How do we differentiate #"chemical change"# from #"physical change"#?

#"Chemical change is characterized by the....."#
#"formation of new substances and the making and breaking"#
#"of strong chemical bonds."#

And this is the definition that would be used by a university graduate AND an high school student....

And here is a clear example of chemical change....

#H_2(g)+1/2O_2(g) rarr H_2O(l)#

A new substance, water, is formed by breaking STRONG #H-H# and #O-O# bonds, and forming stronger #H-O# bonds....

#"Physical change is LARGELY restricted to....."#
#"changes of state, i.e. solid to liquid, liquid to gas. etc."#
#"in which the physical state of the material is CHANGED,"#
#"but chemical bonds are neither broken nor formed."#

And a clear example of physical change....

#H_2O(g) rarr H_2O(l) rarr H_2O(s)#

The state of the water is changed, steam, to liquid, to ice....but no #H-O# bonds have been broken....Both chemical and physical changes can be reversible....

Basically I am not the biggest chem wiz out there. But I recall these definitions, the easiest way to think of it is like this.

A chemical change happens on a much smaller scale and is irreversible.

One example of chemical change is burning wood. When you light up a campfire, the wood burns. Most of the wood turn black or into ashes and cannot be used anymore. You cannot turn burnt wood back into normal healthy wood.

A lot of chemical changes show signs such as colour change or change of smell but most the time it happens in steps and you won't recognise it because it happens on such a small scale

A physical change is much easier to understand, imagine crushing a can of soda. All you did was alter the shape of the can. Or melting an ice cube, you can just freeze it back into place.

Hope this helped, I study mechanics so chemistry is not my thing :)