What is a chemical change? and what is a physical change?

In a chemical reaction, there is a change in the composition of the substances in question; in a physical change there is a difference in the appearance, smell, or simple display of a sample of matter without a change in composition.

A chemical change is a change where something new can be formed and normally can not be changed back. Examples of a chemical change are precipitates being formed, gases being formed, quick temperature change rust, bubbling, foaming, burning, etc. A physical change is a change where nothing new is formed and normally can be changed back. For example you painted a wooden block red. If you want, you can always change the color again. Nothing chemically or permanantly happened to it. More examples of a physical change are; color change, change of state, luster, shape, size, hardeness, etc.      

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it means there are 3 atoms of oxygen