A running bath naturally creates air bubbles, but these pop quickly as they’re pulled back into the main body of water by the high tension at the water’s surface (this ‘surface tension’ results from the attractive forces between water molecules).
Bubble bath contains chemicals called ‘surfactants’ that weaken the attraction between the water molecules and lower the surface tension, meaning that the bubbles can last for longer. The surfactants also increase the bubbles’ elasticity, which makes them more resilient to being squeezed and deformed. As the bubbles grow in number, your bath becomes a frothy delight.
Dr Emma Davies is a science writer and editor with a PhD in food chemistry from the University of Leeds. She writes about all aspects of chemistry, from food and the environment to toxicology and regulatory science.