My experience is that the types of questions you ask students about an experiment are very dependent on the experiment. Usually, I've asked them what would happen if they changed one of the variables in the experiment. They hypothesize, then test their hypothesis by changing the variable (e.g, expose a plant to 8 hours of sun a day vs. 2, use oil instead of water with food coloring mixture, etc.).
Science Buddies has a catalog of science experiments. Many of the experiments are very detailed and include a list of questions that kids can answer as they research and conduct an experiment. You may find some ideas there. They also have a teacher resource section that may help you:
I am not a teacher but a mom and I find that it is easiest is to find the experiment then ask the questions. When my kids and I are trying to figure out what they will do for their science experiment, we find the experiment first then ask what does this prove.
I ask my students to think of things they are interested in already. Then they try to find something they observe and question. It seems like they have more fun than copying something somebody else did. This also has the advantage that the students are thinking through the scientific method for themselves instead of copying another person's reasoning.
I have had a 9th grade hockey player test if the sides of the rink give the angles geometry predicts before and after a game. A fourth grader tested her horse to determine which part of its anatomy grew the most bacteria. One of my 6th grade students poked holes in different outdoor hoses to determine if the distance between the holes affected the height of the water more than the width affected the height of the water. All the students had a blast and had to think fairly hard about how to control the variables in order to get an accurate conclusion.
Well one scientific question I made up was " what happens if you plant three plants and you put one in hot air and one in cold and one in regular air temperature, what would be the results of the plants after the experiment?
My 8 year old daughter borrowed this book from the school library and my kitchen hasn't been the same ever since. She has turned into a regular scientist. This book is FULL of real life pictures and NOT drawings. It is a large size book so it is even easier for her to read. She made a thermometer with a clear shampoo bottle, water, food coloring, and a straw. When she witnessed the water rising and falling through the staw she started screaming,"I'm a genius, I'm a genius." This book is very kid friendly and shows how to do experiments step by step with a real life picture for each step. It even shows how to make a real camera with a paper towel tube, tussue box and magnifying glass. One of my favorite experiments is how to make your own flashlight with batteries, an empty bottle, aluminum foil, and a small bulb.