What are the evidences that a chemical change has taken place or is taking place?

Question

i need an answer as soon as possible.

THANKS IN ADVANCE! :)

ALSO, can i bring the color of the copper wire back after i heat it in a non luminous flame (blue flame)? of course it has a black substance on it. can i scrape it off to bring it back to its original color?

thanks. please answer this.

Answer 1

Evidence for a chemical change includes color changes, temperature changes, gas production, odor, formation of a precipitate, or more rarely, flames or light emission.  Note that these on their own to not guarantee that a chemical reaction is occuring - except for flames, all can also be produced by physical changes.

The black color of the copper wire is caused by cupric oxide (CuO).  This will form if copper is heated in the presence of air.  You can try to scrape it off, or dissolve it in hydrochloric or another strong acid, but a layer of dull red/brown cuprous oxide (Cu2O) will still be covering the copper (this is difficult to scrape off, but can be done with sandpaper, etc.).

There is an easy way to restore the color, at least temporarily.  Hold the wire in the lower part of the flame.  This part is rich in unburned hydrocarbons, hydrogen, and carbon monoxide.  These substances act as reducing agents, and will reduce the copper oxides back to Cu metal - hence, this part of the flame is known as the reducing part (the oxidizing part of the flame is the upper part - more air is present here, and so if you hold the wire here, it will turn black again).  However, when you attempt to remove the wire from the reducing part of the flame, it will still be hot when it reaches the air outside of the flame, and may oxidize again.