Kitty assumes that he is going to ask her to dance the mazurka, because they always do that, and it's the most important dance of the ball.
Although Vronsky hasn't actually asked her yet, she refuses five other partners for the mazurka.
While dancing the final quadrille (another kind of ballroom dance)with someone else, Kitty happens to glance over and see Anna and Vronsky.
Anna has that happy glow people get when they're around someone they really click with.
Kitty tries to figure out who is responsible for Anna's "I'm-into-you" aura. To her horror, she realizes that it's Vronsky, who is wearing a similar expression, an expression that Kitty has never seen before.
Even though Anna and Vronsky are just making small talk, it's clear that the conversation means a lot to both of them.
Kitty feels nauseated.
The mazurka begins, and now she doesn't have a partner.
Kitty sits down. Countess Nordston comes over and says that he asked her for the mazurka.
Kitty is feeling awful: she just turned down one marriage proposal (Levin) because she trusted that another man (Vronsky) was in love with her. Now it turns out he's not.
Countess Nordston is also at the ball. She sees that something is the matter between Kitty and Vronsky, and so she finds Korsunsky, asking him to dance the mazurka with Kitty. He does invite her, but while she's on the dance floor, she bumps into Vronsky and feels crushed by his lack of interest.
Kitty watches Anna and Vronsky and broods. She thinks to herself that there is something "alien, demonic, and enchanting" (1.23.21) about Anna.
Although Korsunksy, the host, pressures her to stay for supper, Anna refuses, saying that she is leaving tomorrow.