Hello there. Considering the water to be pure, it's having a exact ph7 i.e. neither basic not acidic so it is tasteless. But mineral water has sweet taste that is all because of the mineral present inside the water itself. Depending on the minerals present, the water may taste different. It is commonly seen that water has different taste depending on various areas. Somewhere it is sweet, somewhere it's bitter and so on. So that is all because of the composition of minerals that are present in that area water. 

Another thing is the tap water. The tap water usually have a different taste from that from r.o.Why? It's all because tap water is usually the water that comes directly from underground due to which sand particles get mixed in water which changes the taste of water and when same water goes in r.o., r o. filter out those particles making it taste different.