in a situation where a jewelery piece is a combination of sixty per cent copper 
and forty per cent zinc.
the piece will be magnetic ? yes or no
and if a mild acid solution usually used for testing for gold content
was accidently placed on this piece, it would dissolve the metal .
true ?

Hello poetic,

To answer your questions:
1) The piece is not magnetic. A metal alloy of 60% copper and 40% zinc
is brass, and brass is not magnetic.

2) Under a chemical activity series, it is known that copper will not
react with acids, but zinc will. Therefore, when exposed to any acid,
the zinc atoms in the metal alloy will dissolve in the acid to form a
zinc salt (such as zinc chloride, zinc carbonate, etc.) In other
words, the acid will dissolve the zinc in your jewellery piece and
ruin it.

A company that manufactures brass, National Bronze and Metals, has a
page ( http://www.nbm-houston.com/metals/brass260.html ) about how a
specific type of brass, Cartridge Brass, is vulnerable to all sorts of
acids like acetic acid, chromic acid, hydrochloric acid, and nitric
acid. Acetic acid is the chemical name for vinegar and is a weak acid.
Since Cartridge Brass is 70% copper and 30% zinc, its acid reactivity
can be generalized to your brass jewellery piece as well. Clearly,
brass will react with acid.

Do know that weak, dilute acids, such as vinegar, react slowly with
zinc. When the react begins, hydrogen gas is produced. If you do
decide to place acid on the jewellery, then bubbles of gas will be
observed. This confirms that the metal is reacting with the acid.

I hope this answers your question!


References

Bronze and metals Information
http://www.nbm-houston.com/metals/brass260.html

Clark Craft Fasteners
http://www.ccfast.com/ccfalloys.html

Fundamentals of Chemistry by James E. Brady and John R. Holum. John
Wiley & Sons (1998).

GamGramNo22
http://www.gammontech.com/gg22.htm


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