Both sentences are correct. But the nuance is different. 이 음식이 매워요? Is this food hot? The situation : There are two foods. One food is hot and another food is not hot. You want to eat hot food. You can ask to someone in order to confirm that it is hot. 이 음식은 매워요? Is this food hot? The situation : There are some foods. You want to eat a food but you are afraid of eating hot food. You can ask to someone whether it is hot or not. For more information ,refer to http://www.italki.com/question/251654