We all love relaxing after a long day, and what better way to do this than to have a stress-free soak in the tub with your favorite bath bomb? But these spheres of color and fizz are more than just a bath-time treat; they are a complex ball of chemical reactions. A lot of chemistry takes place beneath the bubbles in your bathtub, and it starts with two key ingredients: sodium bicarbonate and citric acid. Keep reading to learn more about bath bomb science.
The wow-factor of a bath bomb is in the initial drop. When it enters the water, it immediately begins fizzing and releases a kaleidoscope of colors.
This happens because of the interaction between what will soon be your favorite duo: sodium bicarbonate and citric acid. While these two substances are inactive when dry, they undergo an important chemical reaction in water.
Most commonly known as baking soda, sodium bicarbonate appears as a fine white powder. Chemically, it is a weak alkaline that doesn’t fully ionize in water. Therefore, it needs a helping hand in order to foam. This is where citric acid steps in.
We cross paths with citric acid every day in the form of citrus fruits. As the name suggests, it is a weak acid that is used to react with sodium bicarbonate. This is known as a neutralization reaction because the basic sodium bicarbonate and citric acid neutralize the effect of each other.
When this happens, new molecules are formed, including carbon dioxide and sodium citrate. Carbon dioxide escapes as rapidly occurring gas bubbles, and this is what causes the bath bomb to fizz!
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Chemicals that Stick Together, Fizz Together
These bubbles of carbon dioxide do more than just put on a show. The fizzing caused by the acid/base reaction helps to distribute the colors, fragrances and softening agents throughout the bathwater.
However, sodium bicarbonate and citric acid aren’t acting alone. Among essential oils, fragrances and colorants, there is a diverse range of chemical products that all have a role to play. These all stick together in a compact ball because of the presence of binding agents, which also help to control the fizz when the bath bomb enters the water.
Several chemicals are used as binding and thickening agents in a bath bomb. Among these ingredients is potassium bitartrate, more commonly known as cream of tartar. This is used as a binding agent and helps to harden the bath bomb, ensuring that it stays compact when dry.
Cornstarch is used to control viscosity and fizz. In bath bombs, it helps to thicken other ingredients and bind everything together. It does this by trapping water between its long starch molecules. These bonds break once introduced to water.
It also behaves as a dry “filler” that helps to control the fizz created by the acid/base reaction. The less cornstarch in a bath bomb, the more bubbles and fizzing there will be.
As well as acting as a binding agent, isopropyl alcohol (IPA) is also used for its high volatility. Because IPA evaporates quickly, it helps to prevent an early acid/base reaction. Like cornstarch, this helps control the fizz of the bath bomb.
Sometimes, especially in humid climates, the moisture in the air causes the sodium bicarbonate and citric acid to react out of the bath. This is because the water molecules in the atmosphere are enough to wet the ingredients.
Isopropyl alcohol prevents this because it evaporates off quickly, keeping the ingredients dry and protecting against any premature fizzing.
Sodium Lauryl Sulfoacetate (SLSA)
At this point, you may be considering swearing off bath bombs altogether. “Did she say Sodium Lauryl Sulfate?” you ask in horror. Well, there’s no need to fret! Sodium Lauryl Sulfate (SLS) and its cousin, Sodium Lauryl Sulfoacetate (SLSA), are actually very different.
Unlike its controversial counterpart, SLSA is generally considered safe to use. Unlike its controversial counterpart, SLSA is generally considered safe to use. It is a hydrophilic compound widely used in the cosmetic industry for its foaming and antibacterial properties. A viscous chemical, SLSA creates lathers, increases and stabilizes foaming, and removes bacteria without irritating the skin.
Commonly found in shampoos and cleansers, SLSA is also used in bath bombs to help remove bacteria from the skin, to reduce surface tension, and (most importantly) to support lots of bubbles!
There’s more science in your bath bomb than you would expect. So, next time you find yourself in need of a new bath bomb, why not try using chemistry to make one yourself?
This article was originally published on 5/8/18 and was updated on 5/29/19 to include an explainer video.
Editor’s Note: This article was contributed by Lucy Bell-Youngof ReAgent Ltd. Lucy is the Marketing Copywriter at ReAgent Ltd. When she’s not writing about chemistry, she can be found writing creative short stories.