Will Soup Thicken Up As It Cools Down?

You've just finished making a batch of homemade soup, and you're letting it cool down a bit before you eat it. But you might be wondering if the soup will thicken up as it does, because the one you made is a little on the thin side. The answer is (don't hate me) — it depends on what's in it. Recipes that contain starches, like potatoes, beans, flour, rice, or cornstarch, will indeed thicken your soup over time as it cools. Starch molecules will soak up the water and eventually swell up, which will cause your liquid to thicken. Depending on how much starch is present in your soup, your soup will tighten up a bit as it cools off. 

If you've made a soup with pasta in it, like chicken noodle soup, you'll notice your noodles begin to swell and soften quite a bit, and the liquid portion might gain a little bit of extra viscosity. Keep the soup in the fridge overnight, and that pasta is going to get much bigger and softer. Not everyone prefers this, which is why some recipes recommend you make a fresh batch of noodles to stir into your soup just before serving it. If it makes you feel any better, that's a bit tedious for me; I tend to just let the noodles swell up and do their thing. 

If you're concerned your soup is too thin, you do have some last-minute options, like adding Wondra (a pre-gelatinized flour available at the supermarket), or even instant mashed potato flakes. Those should add some body, but just add a little at a time, as it's easy to overdo it.

Soups with a broth base that have had meat simmering in them for long periods of time can contain a lot of natural gelatin, due to the breakdown of collagen from bits like meat scraps and connective tissue. Gelatin gives those types of broths a rich body without being thickened by starches (and it's also the secret ingredient for perfect French onion soup). If you chill that soup in the fridge overnight, you'll find that it'll have solidified completely into a wobbly jelly — that's perfectly normal, and it means you've made a gelatin-rich soup that's rich and hearty. If you heat the gelled soup back up, you'll discover it'll melt back down to the same soup you had before. (A little chef's trick to add body to a store-bought broth is to add a packet of unflavored gelatin, by the way.)

So yes, it's definitely possible your soup will thicken as it cools, it just depends on what you put in it in the first place. Anything starch-heavy will likely set up a bit, and anything with a lot of gelatin will potentially gel up a bit when chilled.