There are a few posts regarding when to add oil to a pan, but I was wondering if the same applied to making soups with a pot.
When making a fresh pot of soup, would I heat the pot first (using the water-drop test), then add, say vegetable oil, then the ingredients? Or does it not matter?
On the other hand, if I'm reheating a soup that I've stored in the fridge, is it okay to heat it with the pot or should I heat it and then add the soup?
First off, when reheating soup, it won't matter much. If you get the pan too hot before you add the soup, you could get some spattering, but that's about it.
As for making a fresh batch, there are those who believe that browning the ingredients before adding the liquids will develop more flavor. This will be a lot easier to do with a hot pan, than trying to get a cold pan full of stuff up to a heat sufficient to cause browning.
Overall, it's a tradeoff. If you're in a hurry, it might be worth saving the few minutes to just throw everything in the pan and heat it up. If you really want to develop every last bit of flavor, you'll probably be better off heating your pan, browning ingredients, then adding liquid.
There is no difference for sweating, frying, or sauteeing application. In some sense, the words pots and pans are interchangable.
As far as reheating soup, you have to consider the properties of heat transfer. The burner or flame transfers heat to the metal pot; the pot conducts heat into the soup. The liquid in the soup then convects and spreads the heat throughout the body of the liquid.
If you start reheating with a cold pot, then everything happens at a reasonable pace, and the convection will prevent the soup from overheating locally at the bottom surface and burning.
If you preheat the pot—at least with a very strong heat so that it is very, very hot—when you add the soup, it could flash into evaporation and burn locally. This does no good for your product, although it isn't terribly likely.
In general, however, in reheating, you don't preheat the pot or pan.
