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:
 
http://www.sciencebuddies.org/science-fair-projects/project_ideas.shtml?From=Tab
 
 
Science Bob also has a list of experiments, many with questions that turn a demonstration into an experiment:
 
http://www.sciencebob.com/experiments/index.php
 
More resources for science projects and experiments are listed on LearningReviews.com:

Hi Anaiz,
 
Another resource I'd suggest you peruse is this Education.com info center related to helping children with science fair projects:  http://www.education.com/topic/help-child-with-science-fair-projects/
 
One article in particular I would recommend is: "Understanding the Scientific Method":
 
http://www.education.com/reference/article/understanding-scientific-method/
 
This article gives a step-by-step of the method in a very clear way that will help you and your students in crafting hypothesis questions or statements for science experiments.
 
You'll also find 6th-grade level science activities that include potential hypothesis questions in this section of Education.com:
http://www.education.com/activity/middle-school/science/
 
More ideas are here:
http://www.education.com/topic/great-science-fair-project-ideas/

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.