Toilets, Sinks, Showers, Tubs and Disposals - White residue collecting only at certain taps

This is a mother-in-law question: I'm describing her description of the problem, so hopefully nothing will be lost in translation?

M-I-L claims white residue is collecting in the aerators in the master bath sink and hallway sink: not in the kitchen sink, not at the shower head. I did a search and asked her to collect some of the residue and add it to a bowl of white vinegar to see if it would dissolve- it did not.

From the reading I did, the vinegar test determines if the white residue is lime / calcium / scale from the water or disintegrated dip tube from the hot water heater.

Why would the residue collect only at certain taps? And is there a way to test the residue to see if it is material from the dip tube- for instance, will it melt in gasoline, as other plastics will?

Thanks,
Dave O

Is it possible that the sinks without the residue have larger aerators or perhaps no aerator to collect the residue? Maybe they just allow the water to flow with less restrictions so that any residue is just washed out?

How about a theory that some sinks are more likely to use more hot water than others and therfore may exhibit more residue than other sinks.

Sounds like the dip tube to me.

'"Residue" is sort of one of those ill-defined terms. I am not really thinking of small particles when I hear the term residue. I think more of a "film". Perhaps a slime. Water pipes can collect or build slimey stuff in them. Did you rub the stuff in your fingers to see if it has any grittyness to it?, or is it just slimey? Can you observe if the stuff will float? Let some dry out, and then try to identify further by texture, floatability, perhaps it's reaction to static electricity (the comb test...see if the stuff leaps up to the comb).

I guess our concern is if the dip tube has disintegrated. If it's not, then the residue could be anything. So a person then has to ask what the symptom is of the operation of the water heater if it loses it's dip tube. If it failed, cold water entering the water heater would not just go to the near-bottom of the tank, as it should. It could instead mix with the hotter water up top up by the upper element and by the hot water discharge, perhaps causing cold water to enter and mix with the hot water pipe's water that goes to your fixtures.

It be interesting if someone here really familiar with dip tube failure could get into the specifics so everyone knows symptoms indicitive of such failure. This must wreak some kind of havoc on when the thermostats send power to the elements, one would think. I'm really not positive, and like my ramblings just above, I would be theorizing.