chem自學(2) - intro of chemistry

1. SHOULD be D.
element is defined as "substance that cannot be chemically broken down". silver cannot be chemically broken down, thus it's an element.

C is NOT correct. though silver can exist as free element naturally, it's not because "it is an element". think in this way: salt (sodium chloride) exists naturally, but salt is not ana element -- salt is a compound composed of sodium and chlorine, and can be chemically broken down (by electrolysis).

A only shows that a chemical reaction occurs. it does not necessarily shows that an element is formed.
B is not correct. carbon monoxide can combine with oxygen to form a new compound (carbon dioxide), but carbon monoxide is Clearly nNot a compound.

(However, according to an article in Journal of Chemical Education, element is suggested to be (re-)defined as "atoms of same kind that having same no. of protons in each of their nucleus". that's a good point to bear in mind. )


2. Second one is correct. it's the definition of compound.

"Air is an example of compound." is NOT correct. air is a mixture of elements (oxygen nitrogen, argon, etc.) and compounds (water, carbon dioxide...).

"When an element is broken down chemically, compounds are formed." is NOT correct. elements cannot be chemically broken down.
Even compounds are chemically broken down, elements are not necessarily formed -- when glucose (a carbohydrate) is chemically broken down in our bodies, carbon dioxide and water are formed. they're not elements but compounds.

1. c
2. A compound must be composed of at least two elements.