Why a fizzing tablet on the shower floor carries scent
Putting something fragrant near the water at the end of the day is an old habit. The chemistry and the nose behind it are simpler than the packaging suggests.
The fizz is a plain reaction
A shower steamer is mostly baking soda, which is sodium bicarbonate, and citric acid, pressed into a tablet with a little binder and fragrance oil. Dry, the two powders sit side by side and do nothing. Once water reaches them they dissolve, meet, and release carbon dioxide. That is the fizz. It is the same reaction as in a bath bomb or an effervescent tablet dropped in a glass.
The bubbling has a practical purpose. It breaks the tablet down gradually and pushes the fragrance oil into the air instead of leaving it in a lump. Set on the floor of the shower, out of the direct stream, the tablet is misted and splashed rather than drenched, so it lasts through a whole shower instead of vanishing in a minute. Placement is most of the trick.
Why steam carries scent
Smell is a matter of molecules reaching the nose. Essential oils such as lavender, eucalyptus, peppermint and orange are volatile, which means they evaporate easily, and warm, moist air speeds that along. A small tiled bathroom with the door shut holds those molecules close. For scent, it is about as efficient as a room gets.
Why it works as a signal
Smell has an unusually direct route to the parts of the brain tied to memory and emotion. That is why one scent can bring back a particular kitchen or a particular winter. Used at the same point each day, say after dinner on a weeknight, the same fragrance turns into a cue that the day is changing speed. The repetition does as much of the work as the oil does.
What the tablet cannot promise is a measurable effect on anyone’s mood. Whether lavender smells calm or like an old sachet drawer is a matter of taste. Eucalyptus reads as sharp and clean to many people and medicinal to others. Citrus is bright, and some find it too bright at ten at night. Choose by standing in it and noticing what you like.
One practical note for small flats. The tablets react to any moisture, so they keep best dry and away from the tub spray until they are needed. A bathroom with a vent or a cracked window also helps if a strong scent tends to give you a headache.