Thank you for you reply Robb, you are always so prompt to answer queries. Thank you!
Further to Robb’s reply. In between the time when I posted the query and Robb replying I found extra information at
http://scifun.chem.wisc.edu/HomeExpts/TARNISH.html
Basically it explains that the tarnish on silver is silver sulfide.
When silver is cleaned with abrasive polishes you remove the silver sulfide compound, which is a physical change.
However when you put the silver, which is tarnished into the salt solution with the aluminium foil, you create an electrochemical cell and the chemical reaction as follows:
3 Ag2S + 2 Al 6 Ag + Al2S3
Hence it is better to clean silver using the latter method as you don’t lose silver, which is especially important if the item is silver coated.
This could be used as an example of the difference between physical and chemical change.
I found this interesting and thought it might be of interest to others too.
Best wishes for 2007
Kind regards
Teresa