Make a unique present for friends and family this holiday season.
What’s more fun than bathtub bubbles? Bath bombs have been around since 1989 and have recently found a resurgence in popularity. With this activity, you can customize all your scents and colors for custom science gift-giving!
- Silicone molds (soap/candy molds, muffin tins, or plastic eggs also work well)
- Mixing spoons
- Wax paper
- Large bowl
- Small bowl
- Measuring spoons
- Measuring cups
- Eye protection
- ¾ cup baking soda
- ½ cup cornstarch
- ½ cup powdered citric acid (buy online, at your local craft store, or local grocer with canning supplies)
- ½ cup Epsom salt
- ½ tablespoon water
- 2½ tablespoon oil (coconut, olive, or other vegetable oil)
- Optional: portable fan
- Optional: 4 to 6 drops of food coloring (powdered food coloring works well)
- Optional: 2 teaspoons skin-friendly scented essential oils (lavender, eucalyptus, rose, lemongrass, coffee, cinnamon, vanilla, peppermint … whatever you like! Available online or at your local craft store).
- Put on your safety gear, including gloves and eye protection. Powders can be irritating.
- In a large bowl, combine dry ingredients (baking soda, Epsom salt, citric acid, and cornstarch) and mix until combined and clump-free.
- In a small bowl, combine the wet ingredients (oil, water, scented oil, and food coloring). If you use scented oil, stick to about two teaspoons. Roughly five drops of food coloring work well, but feel free to mix up different pigments and experiment with the color.
- Very slowly add the liquid to the dry mixture — about one teaspoon at a time. Use the whisk to mix as you go. Slow down if you start to see the mixture fizz. The final mixture shouldn’t really clump together and should have the texture of damp sand.
- Spread the mixture into the silicone molds (or plastic eggs) and press firmly until it is fully and evenly packed in each section. Depending on size, this can make about 10 bath bombs.
- Place the packed molds in front of the blowing fan or just set them out to dry. This can take up to a day.
- When your molds are set, flip them over and very gently press the fizzies out onto a piece of wax paper. If they start to crumble as you do this, flip them back over and let them continue to set in the molds for a while longer.
- Test them! Once the bath bombs are set, try one out by tossing it into a tub of water. What do you observe?
- To gift them, gently wrap your bath bombs in decorative bags. If the fizzy does crumble, have no fear! The mixture can be poured into a bath and produce the same great effects.
The key ingredients here are citric acid (a weak acid) and baking soda (or sodium bicarbonate, a weak base). When the two ingredients are dry in their powdered form, they don’t react. But when you put the bath bomb into the tub, water causes the two key ingredients to mix, resulting in an acid-base reaction. Is there any change if you put the bath bomb in cold water versus hot water? How does the size of your bath bomb affect its fizziness? How does the color of the bath bomb affect the water?