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Experiment

The Bath Bomb Fizz Lab

Mix baking soda and citric acid into a simple bath bomb, then drop it in the tub to watch it fizz apart.

Ages 6–1025–45 min1–3 kids

Mix baking soda, citric acid, and a few drops of water into a dough that holds its shape when pressed into a mold, then let it dry before dropping the finished bath bomb into a tub of water to watch it fizz and dissolve. Compare a few different mold shapes and colours across attempts.

What you need

  • baking soda
  • citric acid
  • a small spray bottle of water
  • food colouring (optional)
  • a small mold (a cup or ice tray works)

How to do it

  1. 1Mix baking soda and citric acid together in a bowl, roughly three parts baking soda to one part citric acid.
  2. 2Lightly mist the mixture with water, a spray or two at a time, stirring until it just holds together when squeezed.
  3. 3Press the mixture firmly into a small mold and let it dry for at least an hour.
  4. 4Drop the finished bath bomb into a tub of water and watch it fizz apart.

Variations

  • If drying time feels like too long a wait: a version pressed straight into a cup and dropped in wet still fizzes, just less dramatically.
  • How it ends: the full fizz-out in the bath is the natural, satisfying finish.

💡 Parent tip

Citric acid is available in the baking or canning aisle of most grocery stores if it isn't already in the kitchen.

What your child develops

  • Acid-base chemistry — the same reaction as the volcano lab, applied to a self-care product this time
  • Precision measuring — getting the ratio right takes real attention to proportion
  • Delayed gratification — waiting for the bomb to dry before testing it builds patience toward a payoff

Safety notes

  • Citric acid can irritate eyes and broken skin -- avoid contact and wash hands after mixing. Not intended to be eaten.

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