I am preparing my dogs food… When the pot has watery broth and big chunks and it’s on the lowest flame, it never actually boils. When the pot is filled with very thick liquid made up of broken down foods and fats I can put it on the lowest flame and it will boil.
Why is that?
The real question is “why are you boiling your dog’s food?”
Two guesses: 1, the thicker liquid has less water and thus less heat capacity, so it takes less energy to bring it to a given temperature. Two, there is less movement cause by convection in the thick liquid so the bottom, hotter, part, is unable to transfer its energy to the top part and so boils quicker even when the total energy is the same.
There can be better heat transfer through the liquid and not the solids that give the impression that it is boiling sooner, but the average temperature of the mix may be no greater. Also currents develop in liguids distributing heat more evenly over time. In thicker substances those currents don’t form as well and the stuff at the bottom of the pot reaches a high temperature and bubbles up, but again the average temperature is still below boiling.
I don’t think the thick liquid is boiling. That is, if you measured the average temp of the liquid, it would be well below the boiling point of water.
Here’s what’s happening: At the bottom of the pan, a small bit of water gets vaporized. It can’t easily mix in with the rest of the liquid (due to the general thickness) so instead it forms a bubble that rises to the surface and pops. When this happens regularly it looks like a boiling liquid, but it’s not.
Pure speculation but based on a lot of time spent putting liquids over burners (professionally!).
ETA: What TriPolar said.
In both pots, the layer of water on the bottom is being heated. In the thin pot, that layer of hot water can easily rise to the top and dump that heat into the air. In the thick pot, the layer of water on the bottom cannot rise as quickly, so it will heat to boiling.
I make my dog’s food from scractch, using cheap meats and veges like yams, potatoes, carrots, etc.
Thanks all.
Most of the explanation was captured above, i.e. stronger convection causes the hot water at the bottom to rise to the top. The one missing point is that in plain water, lots of heat is escaping due to rapid evaporation as the hot (but below boiling point) water reaches the surface.
This process is exactly what boiling is.
Except in the viscous fluid, it’s highly localized. Only the very bottom layer is boiling, and convection isn’t spreading the heat uniformly so steam bubbles form early. That’s the “early boiling” described.