If soup was served at a meal on yom tov and there is leftover soup in the pot, can one freeze the soup after the meal so it stays fresh for the next time it will be eaten? The purpose of freezing is not necessarily to prepare for a future day but rather just to prevent the soup from spoiling if left in the fridge for days.
In general, it is permitted to perform actions that prevent food from spoiling on Yom Tov. For instance, it is permitted to place foods in the fridge to ensure that they should not spoil (some permit this for other reasons–because placing the soup in the fridge is part of clearing it away). However, Ashkenazim may not place soup in the freezer on Yom Tov, because this would involve changing its state from liquid to solid, which the Rema prohibits. Instead, the soup can be kept in the fridge until after Yom Tov, and then frozen.
For Sephardim it would be permitted, though there may still be room to question whether the act of freezing is considered an act of preservation or preparation, and this would depend on how urgent the need for freezing. If it can wait until after Yom Tom, the act might be interpreted as preparation (hachanah) rather than preservation.
Sources: See Shulchan Aruch, Orach Chaim 320:8 and 318:16 (permits nolad), Rema 318:16 (prohibits nolad), and Mishnah Berurah 320:35. See also Shemiras Shabbos Kehilchasah chap. 10, no. 4-5, and notes 14-15. Concerning preventing spoilage on Shabbos see Machazeh Eliyahu no. 41, and no. 42:3.