Question in the title.
Tiny air bubbles. Or perhaps that’s not water and that’s not a tap.
Do you live in Flint, MI?
Tripolar got it. Lots of tiny air bubbles.
Your openness to and creativity of possible solutions is truly impressive.
Atmospheric gasses are less soluble in hot water than in cold water. When the water heater heats the water, these gasses would normally come out of solution, but cannot because of the water pressure. When you turn on the tap, the pressure on the water goes from about 60 PSIG to zero, so these gasses do come out of solution. So, as TriPolar says, you see a lot of tiny, tiny bubbles, which looks like it’s white. If you let it sit for a minute, they rise to the top and the water is clear.
This seems to be a phenomena in only some homes, usually older ones. My house is 60 years old but has mostly brand new plumbing and water heater. Hot water comes out of the tap crystal clear. I always think of this as Grandmas’ hot water as I first noticed this as a child at her home. I also find that cloudy hot water has a certain mild odour to it, not bad just different.
I have assumed it is to do with sedimentation in the old hot water tanks - providing nucleation sites perhaps?
Or you simply have an aerator and it is working as intended and will clear up if you unscrew it. But don’t stick your hands under there right away - sometimes crud builds up behind it.
“As I bit into the nectarine, it had a crisp juiciness about it that was very pleasurable - until I realized it wasn’t a nectarine at all, but A HUMAN HEAD!”
What happens if you run that hot water slowly and gently down the side near the rim rather than letting it splash in the center of the water that’s already in the glass? Is it still bubbly?
In my apt I find it does that periodicaly, more so after the water has been shut off for some reason.
Just speculation, but it might be pure chemistry. In my town half of the water comes directly from the carstic plateau, so it chalky white (and destroys washing machine in a half of its lifetime). Other half of the town gets water from the round rock river undergrounds. It is crystal clear.