Law of conservation of mass question

[land_of_ice]

" In a chemical reaction matter can be neither created nor destroyed "

So does this law only apply to chemical reactions or can matter ever be destroyed or created in any way shape or form?


and do people believe that something that has been burned can be unburned?
It says that when metal is heated it leaves behind calx and that if you want to remove the calx and combine it with charcoal you can then heat the calx will become metal again, this seems untrue, this is in a science book, not sure whether or not they are talking about a theory that existed at one time or if this is believed to be true now a days that something can be un burned ?

 

[Borek]

" In a chemical reaction matter can be neither created nor destroyed "

So does this law only apply to chemical reactions or can matter ever be destroyed or created in any way shape or form?

Mass conservation has been replaced by mass&energy conservation - that is, mass can be converted directly into energy and vice versa. But for most practical purposes mass conservation still holds, it becomes important when you are close to c or when you are talking about nuclear eactions.

and do people believe that something that has been burned can be unburned?
It says that when metal is heated it leaves behind calx and that if you want to remove the calx and combine it with charcoal you can then heat the calx will become metal again, this seems untrue, this is in a science book, not sure whether or not they are talking about a theory that existed at one time or if this is believed to be true now a days that something can be un burned ?

Calx is usually a metal oxide - and it can be decomposed back into metal and oxygen. It doesn't have to be easy, but it is always doable.

 

[Mapes]

It says that when metal is heated it leaves behind calx and that if you want to remove the calx and combine it with charcoal you can then heat the calx will become metal again, this seems untrue, this is in a science book, not sure whether or not they are talking about a theory that existed at one time or if this is believed to be true now a days that something can be un burned ?

This is called reduction (the opposite of oxidation). The heated charcoal produces carbon monoxide, which steals the oxygen from the metal oxide, turning it back into metal again.

It's how people have produced metal from ore for ten thousand years.