Sopa vs. caldo

In Spanish, the words 'sopa' and 'caldo' both relate to what we commonly understand as soup, but they are used differently depending on the context and the type of dish.

Sopa

'Sopa' is a thicker soup that usually contains a variety of ingredients such as vegetables, meats, noodles, or legumes. It is a more substantial dish compared to caldo.
Me encanta la sopa de lentejas que hace mi abuela.
(I love the lentil soup my grandmother makes.)
¿Podrías pasarme un plato de sopa de pollo?
(Could you pass me a bowl of chicken soup?)
La sopa de fideos es perfecta para los días fríos.
(Noodle soup is perfect for cold days.)

Caldo

'Caldo' refers to a clear broth or stock, generally lighter than 'sopa'. It's often used as a base for other dishes or consumed as a light meal by itself.
Voy a hacer caldo de huesos para la cena.
(I'm going to make bone broth for dinner.)
El caldo de pollo es bueno para el resfriado.
(Chicken broth is good for colds.)
Prefiero un caldo ligero después del ejercicio.
(I prefer a light broth after exercise.)

Summary

'Sopa' generally refers to a thicker and more ingredient-rich type of soup, often including solids like vegetables, meats, or noodles. On the other hand, 'caldo' typically denotes a clear broth or stock that serves as either a light standalone meal or a base for other dishes.