Different ways minerals deposit on water bowls

[Stephen Tashi]

TL;DR Summary

Why do minerals from city water deposit as distinct grains on Nordicware brand cake pans that are used as water bowls for cats?

Minerals in my city water deposit as white uneven layers on the metal bowls typically sold for watering dogs and cats. They also deposit that way on the metal bowls that I use for feeding humans. However on my two Nordicware brand cake pans (used as water bowls for cats) minerals deposit as distinct white particles resembling grains of sand. What could explain this difference?

 

[BillTre]

You might want to contact your local water company to find out what is in the water chemically.
They should be able to point you to a recent water analysis.
Presumable you have hard water.

How rapidly does the precipitate show up?
Does this precipitation only happen in metal containers?
A clear container on a dark background and illuminated from the side could show some precipitation.
Do you have particle filters on your water supply line?

 

[Borek]

Matter of presence of nucleation sites on the bowl surface. If there are plenty of them crystals can easily start to grow everywhere on the surface, and cover it with a thin layer. If the surface has no nucleation sites, solution will get to the point where it is concentrated enough precipitate will overcome the nucleation activation energy in the bulk of the solution and will start grow there.

 

[Stephen Tashi]

How rapidly does the precipitate show up?
Does this precipitation only happen in metal containers?
A clear container on a dark background and illuminated from the side could show some precipitation.
Do you have particle filters on your water supply line?

The city does have "hard" water. (2021 Water quality report: https://www.lascruces.gov/ArchiveCenter/ViewFile/Item/1901 )

The precipitation also happens to plastic containers if they are used as water bowls. If they are only used to hold water momentarily (like plates and glasses) they don't get the white deposits. The precipitation on plastic is also in rough layers.

There are no filters on the cold water supply to the house. That's the water used to water pets. The white grains appeared in the Nordware cake pans on first use. They appeared on the bottoms of the pans. By contrast, the white deposit on other metal bowls appears near the waterline.

 

[BillTre]

An uncovered dish of water may on time lose CO₂ by diffusion to the air.
The process could be sped up by aerating the water.
This can change the pH of the water which might lead to precipitation.

You could get a bunch of water and let the precipitate form, decant (pour off and collect) the supernant (the liquid above the precipitate) and use it. No precipitate may form if the precipitating chemical was all used up.

 

[Borek]

The white grains appeared in the Nordware cake pans on first use. They appeared on the bottoms of the pans. By contrast, the white deposit on other metal bowls appears near the waterline.

That's precisely what I would expect looking at the mechanism - layers close to the surface are the first one to get higher concentration/lower temperature/lower concentration of CO₂ (whatever drives the precipitation). If the are nucleation sites on the surface - you will initially get the deposit there. If there are no nucleation sites on the surface, and precipitate appears in the bulk - solid will sink and settle down near the bowl center.

 

[sophiecentaur]

Matter of presence of nucleation sites on the bowl surface. If there are plenty of them crystals can easily start to grow everywhere on the surface, and cover it with a thin layer. If the surface has no nucleation sites, solution will get to the point where it is concentrated enough precipitate will overcome the nucleation activation energy in the bulk of the solution and will start grow there.

Did anyone consider that there may be stuff from the pet's mouth which sticks to the bottom of the bowl and forms nucleation sites quicker than in a bowl which is totally undisturbed. Or it may just be dust, blown along the ground, which is less of an issue for a bowl at table height. My newly washed car roof soon gets traces of dust on it which seems to stay there - how annoying.

Looks like an opportunity for a set of experiments . . . . . .

 

[Borek]

Did anyone consider that there may be stuff from the pet's mouth which sticks to the bottom of the bowl and forms nucleation sites quicker than in a bowl which is totally undisturbed.

Quite possible, still related to the nucleation and nucleation activation energy, so still the same mechanisms at work. My understanding is Stephen was looking for some general scheme of what is happening.