Distributing a chemical that signals the presence of the agent by turning dark, she invites the students to test the liquids now in their test tubes.
(a chemical; signals; the presence of the agent by turning dark) (she; invites; the students; to test the liquids now in their test tubes) (the students; to test; the liquids; T:now; L:in their test tubes)
a chemical she the students
signals invites to test
the presence of the agent by turning dark
the students to test the liquids now in their test tubes
the liquids T:now L:in their test tubes