Chemical change. Crusty scales in this context are limescale. Limescale are found in kettles, hot-water boilers and central heating systems. They are deposited by water on the inside of pipes, kettles, etc. When limescale is introduced to vinegar, the acid in the vinegar helps break down the limescale build-up which creates a chemical reaction. Bubbles will begin rising off and a little heat will be produced. This is because calcium or magnesium carbonate is a base and vinegar is diluted acetic acid ( a weak acid). There's always a chemical reaction between an acid and a base. Heat is always produced between an acid and a base which is a part of what a chemical reaction entails.
