Chemical Concept Demonstrated
- The difficulty in classifying a substance as a liquid or a solid
Through a variety of different procedures, determine whether the corn starch and water solution that the beaker contains is a solid or a liquid. For example:
- Hit the surface with a stirring rod.
- Set the end of the rod on the surface.
- Scratch the surface.
- Stir the solution.
- Submerge a finger in the starch solution.
- Try to yank it out.
- Feel it with your palm.
- Now, feel it between the forefinger and thumb.
- Drop some into your hand.
- Drop some onto the bench.
- Pick up the puddle.
- Like a solid
- It will sink
- It will crack.
- Like a liquid
- It will sink.
- Pulling needed
- It feels wet and forms a puddle.
- It feels like a powder.
- It can break into pieces.
- It forms a puddle.
- Like a solid
When pressure is applied to this solution, it exhibits the properties of a solid. When the pressure is released, it exhibits the properties of a liquid. The starch granules reversibly absorb and release water depending upon the external pressure applied. Because the mixture should look like a liquid when properly made, the confusion over whether or not to call it a solid or liquid is only amplified.
It should be noted that creating this ambiguous solution can be rather tricky. Adding too little cornstarch will provide the properties of a liquid, and adding too much will provide the properties of a solid. The middle ground is difficult to obtain, so care should be taken during the preparation.
This demonstration has been used in the past to make students realize that some solids and liquids are difficult to classify as one or the other.