Portal Part 3 - ( Chapter 10 to 11 )
(Test Chamber 18) This is often considered the hardest chamber. Turrets reappear here. At the end, GLaDOS says that "the next test requires exposure to uninsulated electrical parts, that may be dangerous under certain conditions," which comes into importance later.
(Test Chamber 19) The last chamber. A simple energy ball trick is required, followed by a time-limited sequence on a moving platform. As the platform rounds a corner, awaiting the player is not the long-awaited cake but a large incinerator. Chell escapes via portals, much to GLaDOS' apparent displeasure; she briefly babbles electronically. She stutters on her "I," revealing her true nature, before regaining control and attempting, unsuccessfully, to convince Chell that this was "the final test, where we pretended we were going to murder you." Chell escapes into the bowels of the Enrichment Center.