There is a small difference between these two sentences: 食べ物は冷蔵庫の中にありますか？ 冷蔵庫の中に食べ物がありますか？ The first one is asking about the location of some specific food. "Hey, you know that food? Is it in the fridge?" Because the topic, 食べ物 is marked with は, it suggests that you should already know what food we are talking about. Where as the second is asking if there is food in the fridge. That's why the subject 食べ物 is marked with が. So, if you want to ask "Is there food/drink in the fridge?" you would say: 冷蔵庫の中に食べ物/飲み物がありますか？