Water has a pH of 7. If we put acid into it, the solution becomes acidic. Similarly, if we put base into it, the solution becomes basic. For example lime + water tastes a bit sour and bitter gourd + water (what I mean is juice) tastes bitter.
Your body has no taste or smell receptors for water. In order to smell/taste something, that something needs to bind to and activate receptors expressed on the surface of your olfactory receptor neurons or taste buds. When those receptors get activated, you experience the taste/smell.
It makes sense that water is tasteless on tongue because our body is absolutely packed with water, and any receptors for water would always be maximally activated.
But there is a modern concept named "water taste".The idea is that if the tongue is exposed to a particular taste stimulus at a constant concentration for an extended period of time - say, a minute or more - it will experience a short-term adaptation to this stimulus. While this adaptation is in effect, one of three things can occur:
- If the tongue continues to be exposed to the same stimulus at the same concentration, then it does not detect anything.
Ex.: The tongue is continuously exposed to a solution of 0.5% table salt for 1 minute. Shortly thereafter, it is again exposed to a solution of 0.5% table salt. The person tastes nothing.
- If the tongue is exposed to the same stimulus but at a higher concentration, this stimulus is perceived as more intense than normal.
Ex.: The tongue is continuously exposed to a solution of 0.5% table salt for 1 minute. Shortly thereafter, it is exposed to a solution of 2% table salt. This tastes more intensely salty than the same 2% solution would have tasted prior to adaptation.
- If the tongue is exposed to the same stimulus but at a lower concentration, this stimulus is often perceived as being a different taste entirely.
Ex.: The tongue is continuously exposed to a solution of 0.5% table salt for 1 minute. Shortly thereafter, it is exposed to a solution of 0.1% table salt. Rather than being perceived as salty, in the majority of cases, it is perceived as bitter.
Also, Saliva is mostly water, but it also contains low concentrations of various minerals and enzymes that are unique to each person, including a very low concentration of salt. It doesn't taste salty to you because you have adapted to it. When you drink very pure water, your saliva is temporarily diluted, which is similar to situation #3 above.
This is why some people identify pure water as tasting bitter or otherwise unpleasant. They are likely responding positively to water with a mineral composition that is similar enough to their saliva so as to not create the perception of bitterness. It's even possible that they have found a bottled water that they perceive as sweet, since bitter- or sour-tasting components at concentrations below the adaptation level can be perceived as sweet in the same way that salt can be perceived as bitter. In other words, anything naturally in your saliva with a bitter or sour taste could potentially be perceived as sweet at a more dilute concentration.B