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Name: Donald C.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 10/1/2003


Question:
I teach 8th grade science. We are currently working on the concepts of pure vs mixture and solubility. I am creating a homework assignment to involve parents and have discovered that I am unsure of the definition of a homogeneous mixture. I know that homogeneous is a mixture that is uniform throughout. I know that a solution is a homogeneous mixture. Examples of kitchen solutions would include Kool-Aid, sugar water, honey, coffee, tea, and soda. I know that salsa or chicken soup, for example, would be a heterogeneous mixtures.

Question: Would powdered sugar (sugar and corn starch) be considered a homogeneous mixture or heterogeneous? Phrased another way, does a homogeneous mixture have to be a solution?


Replies:
Hello Donald,

"Uniform throughout" refers to a lot of length scales. For example, a mixture of sugar and salt is definitely a heterogeneous mixture because the sugar and salt crystals are physically separate from one another (at least under a magnifying glass you can see the difference if not with the naked eye). A more sweeping definition of a homogenous mixture is that it is uniform at all macroscopic levels and methods of observation (or if you prefer, all macroscopic length scales). A heterogeneous mixture is not macroscopic at all levels of observation. So an alloy of two or more elements (like nickle and zinc) is a homogenous mixture, whereas a bunch of nickle and zinc filings mixed together in a bowl would be a heterogeneous mixture.

All that said, I believe that powdered sugar is a heterogeneous mixture because I think that close examination would reveal crystals of sugar distinct from the from the flakes of cornstarch. However, if I am incorrect and the sugar is literally dissolved into the cornstarch, it would be a heterogeneous mixture.

Finally, "solution" is the term chemists usually use synonymously with "homogenous mixture." However, "solution" is not as precise a concept and is sometimes used to describe heterogeneous mixtures as well.

Hope this was helpful,

Prof. T



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