end to both kinds of chemistry experiments.