Well. I was making muffins today & both baking soda & orange juice were on the list of ingredients. This got me thinking to that fun childhood experiment of mixing baking soda & vinegar to make a foaming volcano...
So why does it foam?
Vinegar & orange juice are acids - and since they're liquids, they're basically dissolved in water (your vinegar is definitely not 100% acetic acid). This means there are free hydrogen ions in the solution (this is a property of acids) - the more hydrogen ions released in solution (this depends on the rest of the molecule, not necessarily how many hydrogens there are), the more acidic the chemical. The concentration of hydrogen ions is what pH measures.
Anyway (this was supposed to be short, if not for all the brackets), baking soda (sodium bicarbonate) has the following chemical formula: NaHCO3. The hydrogen ions from the acid can replace the sodium in this compound, giving us H2CO3. Otherwise known as carbonic acid.
This acid is special. It doesn't produce hydrogen ions when dissolved in water (H2O) - it reacts!
H2O + H2CO3 ------> 2 H2O + CO2
CO2 is carbon dioxide - the gas you see bubbling, or foaming out of your "volcano".
And yes, the muffins are delicious (no foam though).