Tomato sauce is mainly made up of tomato puree and spices while condensed tomato soup is made of tomato puree, flour, and sugar. Tomato soup is sweeter than tomato sauce and also has a thinner texture. If you're going to substitute tomato soup for tomato sauce, you'll need to remove 1/4 cup of liquid from the ingredients for every 10 ounce can used. And don't forget to take into account the sweet flavor of the soup.