HI Sarah, I created a unit when I was still teaching 8th grade that hits on this topic in a project approach. It can easily be adapted to 6th grade and might give you a "kernel" of an idea to create your own unit. It doesn't include all the properties but you could tweak the unit to include the ones you need. If nothing else, hopefully this unit can serve as an example of taking what might appear to be mundane concepts and make them fun and exciting. You can view the unit, CFQs and download the resources at http://educate.intel.com/en/AssessingProjects/AssessmentPlans/SecondaryAssessmentPlans/AccidentalDiscoveries/
Also, for other project ideas, review our Designing Effective Projects site at http://educate.intel.com/en/ProjectDesign/UnitPlanIndex/GradeIndex/
and Assessment Plans which also have resources and project ideas with a strong emphasis on integrated assessments: http://educate.intel.com/en/AssessingProjects/AssessmentPlans/
Feel free to adapt any of these resources for your classroom.
Intel K-12 Education
I don't have answers to all of your questions, but I will share a couple of links that might be included into the content that you have mentioned.
Solid, Liquid Gas
Meet the Elements
6th graders will enjoy these!
Here are a couple of thoughts for your unit and some resources that may be of assistance.
Essential Question----- Is Change Necessary? (Important) Why Change?
If you have access to Discovery Education, you will find great videos there on matter. There are also ways to set up quizzes and lessons for your students.
If you do not have access to it, there are a number of free video sites that have movie clips for introductions or reviews.
Good luck with your lesson. I would love to see the finished project.