Back on the other manipulation front, Roderigo is getting tired of Iago taking all his money and not delivering the goods (i.e., Desdemona), as promised. Iago tells him to cool his jets, and also to kill Cassio when the opportunity arises, which, according to Iago, will happen that night between midnight and 1:00 AM.
Meanwhile, Desdemona and Emilia are talking together, and Desdemona begins to act strangely, foreshadowing her own death. She sings of it, too. Emilia, meanwhile, defends the act of cheating on one's spouse, especially if there's a good reason for it.