A bath bomb is mostly baking soda and citric acid, and the fizz happens when those two ingredients dissolve in water and release carbon dioxide gas.
Drop one in a full tub and it spins, foams, and colors the water, then goes quiet. That whole show comes down to two cheap powders reacting the moment water reaches them, plus a few extras that control how fast the reaction runs. Once you know the recipe, you can read any label in a store aisle or mix your own batch in a kitchen.
The Chemistry Behind The Fizz
The fizz is an acid-base reaction between sodium bicarbonate (baking soda) and citric acid, and the carbon dioxide it produces is what you see bubbling up.
Chemistry teaching resources from the Royal Society of Chemistry describe the reaction as the two dry powders dissolving in water and reacting to form carbon dioxide gas. Leave a bath bomb in a sealed, dry wrapper and nothing happens — the ingredients sit side by side without touching, which is why the reaction stays dormant until it hits the tub.
The moment water soaks in, both powders dissolve at once. That releases carbon dioxide, and the gas forms bubbles that force their way out of the tablet. The foam you see is the same reaction, just trapped and stretched by the other ingredients in the mix.
Ingredients In A Typical Bath Bomb
A standard recipe leans on baking soda, citric acid, and cornstarch, with optional Epsom salt and a small amount of oil, color, and fragrance to round it out.
A commonly published DIY recipe from Arm & Hammer lists roughly 100g bicarbonate of soda, 50g citric acid, 25g cornflour, 25g Epsom salt (optional), and 2 tablespoons of oil. That balance matters: the baking soda sits well above the citric acid by weight, since the two react in a set ratio. Cornstarch, or cornflour, does a different job entirely — it slows how quickly the tablet dissolves, which stretches out the fizzing time instead of letting everything burn off in one second.
Epsom salt is optional and mostly adds bulk and a bit of firmness. Oils, witch hazel, isopropyl alcohol, or water act as binders that hold the powders together in one shape. Food coloring and fragrance are what give each bomb its look and smell.
| Ingredient | What It Does | Optional? |
|---|---|---|
| Baking soda | Base that reacts with the acid to make fizz | No |
| Citric acid | Acid that powers the carbon dioxide reaction | No |
| Cornstarch | Slows dissolving to extend fizzing time | No |
| Epsom salt | Adds bulk and a solid feel | Yes |
| Oil, witch hazel, alcohol | Binder that holds dry powders together | No |
| Food coloring | Colors the water and tablet | Yes |
| Fragrance | Adds scent during and after use | Yes |
Why Cornstarch And Binders Change Everything
Cornstarch and binders are what separate a good bath bomb from a crumbly or short-lived one, because they control water contact and structure.
A bomb that is pure baking soda and citric acid dissolves fast and fizzes hard, then quits. Cornstarch slows the water down as it soaks through the tablet, which spreads the reaction across more time and lets the color and fragrance carry further into the tub. Binders keep the whole thing from falling apart in handling — the oils, witch hazel, or alcohol hold dry powders together long enough to set.
Too much water during making is the classic failure point. Add even a little prematurely and the acid and base start reacting before the bomb is formed, leaving you with a flat, weak tablet that barely fizzes at all. Dry ingredients stay dry on purpose, right up until use.
Safety And Buying Considerations
Bath bombs are safe for most people, but added dyes, oils, and fragrances are the parts most likely to bother sensitive skin, since the reaction itself is just baking soda, citric acid, and carbon dioxide.
The ingredients list is the best guide when choosing a bomb. Shorter, more familiar lists tend to suit sensitive skin better than heavy fragrance and dye loads. If you want a shortcut, our roundup of field-tested bath bombs worth buying compares options on exactly those ingredients.
One cleaning note that surprises first-time buyers: strong colors can leave a rim or trace in the tub. Rinse after a colored bomb and it stays a non-issue. That comes down to the dye load, not the reaction.
FAQs
What causes that popping sound when a bath bomb dissolves?
The sound comes from carbon dioxide gas escaping the tablet as it breaks apart. As the baking soda and citric acid react, bubbles form, expand, and pop against the water surface, which produces the crackling effect. A larger, firmer bomb tends to make more noise because more gas releases.
Can you make a bath bomb without cornstarch?
Yes, but the reaction runs faster and fizzing time shortens. Cornstarch slows water from reaching the powders, extending the show. Skip it and you get a hard, quick burst instead of a longer, gentler dissolve. Bump up the binder slightly if you leave it out.
Do bath bombs expire?
They lose potency over time, mostly because moisture in the air slowly starts the reaction. A bomb stored in a sealed bag stays usable far longer than one left open in a humid bathroom. Once it feels soft or crumbly or barely fizzes, it has lost most of its fizz.
References & Sources
- Royal Society of Chemistry. “Making Bath Bombs” Explains the acid-base reaction and carbon dioxide fizz.
- Arm & Hammer. “DIY Baking Soda Bath Bombs” Provides typical ingredient ratios including cornstarch and Epsom salt.
- Wikipedia. “Bath Bomb” Overview of common ingredients, binders, and the reaction.