Question

old pennies become dull when the copper on the outside combines with oxygen in the air. this can take many years. the result is a new chemical called copper oxide.
when you put pennies into a vinegar and salt solution, the acetic acid and salt work together. they dissolve the copper oxide. it becomes part of the solution.
when you put steel paper clips in the solution, some of the iron in the steel dissolves as well. the copper in the solution and the iron that’s left in the paper clip are strongly attracted to each other. so the copper ends up sticking to the paper clip.
and that’s how you use chemistry to make copper - plated paper clips!
fill in the blanks to explain why the experiment works when you follow the steps.
the pennies become shiny because the mixture of salt and vinegar dissolves copper oxide. the paper clips get a copper coating because the leftover copper in the solution sticks to the salt, iron, acid in the paper clips.

Explanation:

From the text, when steel paper clips (which contain iron) are put in the solution, some iron dissolves, and the leftover iron in the paper clip attracts the copper in the solution, making the copper stick to the paper clip. So the blank should be filled with "iron" as the copper sticks to the iron in the paper clips.

Answer:

iron