Test and analysis: the fundamental difference between physical change and chemical change lies in whether new substances are produced, and if new substances are produced, they belong to chemical change; On the contrary, it is a physical change. A, remove the scale in the thermos bottle with vinegar, which is the chemical reaction between acetic acid and calcium carbonate, the main component of scale; B, cutting potatoes into filaments from blocks only changes the shape of potatoes, and no new substances are produced, which belongs to a physical change; C, adding soda ash to remove the sour taste of steamed bread is a chemical change. D, iron reacts with oxygen and moisture in the air when the kitchen knife is rusted and untreated, which belongs to a chemical change. So choose B.