How are physical and chemical changes different? 

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Chemical changes are changes that result in the formation of a new substance. In other words, the composition of a substance changes when the substance undergoes a chemical change. Chemical reactions are chemical changes. Examples of chemical changes (and reactions) are combustion, decomposition, fermentation, and oxidation. Color changes, bubbles, and the formation of a precipitate are often indicators that a chemical change has occurred.

The composition of a substance does not change during physical change, but physical changes may result in a change of the form, appearance, shape, or size of a substance. All phase changes are physical changes. Other examples of physical changes include snapping, smashing, cracking, stretching, cutting, and dissolving.

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Physical changes are those changes in which the phase of the substance may change. This change can be generally observed (especially if it is a phase change). However, there is no change in the composition of the material, that is the starting and ending material is the same. For example, melting of a metal (Say iron or copper) is a physical change, since the state (or phase) has changed. However, the final product is still the same metal (iron or copper, etc.).

In comparison, chemical changes involve a change in the composition of the material. Such changes may be observed through a release of gases, change in color, etc. An example is the rusting of iron. Over time, the pure metal gets oxidized and we see a color change, which is a visual confirmation of oxidation reaction (in this case) and a chemical change.

Hope this helps. 

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