Would iron be considered a substance or a mixture? How about blood?

Iron is:
http://en.wikipedia.org/wiki/Iron
substance is a very general term:
See definition 3:
http://www.m-w.com/dictionary/substance

Now blood is living tissue and as such very complex. In ways it is a mixture of various components such as red blood cells, white blood cells, plasma.
But not in the chemistry sense of the word.
See:
http://en.wikipedia.org/wiki/Blood
I guess you could say blood is a very complex mixture of organic compounds.

I'm sorry, i'm still a little confused. My friend is saying that Iron is a substance, is that true? Then, she said that blood is a mixture. Is that true too?

Substance is a very general term. Every piece of matter in the universe is a substance. On blood, tell your friend to define mixture. did you read the links I left you? Try reading and make your own judgement:
http://www.google.com/search?as_q=mixtures+elements+compounds&hl=en&num=10&btnG=Google+Search&as_epq=&as_oq=&as_eq=&lr=&as_ft=i&as_filetype=&as_qdr=all&as_nlo=&as_nhi=&as_occt=title&as_dt=i&as_sitesearch=&as_rights=&safe=images

I think i got it. Iron is a substance and blood seems to fit the definition of a mixture.

Yes, you are correct. Iron is considered a substance because it is a pure element with a specific chemical composition. On the other hand, blood can be considered a mixture because it is made up of various components such as red blood cells, white blood cells, and plasma. It is not a pure substance like iron because it contains different substances mixed together.

Yes, you are correct. Iron is considered a substance because it is a pure element with a specific chemical composition. On the other hand, blood is considered a mixture because it is made up of multiple components, such as red blood cells, white blood cells, and plasma. While blood may be classified as a complex mixture of organic compounds, it does not fit the definition of a pure substance like iron.