Hot tap water comes out of the water heater, so it has been sitting for a while, thus tasting a bit stale because it’s no longer aerated like out of the tap.
If you have a tank-style water heater, then yes, there are more chemicals (calcium, rust from the tank, etc) in that water, plus nasty things like lead from old pipes, etc. The crud that collects in your hot water kettle is a good sample of what the inside of your hot water heater looks like.
A more energy-efficient way to boil water is to heat a kettle full of cold, frsh water, then pouring that into your pasta pot. The kettle is faster and more efficient than the stove top.
