Key concepts
Chemical reaction
Have you ever had a refreshing bath with bubbles or bath salts? What if you could use chemistry to create a bath-time treat that incorporated both fizzing bubbles and soothing salts? Such a combination does exist, and it's called a bath bomb. If you have never experienced one, these "bombs" become fizzy when they touch the water. Why? It is due to a chemical reaction taking place between different ingredients within the bath bomb. In this activity you'll get to make your own homemade bath bombs and explore how changing the ratio of the ingredients affects how much the bath bomb fizzes when it comes into contact with water. Then you can use your perfected method to make some bath bombs as a gift for Mother's Day!
Bath bombs can have a wide range of ingredients, including bath salts (which can help sooth muscles), food coloring, fragrances and other components. There are, however, a few key ingredients that most homemade recipes have: baking soda and citric acid. Why is this? When baking soda and citric acid are mixed and are then put in water, they undergo a chemical reaction. The reaction produces lots of bubbles, which you see as the bath bomb dissolves in the water. These bubbles that make the water become so fizzy are made of carbon dioxide gas.
Another ingredient that is often used in homemade bath bombs is cornstarch. This ingredient can act as a dry "filler" that gets mixed in with the reactive baking soda and citric acid in the bath bombs. In this activity you'll explore how changing the amount of cornstarch filler affects how fizzy the bath bombs turn out and you'll figure out what goes into making the most impressive bath bomb.

  • Citric acid (This is usually available at grocery stores in the canning section.)
  • Baking soda
  • Cornstarch
  • Water
  • Measuring spoons
  • Vegetable oil
  • Food coloring (at least two different colors)
  • Medicine dropper
  • Forks and spoons for mixing
  • Optional ingredients to add to your bath bombs: Epsom salts and fragrance (such as essential oils).
  • Four bowls
  • A muffin tray or ice cube tray (If you are using a muffin tray, you can dry the bath bombs for 45 minutes in an oven or overnight at room temperature. If you live in a very humid environment, it's recommended to oven dry the bath bombs. If you use an ice cube tray, however, you can only dry the bath bombs overnight at room temperature.)
  • A bathtub to make a hot bath and test your bath bombs in
  • If you are using a muffin tray, preheat the oven to 170 degrees Fahrenheit (or its lowest setting). Always have an adult help when using the oven.
  • Note that the recipe amounts given in this activity are for approximately filling one muffin cup, or about three ice cube cups, but this greatly depends on the exact size of the cups in your muffin or ice cube tray. If you want to make additional bath bombs, you can double or triple the recipes.
  • In one bowl mix one and one third tablespoons (tbsps.) of citric acid, two and two thirds tbsps. of baking soda and two tbsps. of cornstarch. If you are using Epsom salts, you can also add two teaspoons (tsps.) to the mix.
  • In a second bowl mix one tsp. of vegetable oil, one tsp. of water and two drops of food coloring. If you want to include fragrance, also add 15 drops into the mixture. Be sure to rinse and clean the medicine dropper and measuring spoons in between measuring the different ingredients.
  • Using a clean medicine dropper, add a few drops of the wet mixture to the dry ingredients in the first bowl. What happens when you add a drop of the wet mixture? You should see it fizz—this is the bath bomb reaction taking place! Because you don't want the bath bombs to react yet, quickly press down on the fizzy spot with the back of a clean spoon. This should stop the fizziness. Mix in the damp spot with the rest of the ingredients in the bowl. Repeat this process until you have added, and thoroughly mixed in, all of the wet ingredients (a few drops at a time) to the dry ingredients.
  • Tip: Part of the challenge of making homemade bath bombs is adding the right amount of wet ingredients. If you live in a humid environment, you may not need to add all of the wet ingredients. If the bath bomb mixture appears to continue to puff up even after you have thoroughly mixed in some wet ingredients, then the mixture may be too wet. If this happens, start over making the bath bombs from the beginning, but this time use less water in the recipe. If you find that this first recipe works better using less water, adjust the following bath bomb recipe similarly.
  • Use a clean medicine dropper to drop one drop of vegetable oil into each cup on the tray that you will be using. Then use a finger to spread the oil all around each cup's surface.
  • Fill one of the tray's cups with the bath bomb mixture. Add a spoonful at a time and use the back of the spoon and/or the palm of your hand to press the mixture down into the cup continually as the mixture is added to the cup. If you are filling multiple cups, evenly divide up the mixture between them.
  • In a third bowl combine one tbsp. of citric acid, two tbsps. of baking soda and three tbsps. of cornstarch. If you are using Epsom salts, add in two tsps.
  • In a fourth bowl mix one tsp. of vegetable oil, two and one half tsps. of water and two drops of a different food coloring. If you want to include fragrance, also add 15 drops. Be sure to rinse and clean the medicine dropper and measuring spoons in between measuring the different ingredients.
  • Use a clean medicine dropper to slowly mix the wet mixture with the dry ingredients in the third bowl, one drop at a time, as you did before, using the spoon to press down on fizzy spots and continually stir the mixture. Fill the tray's cups as similarly as possible to how you filled them for the first bath bomb recipe.
  • Let the bath bombs dry. If you are using a muffin tray, dry the bombs overnight or turn off the oven (which was preheated to 170 degrees F) and let them stay in the (turned off) oven for 45 minutes with the oven door closed. If you are using a plastic ice cube tray, dry the bath bombs overnight at room temperature. Once the bath bombs have dried, carefully remove them from the cups.
  • Tip: If the bath bombs are very crumbly, the recipes may not have had enough water in them. To fix this, you can remake the bath bombs but try using a little more water.
  • Get ready to toss the bath bombs into a bath! Fill a tub with hot (but not scalding) bathwater. Then place the bath bombs in the tub. What happens when the bath bombs are placed in the water? Is a bath bomb made from one recipe fizzier than a bath bomb made from the other recipe? Does one take longer to dissolve than the other one? Which do you think worked best? How do you think the amount of cornstarch in the recipes is related to your results?
  • Extra: In this activity you tested your bath bombs in hot bathwater, but they might behave differently in colder water. You could use a stopwatch or timer to see how long they take to dissolve in hot water and then compare it with placing them in cold water. Do the bath bombs take a different amount of time to dissolve depending on water temperature?
  • Extra: Instead of citric acid, you could experiment with making bath bombs using citric acid substitutes, such as cream of tartar or lemon juice. How do bath bombs made using a citric acid substitute compare with those made using citric acid?
  • Extra: In this activity you tried varying the amount of cornstarch but you could try making bath bombs without any cornstarch. If bath bombs are made that don't have cornstarch, how do they compare with those made with cornstarch? Are they very different?

Observations and results
Did the bath bomb made using more cornstarch (following the second recipe) take longer to dissolve than the one made with less cornstarch (following the first recipe)? Did the one made with less cornstarch fizz more?
When a bath bomb comes in contact with water, the baking soda and citric acid react to make carbon dioxide bubbles. This is an acid–base reaction, where baking soda (also called sodium bicarbonate) is a weak base and citric acid is a weak acid. The cornstarch acts as a "filler" to control the reaction between the baking soda and citric acid. In this activity the second recipe used more cornstarch, and less baking soda and citric acid, compared with the first recipe. Consequently, you should have seen that a bath bomb made using the first recipe produced more vigorous bubbles and impressive fizzing, and dissolved much faster, compared with a bath bomb made using the second recipe. (The size of the bath bombs also affects how long it takes them to dissolve, because larger bath bombs will typically take longer than smaller ones to dissolve. But because the bombs from the different recipes should have been similar in size, this factor should not have greatly affected the comparison.)
If you have extra bath bombs and want to save them for later, put them in a sealable plastic bag. Once you've settled on your favorite recipe, you can also make them and give them out as gifts!
More to explore
What's New, CO2? Get to Know a Chemical Reaction (pdf), from the American Chemical Society
Try This: A Chilling Recipe, from the Commonwealth Scientific and Industrial Research Organization
Fun, Science Activities for You and Your Family, from Science Buddies
Shimmy, Shimmy Soda Pop: Develop Your Own Soda Pop Recipe, from Science Buddies

This activity brought to you in partnership with Science Buddies