Ask A Scientist Chemistry Archive

name         Donald C.
status       educator
age          40s

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?
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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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