Activate text to speech I want to activate text to speech in Google hangouts in my nexus 5 phone so that messages can be read aloud . I couldn't figure it out. Please guide me. 
Can this service be used in Gmail and whatsapp. If yes then how
It sounds like you talking about TalkBack, the accessibility program that comes with a lot of Android devices.

In order to activate TalkBack (assuming it is installed), open your phone settings, then go to the Accessibility section. Select TalkBack and enable it (and accept the warning that TalkBack will be able to see sensitive information).

After it is enabled, you should look through the settings for TalkBack and configure it to your needs.