What constitutes a chemical change? Sometimes what appears to be a chemical change is actually just a physical change. For example, adding a "Menthos" type candy to a glass of soda pop. What is observed is an exciting increase in "fizz" or release in carbon dioxide. Two possible explanations for this phenomenon are:
The surface area of the candy provides nucleation points for the gas to come out of solution.
The guar gum in the candy is dissolved into solution and reduces the surface tension of the water.
Probably these two factors work together to produce a physical change in the soda pop (i.e. gas release is increased).
Demonstration:
The video clip below shows the menthos and cola classroom demonstration. Add two or three "Menthos" candies to 2 L of soda pop. The results are spectacular. Make sure you have a pan large enough to catch the overflow - if you don't have a large pan, it would be wise to take the demonstration outside (press play to start the video):
Menthos candy added to cola (QuickTime 1MB)
You may wish to modify this demonstration by putting a hole in the bottle cap (a cork borer works well for this task). After adding the candy, quickly screw the cap back on. It is recommended to perform this demonstration outside or you might decorate your ceiling!
It can be very difficult to get a the cap with the hole in it back onto the bottle without getting sprayed yourself, so you may want to try a more creative way of adding the candy to the cola. First, obtain a large paper clip and use pliers to put bends in the ends:
Now bend the rest of the paper clip into a basket shape. The top bent ends will hold the paper clip hanging by the hole in the bottle lid. The paper clip basket should extend about 2 cm into the neck of the bottle. Test it with a empty bottle to ensure that it slides easily into the neck and holds 2 or 3 menthos candies without falling into the bottle:
Because most 2 L soda bottles use the same bottle caps, you can use the cap with the hole in it and paper clip basket many times.
When it comes time to do the demonstration, slide the basket (suspended from the bottle cap) into the neck of the bottle, place in a few Menthos candies, then carefully screw the cap on tightly. Now watch what happens when you squeeze the clips and tap the top of the paper clip so that the basket and candies fall into the cola (press play to start the video):
Menthos added to cola with small hole in cap (QuickTime 2.2 MB)
Task:
Questions Leading to Further Investigation
Try this activity with different candies and different brands of soda pop. Is there a difference? Does one work better than the other when compared to the initial demonstration?
Does varying the number of candies added make a difference?
Does temperature of the soda pop make a difference?
Does the type of container holding the soda pop make a difference?
Does changing the shape of the candy make a difference?
