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The major difference between physical and chemical changes is the change in composition. For a change to be termed physical, there should be no change in composition of constituents. In other words, no elemental changes take place and no new substance is formed. On the other hand, chemical changes are due to formation of new substances. Thus, color and temperature changes are physical changes, while oxidation and hydrolysis are chemical changes.
Odor is generated when substances change composition. A very common example is rotting of eggs, that have a very different odor from fresh eggs. During the process of rotting, chemical reactions take place and egg's composition changes. Another example is that of odor from the sewers. The wastewater flowing in sewers rot, generally anaerobically, and this generates could gases such as hydrogen sulphide, which causes malodor. Fresh sewage, on the other hand, does not have any noticeable odor. Hence, odor is a chemical change.
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