So far, much of our study of the chemical world has focused on investigating elements, compounds, and reactions. We have assessed the characteristics of elements and studied how they combine through chemical reactions to produce new compounds. Now, we are going to shift our attention away from compounds and reactions and look at what happens to matter when it changes state. We are going to study why substances exist as solids, liquids, or gases. We are also going to look at what takes place on the molecular level when matter changes state from one form to another. As you might already imagine, some substances exist as solids at room temperature, others exist as liquids, and still others exist as gases. Life as we know it would be much different, for instance, if water was solid at room temperature, or if oxygen and carbon dioxide were liquid. This chapter will help us understand the different states of matter.