Ms. Rowling decided to use her 12 Days of Christmas on Pottermore to explain once again that she still hates Chemistry. What a magnificent role model for children everywhere. But at the same time she 'splains that she loved writing about Potions and herbal medicine that are based on chemistry, as well as alchemy.
You could say that Alchemy has been J.K. Rowling's "bread and butter" all these years, since her first book was about Nicholas Flamel, one of the most famous of the alchemists. She also doesn't mind dropping the name of Paracelsus here and there either. Also, without Potions, Harry probably wouldn't have survived his years at Hogwarts. Did I mention her husband is a doctor?
But keep on hating the Chemistry, Ms. Rowling!
However, I do like her description of Snape as a "brooding, slow-burning personality." :)