last month i have sold my phone to someone. and now i got the information that my whatsapp chat was visible on that phone. Now, is it possible that my current chats on whatsapp(the chats on my new phone) will still be visible on my old phone???
There are other apps which allow users to view Whatsapp conversations without starting the official Whatsapp application. Whatsapp also performs a backup of your conversations and stores it on the SD card (internal or external, depending on device). It's possible that when the new user entered his credentials, the app detected a previous backup and allowed the new user to import the old conversations in.
AFAIK, Whatsapp authenticates the app with the SIM card that is present in the device. So, your new chats should NOT be visible. In any case, prudence dictates that when selling a phone, ALWAYS perform a Factory Reset AND wipe the data off the internal / external SD card(s).