Black Snake Version 1 (Fire)
.Sodium bicarbonate (baking soda)
.Lighter fluid or alcohol
Mix together a batch of 4 parts powered sugar to 1 part baking soda. Add a little fuel to the mixture and ignite.
After the black snake is lit, the firework smokes and creates ash resembling a snake. This happens due to an intumescent reaction. The sodium bicarbonate produces carbon dioxide gas, while the sugar creates carbon containing ash.
Your snake will stay on the ground, and will not do any fancy fireworks stuff like emit sparks or flares, and won't produce any sound, but their will be smoke.
Black Snake Version 2 (No Fire)
.Sulfuric acid (drain cleaner)
.This experiment should be performed outside or in a fume-hood
In a glass beaker or other sturdy canister with sugar lining the bottom, pour in a little sulfuric acid until the sugar is completely covered, then just sit back and watch the reaction. The sugar will change from white to dark brown, then to black carbon.
Sugar is a carbohydrate, so when you remove water from the molecule using the sulfuric acid (an excellent dehydrating agent), you're basically just left with elemental carbon.