Dry ice (solid carbon dioxide, CO2) is quite an interesting substance. To remain in its solid form, dry ice must be kept very cold, below about -80°C. Your experience tells you that a solid substance will melt when its temperature is raised. However, dry ice instead changes directly from a solid to a gas in a process called sublimation. Dry ice is used frequently in “fog machines,” where large chunks of dry ice are dropped into water. The rapid warming generates large amounts of gaseous CO2, which immediately sinks because it is more dense than air. In this chapter, you will learn about the kinetic-molecular theory, which is a series of assumptions that provide a general description of the particles of matter when they are in the solid, liquid, or gas states. Along the way, you will discover how these particles behave when they undergo changes from one state of matter to another.
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