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Are changes in state "physical" or "chemical" changes for molecular solids, and why?

Changes in state are physical changes. Let's take water as an example. H2O. In the gaseous state, we sometimes call it steam or water vapor; it's chemical formula is still H2O. In the liquid state and in the solid state (ice), the chemical formula is H2O.

In a physical change, the original substance is still there - it may be in a different state or shape, but there are no new substances formed. For a chemical change to occur, new products with new chemical formulas must be produced. If sodium is added to water, a chemical change occurs because hydrogen gas and sodium hydroxide are produced. The atoms of the reactants have been rearranged to form new products.

When CO2 is a solid, we call it dry ice. If left out at room temperature, the CO2 will sublime. It will go directly from a solid to a gas. The chemical formula is still CO2; thus a physical change.

Very easy explanation with additional info here http://www.chem4kids.com/files/matter_chemphys.html

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