Bath bombs are spherical or decoratively shaped solid bath additives that dissolve fizzingly in the bathwater, releasing fragrances, nourishing oils, and often colors. They are primarily used for skin care and relaxation. Their fizzing effect is based on a chemical reaction that occurs upon contact with water. In addition to the carbon dioxide produced, which creates the characteristic fizz, sodium citrate and water are also formed. This guide offers a comprehensive scientific exploration of the chemical principles, the involved reaction mechanisms, and the practical implementation for producing high-quality bath bombs. The aim is to provide an understanding of the underlying processes and enable a methodical approach.


Chemical Principles

1. The Core Reaction

The fizzing effect of bath bombs is based on an acid-base reaction1[An acid-base reaction is a chemical process in which an acid donates a proton (H+) to a base, which accepts it, often resulting in the formation of water and a salt.] between sodium bicarbonate (NaHCO3, commonly known as baking soda) and citric acid (C6H8O7). When water is present, the following products are formed: carbon dioxide (CO2), which creates the characteristic fizz, sodium citrate (Na3C6H5O7), and water (H₂O).

Reaction equation: NaHCO3 (s) + C6H8O7 (aq) → Na3C6H5O7 (aq) + CO2 (g) + H2O (l)

In simpler terms: baking soda and citric acid react with each other when they come into contact with water. Water specifically triggers the reaction because it acts as a solvent, dissolving both baking soda and citric acid into their ionic components2[Ionic components are charged particles (ions) that form when atoms or molecules gain or lose electrons, consisting of positively charged cations and negatively charged anions.]. This allows the hydrogen ions from citric acid to react with the bicarbonate ions from baking soda, releasing carbon dioxide gas. In other solvents, such as oils or alcohols, the reaction does not occur because they cannot ionize the reactants in the same way. This produces a gas (carbon dioxide) that creates the bubbles and a substance (sodium citrate) that is formed from citric acid and baking soda.

Calculation of the Released CO2 Amount:

If the total reaction mixture is divided into 8 bath bombs, the mass and volume of the carbon dioxide produced per bath bomb can be calculated as follows:

Given values:

  • 250 g sodium bicarbonate (NaHCO3)
  • 125 g citric acid (C6H8O7)
  • Molar masses: NaHCO3 = 84.01 g/mol, C6H8O7 = 192.12 g/mol, CO2 = 44.01 g/mol

Calculation steps:

  1. Molar amounts: Baking soda: 250 g / 84.006 g/mol = 2.976 mol
    Citric acid: 125 g / 192.124 g/mol = 0.651 mol
  2. Limiting reactant: Citric acid is the limiting reactant, as 1 mol of citric acid requires 3 mol of baking soda.
  3. CO₂ calculation:
    n(CO2)3[A mol is a unit of measurement in chemistry that represents 6.022 × 1023 particles (atoms, molecules, or ions) of a substance, known as Avogadro’s number.] = 3 × 0.651 = 1.953 mol
    m(CO2)4[Mass in Gramms]: 1.953 mol × 44.01 g/mol = 85.90 g
    V(CO2)5[Volumne in Liters] (under standard conditions)6[Standard conditions refer to predefined reference values in chemistry, typically 0°C (273.15 K) and 1 bar (100 kPa), used to ensure consistency in measurements and calculations.]: 1.953 mol × 22.414 l/mol = 43.75 l
  4. Per bath bomb (divided into 8):
    Mass of CO2: 85.90 g / 8 = 10.74 g
    Volume of CO2: 43.75 l / 8 = 5.47 l

Why no danger arises: The amount of released carbon dioxide is harmless. Even with multiple bath bombs, the gas disperses quickly in the room or dissolves in the water. It is neither toxic in these amounts nor poses a risk of dangerous accumulation.

Fig. 1: Sodium bicarbonate (NaHCO3, commonly known as baking soda) and citric acid (C6H8O7)

2. Stability and Binding

The addition of cornstarch (C6H10O5) contributes to the mechanical stability of the bath bomb and reduces the reaction rate. Coconut oil (primarily composed of triglycerides such as lauric acid triglyceride) serves as both a binding agent and a nourishing substance. Coconut oil, in addition to these functions, plays a crucial protective role for the reaction mixture. Since the mixture of sodium bicarbonate and citric acid is highly hygroscopic, it would prematurely react upon absorbing moisture from the air, compromising the shelf life of the bath bombs. During production, melted coconut oil forms a thin coating around the powdered components, effectively protecting them from moisture.

The low melting point of coconut oil, at approximately 23–26 °C, ensures that this protective layer dissolves in the warm bathwater. This allows the dry powder components to come into contact with water, initiating the desired acid-base reaction and producing the characteristic fizzing effect. This makes coconut oil an essential component that ensures both functionality and shelf life of the bath bombs.


Materials and Chemicals

Required Chemicals:

  • 250 g sodium bicarbonate (NaHCO3) – the base of the reaction.
  • 125 g citric acid (C6H8O7) – the acid that reacts with baking soda.
  • 60 g cornstarch (C6H10O5) – serves as a stabilizer and binder.
  • 60 g coconut oil – acts as a binder and nourishing component.

Experimental Protocol

1. Mixing the Dry Ingredients:

In a large mixing bowl, baking soda (NaHCO3), citric acid (C6H8O7), and cornstarch (C6H10O5) are carefully mixed. The mixture should be homogeneous to ensure a uniform reaction.

Fig. 2: Mixing of dry (water free) substances

2. Melting the Coconut Oil:

Coconut oil is melted in a water bath until fully liquefied. The temperature should not exceed 30–35 °C to prevent premature reactions during mixing.

3. Combining the Components:

The melted coconut oil is gradually added to the dry mixture while continuously stirring to prevent clumping. The consistency should resemble damp sand – moldable but not liquid. If desired, essential oils (e.g., lavender oil) or food coloring (water free) can be added at this stage. These additives should be used sparingly to maintain the chemical balance.

Fig. 3: After adding the melted coconut oil. During the addition of fragrance oil.

4. Shaping the Bath Bombs:

The prepared mixture is pressed into molds. Silicone molds, egg cups, or other small containers can be used. The mixture is pressed firmly but not excessively to avoid crumbling.

5. Drying:

The shaped bath bombs must dry for at least 24 hours in a dry place to harden fully. Once dried, they can be removed from the molds.

Fig. 4: Fully hardened bath bomb

Results and Observations

The produced bath bombs exhibit a distinct fizzing reaction upon contact with water, due to the release of carbon dioxide. Additives such as essential oils and dyes do not significantly affect the reaction rate or the stability of the final products.

Typical reaction parameters:

  • Optimal mixture: 2:1 ratio of baking soda to citric acid
  • Reaction duration in water: 30–60 seconds
  • pH value of the solution: slightly acidic (approx. 6)

Discussion

The production of bath bombs illustrates fundamental principles of chemistry, particularly the reactivity of acids and bases. The choice of binding agents (e.g., coconut oil) and stabilizing components (e.g., cornstarch) influences both the shaping process and the shelf life of the products. Future experiments could focus on optimizing the reaction rate by varying the particle sizes of citric acid or using alternative acids (e.g., tartaric acid). Additionally, experiments could explore the incorporation of sodium lauryl sulfate (SLS) as a foaming agent, allowing for the creation of foam bath bombs. SLS could enhance the bathing experience by producing stable, long-lasting foam while maintaining compatibility with the skin when used in appropriate concentrations.

Safety
Citric acid is a weak acid that can irritate the eyes. Use safety goggles.




  • 1
    [An acid-base reaction is a chemical process in which an acid donates a proton (H+) to a base, which accepts it, often resulting in the formation of water and a salt.]
  • 2
    [Ionic components are charged particles (ions) that form when atoms or molecules gain or lose electrons, consisting of positively charged cations and negatively charged anions.]
  • 3
    [A mol is a unit of measurement in chemistry that represents 6.022 × 1023 particles (atoms, molecules, or ions) of a substance, known as Avogadro’s number.]
  • 4
    [Mass in Gramms]
  • 5
    [Volumne in Liters]
  • 6
    [Standard conditions refer to predefined reference values in chemistry, typically 0°C (273.15 K) and 1 bar (100 kPa), used to ensure consistency in measurements and calculations.]