Hi.. Thanks for the query.. If you had dehydration then white patches on tongue can be due to overgrown or inflamed tongue papillae, fungal overgrowth, accumulation of debris and bacteria over the tongue.. You should first of all take appropriate management for dehydration as it can have deleterious side effects.. Drink plenty of water.. Use a tongue cleaner to clean your tongue.. If the tongue is painful gargle with Magic mouthwash which contains numbing gel containing Lignocaine.. Avoid spicy foods.. Chew sugar free chewing gum and instil artificial saliva drops to reduce dry mouth.. Hope this helps.. Regards..