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Mochi Stem Activity #1: Chemistry and Creativity: Making Bath Bombs With Kids

By Mochi Bear

Are you ready to make bath bombs but not sure where to start? We’ve been making bath bombs for a few years now and LOVE it! It’s a great way to involve the kids in some fascinating hands-on chemistry lessons. Plus people of all ages love using bath bombs, making them a wonderful project idea.

Bath bombs are all the craze with everyone enjoying the fizzy bath time fun of bath bombs. Enterprising tweens and teens especially love making their own Homemade Bath Bombs from scratch. It is a fantastic DIY project for teens and a great, practical application of chemistry.

Ready to learn more? I’m sure you have lots of questions. Since we started making bath bombs and bath fizzes about 2 years ago we’ve had tons of questions. Finally, I have gathered up all those inquiries into one place. Let’s get started as we tackle all your bath bomb making questions.

Why make bath bombs instead of buying them?

Although you can buy bath bombs in the stores, there are a few reasons you might prefer to make your own bath bombs.

1) It is much cheaper to make your own bath bombs. It costs me about $1 a bath bomb to make them myself. If I buy them at the store they are anywhere from $5 to $15 each. That’s a massive saving!

2) It is actually fun and not that hard once you learn all the tricks.

3) Learning how to make bath bombs is a fantastic STEM activity as kids work with measurements and chemistry to master their fizzy creations.

4)  bath bombs make incredible gifts that people love to receive.

5) It is an incredibly creative process that gets kids using their imagination and critical thinking skills. There is a huge sense of accomplishment when you slip into a warm bath with a bath bomb you made yourself.

Best of all, this is a zero-waste creative project so you don’t contribute to any landfill waste when making your creations.

What is a bath bomb?

A bath bomb is a hard-packed mixture of dry ingredients that effervesce or fizz when added to a warm bath. They are a special treat that is loved by adults for it’s fragrant, luxurious bath experience, and by kids for its bubbly, fizzy fun as it dances around the bathwater.

What is the science behind a bath bomb?

To turn bath bomb-making into an educational activity, and to increase your successes, you need to understand the science behind the beloved bath bomb fizz.

Bath Bombs are a simple acid-base reaction between baking soda and citric acid (or another replacement acid which we will explore in a moment). In their dried, powdered form you can combine the acid and base and they will not react. We need to add the magical ingredient to trigger the chemical reaction, warm water!

We did a study showing how water temperature affects the fizz. 

One of the results of the reaction between the acid, base, and water is Carbon Dioxide gas (CO2) which is those wonderful fizzy bubbles.

Another fascinating science principle at work here is the role of the emulsifier, polysorbate 80. We all know oil and water don’t mix. That’s until you add an emulsifier. This is a key bath bomb ingredient because without it all your oils, colors, and scents will float on top of your bathwater in a kind of scummy layer that sticks to everything. Make sure you add the emulsifier for a luxurious bath experience and easier clean up after the bath!

What does each ingredient do?

There are many different bath bomb recipes out there, but most share the same common core ingredients. Let’s explore a few of these ingredients.

Citric Acid – The most common acid used in bath bomb making is Citric Acid, however, powered buttermilk can also be used and Cream of Tartar is also a common dry acid to add to the mix. This ingredient is critical for our chemical reaction to create the fizz.

Sodium Bicarbonate – Also known as baking soda, this is the base in our all-important acid-base chemical reaction which gives us our fizz.

Epsom Salts – Epsom Salts are known for helping with sore muscles, aches and pains, and restless leg syndrome. Although not required, they are a nice addition to bath bombs.

Coconut Oil – Coconut oil is the binding agent that holds all of those dry ingredients together. Since it is an oil, it does not trigger the chemical reaction. I always use a good quality solid coconut oil, melt it to make the bath bombs, then when it solidifies again it makes for nice hard bath bombs.

Essential Oils – I like to use a variety of essential oils in my bath bombs based on our needs and what is drawing us in each time. Really, which essential oil you use is based on your preference and what scents appeal to you. Normally I focus on relaxing scents since our bath times are usually right before bed.

Colors – The easiest way to color bath bombs is with mica powder. Although you can use a soap colorant, most of them are water-based and will cause premature reactions in your mix. Mica is dry and will mix to create vibrant colors. Just remember a little goes a long way.

Polysorbate 80 – Polysorbate 80 is an emulsifier that helps the oils and colors in your bath bombs disperse more evenly in the bathwater rather than pooling on top. It also prevents the oils and mica from clinging to the tub and the bather. This is important as oil on the bottom of the tub can make it slippery. We all know oil and water don’t mix, which is why it is so important to use Polysorbate 80 when making bath bombs. 

Sodium Lauryl Sulfoacetate (SLSA) – We don’t always add SLSA but sometimes it is a really nice addition for some extra foamy, bubbly action. SLSA is a surfactant that helps create a LOT of extra bubbles when used in bath bombs. This results in a bath bomb fizzy fun to kick off your bath but then a wonderful, fragrant, bubble bath after. Another big benefit of using SLSA is that it helps trap the fragrance of the essential oils, helping the wonderful aromas last longer.

Witch Hazel or Rubbing Alcohol Spray – These are used to help “wet” the mix. You want your bath bomb mix to be like wet sand, but sometimes it needs a little extra moisture to make that happen. A couple of sprays of these items can help you get the right consistency.

What do I need in a bath bomb making kit?

We make bath bombs all the time and although we like to change up our recipes and use a variety of ingredients there are a few core items that I will always have in my bath bomb-making kit. Click the arrow on each ingredient for more details.

  • 1 cup baking soda
  • 1/2 cup citric acid
  • 1/2 cup Epsom salt
  • 1/2 cup cornstarch
  • Coloring pigment - optional
  • 2 1/2 tablespoons almond oil - or melted Coconut oil
  • 3/4 tablespoon water
  • 12-15 drops essential oil
  • Bath bomb molds

There are a few options for bath bomb molds. The most common are clam style molds to make the traditional round bath bombs, but you can also use silicone molds.

How to make bath bombs with kids

Safety is key whenever we involve kids in these activities. I highly recommend using safety goggles, gloves, and possibly a face mask when mixing up bath bombs, especially if the kids are helping you. Citric acid can be irritating if it becomes airborne (which is easy if your kids are enthusiastic at their pours and stirring). Always use common sense and protect yourself. And of course, kids should always do this project under responsible adult supervision.

With tween and teens, feel free to tackle any technique that strikes their fancy. Although using the clam style bath bomb molds can be a bit tricky, they should be able to figure it out. With younger kids, it might be easier to make silicone mold bath bombs. Then you can also have kids paint their bath bombs for a special and unique bath bomb-making project.


1. Mix together all of your “dry” ingredients–ie, the baking soda, citric acid, cornstarch, and Epsom salt–in a large bowl with a whisk.

Our “dry” ingredients before mixing

2. Then move on to the “wet” ingredients in a separate bowl. You will first need to melt your coconut oil (I usually do this in the microwave for about 30 seconds, checking on it, and then adding 5-10 more seconds until the oil is completely melted). Then, add in any essential oils (if using) to the melted coconut oil. You can also add your colorant into your wet ingredients at this stage if you are making single-colored bath bombs.

Unmelted coconut oil. Since it’s solid at room temperate, it really helps to bind your bath bombs together and makes them super smooth (your skin too!)

3. Now, working VERY slowly, pour your “wet” ingredients” into your “dry” ingredients and incorporate quickly with a whisk, your hands, a food processor, or a stand mixer. Only add in a small amount of the oil at a time to minimize the mix from reacting. You want to end up with a mixture that feels like “wet sand”. If your mixture is a bit on the dry side, spray a few times with witch hazel. You’ll know the mixture is just at the right moisture level when you can form a ball of it in your hands and drop it back into the bowl without crumbling. If your mix starts fizzing, you’ve gone too far!

5″Wet sand” bath bomb mixture. I had to spray a few times with witch hazel before getting the right moisture level

4. If you are making multi-colored bath bombs, I suggest separating the wet sand mix into separate bowls and then adding in the different colors. Make sure to mix really well so that the colors are well-distributed.

5. Now for the fun part! Using your hands (I like to wear gloves to keep them nice and clean), press the bath bomb mixture into your bath bomb molds. If you are making multi-colored bombs, simply alternate the colors you add into the molds. If adding sprinkles, you can mix a dash into your bath bomb mixture, then pour a bit into the bottom of one of the mold halves before filling. If you want your bath bombs to float, DO NOT PACK TOO TIGHT. Just lightly press down the mix, overfilling each half slightly, then press the two halves together. You can either keep the bath bombs in the molds or carefully remove them after allowing them to sit for a few minutes. It helps if you first tap on the metal molds a few times with a metal spoon before removing.

6. Let the bath bombs dry, usually about 24-48 hours (depending on your humidity). To help loosen them, tap on them a bit with a metal spoon before removing them.

7. Add to water and enjoy it!


How many does it make?

I get this question so often! How many bath bombs does this recipe make? Well here is where things get tough because it really depends what size bath bomb molds you are using! My bath bomb mold kit came with three different sizes, so I usually make a variety of sizes. Not to mention if you use silicone molds or muffin tins it gets even harder to provide a clear answer. This makes it so hard to answer the question, “how many does the recipe make?” Suffice it to say that in general, these recipes will make anywhere from 4 to 12 bath bombs on average. I normally go with a smaller bath bomb mold so I get closer to a dozen bath bombs each time. But my kids like to use bath bombs every day if they can, so I need to make my batches last.

How do you use a bath bomb?

Ready to use your bath bomb? Run yourself a nice warm bath. Remove any wrappers or packaging from your bath bomb. Now add your bath bomb to the warm water and relax as it fizzes and bounces around your tub leaving a fragrant bubbly experience in its wake.

If you have some colors from the bath bomb left on the sides or bottom of your bathtub, simply rinse with freshwater or use a little bit of regular bathtub cleaner to remove the residue. 

Storing and Gifting Bath Bombs

Once you have successfully made your gorgeous bath fizzies you are going to want to store them safely. It is best to keep them in an airtight container to protect them from moisture. Remember, any water, even humidity in the air will cause your bath bombs to start reacting, which means less fizz at bath time.

I prefer to use a glass container with a lid to store our personal bath bombs.

When gifting bath bombs I put them in reusable plastic bath bomb molds, or shrink-wrap them in plastic. If you don’t have a way to shrink wrap, you can use a plastic gift bag and just make sure to tightly secure the top.