The Chemistry of Bath Bombs – And How to Make Them Better
When you drop a bath bomb into your tub and watch it fizz, swirl colour, release fragrance and turn your soak into a sensory experience, you’re witnessing real chemistry in action. In this blog we’ll explore how bath bombs work (the chemistry behind them), and then dive into how to make them better, better fizz, better skin feel, better aesthetic and safer for you.
1. How Bath Bombs Work: The Science Behind The Fizz
The key reaction
At the heart of every bath bomb is an acid-base reaction. The most common ingredients: a weak base (typically sodium bicarbonate, aka baking soda) and a weak acid (commonly citric acid). When dry, these two coexist relatively inertly; when introduced into water, the acid and base dissolve, react and produce carbon dioxide gas (CO₂) — which gives the dramatic fizz.
To scientists this is what the chemical reaction looks like:
In essence when citric aside and sodium bicarbonate combines , the CO₂ escapes as bubbles, breaking apart the bath bomb and releasing the other ingredients (oils, colourants, fragrance) into the bath. 
Role of secondary ingredients
- Cornstarch (or similar filler/stabiliser): Slows the reaction, giving longer fizzing rather than a sudden burst. It also helps hold the bomb together until use.
- Moisturisers and oils: These are released during fizzing and enhance skin feel (softness, hydration). They also help bind the dry mix when making the bomb.
- Salt (e.g., Epsom salt, MgSO₄), butters, butters, colourants, fragrance/essential oils: These are “bonus” ingredients to boost sensory experience, skin effect or visual appeal.
- Colourants & dyes: Give the “wow” of colourful bath water, but must be chosen with skin safety in mind.
- Binder / little water or oil / witch hazel spray: Enough to make the mixture hold shape, but too much moisture will trigger the fizz prematurely and ruin the bomb.
What makes the fizz happen (and what affects it)
- The rate of fizzing is influenced by water temperature: warmer water dissolves the acid and base faster → more rapid reaction.
- The ratio of acid to base and total dryness of the mixture matters. If too much filler (cornstarch) or oil, the reaction may be weak or delayed.
- The drying and storage of the finished bomb matter: moisture/humidity in environment can trigger partial reaction prematurely (leading to bombs that fizz prematurely or crumble).
Why bath bombs feel so good
- The visual + audible fizzing adds novelty and sensory satisfaction.
- The release of oils and salts leads to a pampering feel in the bath.
- The combination of fragrance + warm water + colour creates a mood shift / self-care ritual.
So while the core chemistry is simple, the total experience is multi-sensory—making bath bombs both fun and indulgent.
2. How to Make Better Bath Bombs: Tips & Tricks
If you’re making bath bombs (for yourself or as a business), here are ways to improve fizz, stability, skin-feel, appearance and safety.
A. Get your core formula right
1. Basic ratio: A typical starting point is about 2 parts sodium bicarbonate (baking soda) to 1 part citric acid. Many sources suggest something like ~1 cup baking soda to ~½ cup citric acid.
2. Add filler/stabiliser: Perhaps ~½ to ~1 part cornstarch (or more if you want slower fizz). This helps “melt-in-water” not “explode”.
3. Oil / moisturiser / salt addition: Add oils (carrier oil or butters) to enhance skin feel; salts (e.g., Epsom) to add value; colourants + fragrance for appeal.
4. Binding moisture: When mixing, you need just enough moisture (spray of water or witch hazel) or oil so the mixture holds shape when pressed into a mould—but avoid triggering reaction prematurely. This is where many mistakes happen (bombs fizz/crumble early or don’t hold shape).
5. Drying & storage: Let bombs dry thoroughly in a low-humidity environment, then store in airtight packaging (to prevent moisture uptake) until use.
B. Boosting fizz & performance
- Reduce filler: Too much cornstarch will slow down fizz significantly (or prevent it).
- Balanced acid/base: If you have too much oil or too much binder, the reaction will be dampened.
- Consider water temperature: For at-home use, dropping bomb into warm/hot bath helps with fizz. For demonstration or photography, cooler water may slow the reaction for effect.
- Use quality moulds: Press mixture firmly and compact it into moulds; loosely pressed bombs may crumble or fizz weakly.
- Optional performance enhancers: Some makers use additives like polysorbate to prevent colour staining; but note these can alter reaction behaviour and require adjustment.
C. Improving skin-feel & aesthetic
- Choose skin-friendly oils/butters, and keep fragrance levels moderate (too much fragrance or synthetic colourants may irritate sensitive skin). The core chemistry may produce fizz, but you want the aftermath to feel good.
- Use natural / safe colourants where possible to avoid tub staining or skin irritation.
- Consider adding Epsom salts (MgSO₄·7H₂O) for added muscle-relief or spa feel.
- Add essential oils for aromatherapy benefit (e.g., lavender for relaxation, citrus for energy) — just ensure proper dilution and skin safety.
- Packaging & shape: Unusual shapes, layered colours, embed designs and “surprise centres” can make bombs more premium/fun.
D. Troubleshooting common problems
- Bombs don’t fizz much → Possibly too much filler, too much binding moisture, wrong ratio of acid/base, or triggered early reaction due to moisture.
- Bombs crumble / fall apart → Possibly too dry/not enough binder/moisture/oil; or poor compaction into mould; or humidity causing issues. > “my bombs are crumbling and dry … I need more oil or water?”
- Bombs too oily / leave oil film in tub → Too much carrier oil/butter; consider using less or using emulsifier (e.g., polysorbate) to disperse oil in water.
- Bombs crack or expand before use → Likely moisture uptake or hygroscopic ingredients (e.g., magnesium chloride) drawing water in. Dry storage is crucial.
- Colour or fragrance issues → Ensure you use colourants safe for bath use, test for tub staining, ensure fragrance oils are safe for baths. Use emulsifier if oils seem to separate.
E. Safety & skin-friendliness
- While the acid/base reaction itself is mild (given the dilution in bath water) and generally safe for many users, fragrances, synthetic dyes, high essential oil concentrations can cause irritation.
- Do a patch test if you’re using new fragrance oils or colourants on sensitive skin.
- Clean the tub after use if you used heavy colourants or glitter to avoid staining or residue.
-If making for sale, ensure compliance with cosmetic safety regulations in your region.
-Avoid confusions: bath bombs may resemble other bath/cleaning products (there have been mis-uses).
3. DIY “Better Bath Bomb” Mini-Recipe & Bonus Tips
Here’s a simple recipe plus bonus ideas to take your bath bombs to the next level.
Mini Recipe (for 2 bombs approx.)
- 1 cup (≈ 240 g) sodium bicarbonate (baking soda)
- ½ cup (≈ 120 g) citric acid
- ½ cup (≈ 120 g) cornstarch
- 2 Tbsp carrier oil (e.g., sweet almond oil)
- 1 Tbsp Epsom salt (optional)
-15-20 drops essential oil (e.g., lavender)
- ½ tsp water or witch hazel (in a spray bottle — spray lightly)
- Optional: colourant, dried petals, glitter (choose tub-safe)
Instructions:
1. Mix the dry ingredients thoroughly (baking soda, citric acid, cornstarch, Epsom salt).
2. Add the oil and essential oil; stir until mixture is uniform.
3. Lightly spray the water/witch hazel while stirring until the mixture just holds shape when pressed (like damp sand). Avoid overspray.
4. Press mixture firmly into mould(s). Pack tightly.
5. Let dry 12-24 hours in a low-humidity place. Once dry, remove from mould and store in sealed bag or container until use.
6. Drop into warm bath water and enjoy the fizz, colour, scent and skin-feel.
Bonus tips to upgrade:
- Create layered colours: divide mixture, tint each portion a different colour, layer in mould.
- Add surprise embed: small solid piece (e.g., mini bath melt) in centre for “wow” when bomb dissolves.
- Use tub-safe glitters or mica: shimmer effect, but test that it rinses easily.
- Choose skin-nourishing oils/butters: e.g., shea butter, cocoa butter, coconut oil (use wisely).
- Use a drying aid: e.g., use a fan or dehumidifier to ensure bombs fully dry—especially important in humid climates.
- Use emulsifier like polysorbate if you’re adding oils and colourants that might separate or stain. (But adjust your recipe, as this adds liquid)
- Label with ingredients and recommended bath instructions (especially if gifting or selling).
- Use eco-friendly packaging to appeal to conscious consumers.
Bath bombs are a delight because they blend chemistry (acid-base reaction, polymer behaviour of oils/fillers), sensory play (fizz, colours, scents) and self-care ritual (a soak that feels luxurious). By understanding the science behind them and the variables you can control—ingredients, ratios, moisture, drying—you can make bombs that are not just pretty but perform well and delight users.
Whether you’re making them for yourself, gifting them, or launching a side business, focus on:
- strong fizz (via correct acid/base/filler ratio)
- skin-friendly formulation (good oils, safe fragrance, gentle colours)
- aesthetics & novelty (layering, shapes, surprise centres)
- stability & storage (drying, packaging, humidity control)
- safety & tub cleanliness (avoid staining, label ingredients)
With these in mind, you’ll elevate your bath bomb game from “just fizz” to “wow experience”.
Visit our website and try out our Bath Bomb Making Kit or visit our retail store at Craftiviti, Level 2, Centrepoint Bandar Utama.