泡澡球

3 NaHCO₃ + H₃C₆H₅O₇ → Na₃C₆H₅O₇ + 3 CO₂ + 3 H₂O

bicarbonate needed = citric acid × 3 × M(NaHCO₃) ÷ M(citric acid)
                   = citric acid × 1.3118
CO₂                = (citric acid consumed ÷ 192.124) × 3 × 44.009
liquids %          = Σ liquids ÷ total × 100
colour %           = colour ÷ total × 100          # held to its supplier's maximum
bombs              = ⌊total ÷ (V × ρ)⌋            # V = π/6 × d³

• Neutralisation of citric acid by sodium bicarbonate: 3 NaHCO₃ + H₃C₆H₅O₇ → Na₃C₆H₅O₇ + 3 CO₂ + 3 H₂O. Molar masses NaHCO₃ 84.007 g/mol, anhydrous citric acid 192.124 g/mol, CO₂ 44.009 g/mol (IUPAC 2021 standard atomic weights).
• The craft’s 2:1 bicarbonate-to-citric-acid ratio by weight. A convention, and deliberately bicarbonate-rich: the surplus is filler and buffer, not reagent. — Not the stoichiometric ratio, which is 1.312:1.
• Working convention: total liquid ingredients around 1–4 % of the mix. Below it the bomb crumbles; above it the reaction starts in the bowl. — Strongly dependent on humidity, on whether the liquid is water-based or anhydrous, and on how fast it is mixed in. A habit of the craft, not a measured threshold.
• Only a colour permitted for cosmetics: in the EU and Great Britain one listed in Annex IV of Regulation (EC) No 1223/2009 and used under its conditions (Art. 14(1)(c)); in the US one listed for cosmetics in 21 CFR 73 Subpart C, or in 74 Subpart C from certified batches. How much goes in is its supplier’s figure: 21 CFR 73.2496 lets mica colour cosmetics “in amounts consistent with good manufacturing practice”, with no number. — Micas, oxides, lakes and dyes are used at rates too far apart for one figure to stand for them, so the engine has none of its own.
• Packing density of a pressed bath bomb, ≈ 1.1–1.4 g/mL depending on press force. The default assumes hand-packed spheres. — Calibrate by weighing one finished bomb from your own mold.