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If a divided black substance is placed in a glass tube with air, then heated with a...

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kshockley | eNoter

Posted September 17, 2013 at 9:54 PM via web

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If a divided black substance is placed in a glass tube with air, then heated with a bunsen burner the black substance turned red-orange. The total mass of the red-orange substance is greater than the black substance... Is the black substance an element? Is the red orange substance a compound?

I do not understand this question, and I have a drawing to go by....

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ncchemist | eNotes Employee

Posted September 17, 2013 at 11:46 PM (Answer #1)

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We are differentiating between an element and a compound.  An element is the simplest, purest form of matter where all of the atoms in the substance have the same mass number (number of protons in the nucleus).  They can differ only as isotopes (number of neutrons in the nucleus).  An example is carbon (C).  A compound is a substance that is composed of molecules containing more than one type of element.  An example is water (H2O).

When the black substance is heated in the presence of air, it changes color and adds mass.  That is likely because the black substance has oxidized due to the oxygen (O2) in the air.  Air is almost entirely nitrogen (N2) and oxygen (O2), and nitrogen is highly unreactive.  So the substance has been oxidized, or had oxygen atoms added to it (hence the increase in mass).  So the red-orange substance is almost certainly a compound since it would contain more than one type of element (whatever was originally present plus oxygen).

As far as the black substance to begin with, I don't think we can say with certainty whether it is an element or a compound.  If it were an element, it would likely be carbon (black powdery soot).  But oxidized carbon would be either carbon monoxide (CO) or carbon dioxide (CO2), neither of which are colored solids.  So I am tempted to say that the powder is a compound but I don't think that it can actually be proven either way.

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etotheeyepi | Student, Undergraduate | Valedictorian

Posted September 18, 2013 at 12:39 PM (Answer #2)

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A black substance that turns red-orange in hot air: that sounds like copper oxide to me.  Does the red-orange substance form a blue solution when disolved in acid?

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