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anonymous

  • one year ago

The single best demonstration of chemistry: take some coffee, expose to water and heat, it turns into something almost like leather ..amazing!

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  1. abb0t
    • one year ago
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    Or you could freeze water to make ice cubes. That's pure chemistry.

  2. anonymous
    • one year ago
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    Now the real question...is the result from the coffee experiment a chemical change or a physical change? ;-)

  3. cuanchi
    • one year ago
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    @abb0t the change of phase (freezing) it is NOT a chemical change, it is consider a physical change http://www.chem4kids.com/files/matter_chemphys.html

  4. abb0t
    • one year ago
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    He said demonstration of chemistry. A physical change from liquid to solid IS chemistry. It's a branch of chemistry which you might not be familiar with - physical chemistry. I suggest you look into that.

  5. cuanchi
    • one year ago
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    Dear @abb0t: In General Chemistry a difference than in Physical Chemistry the distinction between chemical and physical change is not absolute, and there are examples of changes which teachers find difficult to classify. Students are often expected to distinguish between chemical and physical changes early in their study of chemistry, but some find this quite difficult. Something that several undergraduate General Chemistry books agree is that phase changes (solid to liquid to gas) are physical changes. I do not agree with you to use the freezing of water as the most didactic example of pure chemistry. In turn I may be using this example in Physical Chemistry classes. I usualy use the rusting of an iron nail, or the combustion of a log in a fireplace as chemical changes. 1. A physical change is a change where no new substance is produced. (….a change that does not involve the breaking/forming of strong chemical bonds.) (…..a change where molecules/ions etc are rearranged, but not changed.) 2. A chemical change is a change where a new substance is produced. (…..a change that involves the breaking/forming of strong chemical bonds.) (…..a change where new molecules/ions etc are formed.) 3. Physical change: No new substance is produced (the same molecules are present before and after the change) (the change may readily be reversed) (the energy change involved is modest) 4. Chemical change: A new substance is formed (strong chemical bonds are broken – eg in the oxygen molecules - and new chemical bonds are formed in the metal oxide) (different particles are present after the change - oxide ions rather than oxygen molecules) (this change is not easily reversed) (a great deal of energy is often given out in this change) 5. Physical change: No new substance is formed (NB the solution is not a pure substance, but a mixture) (the same particles are present after the change as before) (this reaction is readily reversed – by evaporation) (the energy change for dissolving is minimal) Note, however, that the ionic bonds in the lattice have been disrupted, which may suggest dissolving could be considered as a chemical change. Note: Energy changes are not the best way to characterize these changes.

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