It is easy to separate cornstarch from water. They are in a "mixture" together. That means that there has been no chemical reaction. The cornstarch is still cornstarch; the water is still water. Therefore you can "physically" separate them using techniques like evaporation, filtration, distillation, etc. In this case, simple evaporation would work. Of course, you will not end up with a nice powder; rather you will end up with a very hard 'clump' of cornstarch. By the way: If you used tap water (which contains dissolved minerals), the minerals will also stay behind when the water evaporates. That will add to the clumping of the original corn starch and would mean that you no longer have pure cornstarch.
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Fun with cornstarch:
Cornstarch and water is, indeed, a mixture ... but a very special one called a suspension (where the particles are suspended in the water and would settle out if given enough time). The cornstarch is not actually wet, even in water.!
This particular suspension has another special property, as it is a "Non-Newtonian" liquid. That means it acts like a liquid if you don't disturb it, but as soon as you do, it acts like a solid.
Example of what happens: If you stir the 'liquid' *slowly* with your finger, it acts just like a liquid. Try it again by stirring real fast and it will be like trying to stir a very thick kind of solid mass!
Search the Web for something like "cornstarch experiment" and you'll get lots of sites that will tell you how to do the experiment. And all it takes is cornstarch and water mixed to make a paste about like pancake batter!