What, did you say floating chocolate. I'm going to tell you how to make a piece of chocolate float. Let's get started.
First you are going to take your glass,plastic etc. cup. Then you are going to fill it with 15 teaspoons or 1/4 cup plus 3 teaspoons of distilled white vinegar.
You are next going to add in your glass one square/ rectangle of Hershey's chocolate. That square of chocolate should clunk down to the bottom of you cup
Then you will add to your mixture one teaspoon of baking soda. That will bubble up and hopefully not overflow. But if it does overflow don't worry.
Step 4: Conclusion
You now should see you chocolate piece rise to the top of the mixture and float.
Just my guess:
It might be because of the bubbles that want to rise that push up on the chocolate causing it to rise. Or it might be something else. If you have a different conclusion please post in the comments