Bath bombs are really popular – they’re also easy to make and a great chemistry experiment!
You will need: Epsom salt, citric acid, baking soda, cornstarch, coconut oil, essential oil (optional), food coloring (optional), silicone molds. I don’t usually have citric acid in my pantry, but you should be able to find this at the grocery store or online. I got ours online.
- Mix dry ingredients in a bowl.
- 1/2 cup baking soda
- 1/4 cup citric acid
- 1/4 cup Epsom salts
- 1/4 cup cornstarch
- Scoop 1.5 or 2 Tbs of coconut oil into a separate bowl and melt in the microwave.
- Optional – you can can add a few drops of food coloring and/or a few drops of an essential oil to the melted coconut oil.
- Pour the oil into the dry ingredients and stir until it looks a little crumbly or clumpy.
- Press firmly into silicone molds. We used bunny shaped molds, but you can use any shape that you have or you can buy specific bath bomb molds. I let ours dry overnight and then put the tray in the freezer for about an hour. (The time in the freezer helps solidify the oil and keeps them together when you pop them out of the mold.)
Here’s what’s going on with these: Your bath bomb is essentially an Epsom salt and coconut oil bath (with optional essential oils) that is packaged with some extra ingredients that undergo a chemical reaction when placed in water (all that fizzing!). It’s basically a more exciting way to pour Epsom salts and coconut oil into your bath tub.
The reaction that you are seeing with bath bombs is from the citric acid and the baking soda. This is really similar to what we see with the baking soda and vinegar reaction that we’ve posted about before. You’ll recognize that the baking soda (sodium bicarbonate, NaHCO3) is the same, but we are swapping citric acid in place of the vinegar (that contains acetic acid). The chemical formula for acetic acid is C2H4O2 and the formula for citric acid is C6H8O7. Both of these are considered weak acids and they have the same job in the chemical reaction. Just like with baking soda and vinegar, the first step with baking soda and citric acid is a double replacement reaction where the sodium (Na) from the sodium bicarbonate switches places with the hydrogen (H) from the acid. One of the products from this reaction is the chemical carbonic acid (H2CO3). Carbonic acid then undergoes a decomposition reaction which means that it decomposes or breaks apart. When that happens, it breaks into water (H2O) and carbon dioxide (CO2). The carbon dioxide is a gas, so you can see it bubble up through the water. This is the fizz that you see with your bath bomb. The biggest difference between this reaction and the one with vinegar is that the citric acid is a solid powder and acetic acid is a liquid solution. This is important because that first double replacement reaction doesn’t really happen until you put all of the chemicals in water. Because acetic acid is mixed in with water in the vinegar, the reaction starts right away. With citric acid we have to plop the bath bomb in the tub (or in any water) to get the reaction to start.
The other ingredients are there to give you a nice bath after the fizzing is done or to help hold the bath bomb together. Epsom salt gives a nice soak and the essential oils give some pleasant aromas. The food coloring is just for fun and color. The cornstarch is mostly just a filler. The coconut oil helps hold all of the powders together and adds some moisturizing to the bath.
Enjoy your experiment – and your bath! 🙂