The Frozen Vinegar Rainbow Fish is a kitchen-science sensory activity built around The Rainbow Fish by Marcus Pfister. Pour vinegar into a fish-shaped silicone ice cube tray, tint each cavity with food coloring, freeze. Set the frozen fish on a tray of baking soda, drip a little water, watch the fish fizz, bubble, and slowly melt into colored puddles. The Rainbow Fish from the book comes to life as a tiny chemistry demo.
It works because vinegar (acidic) and baking soda (alkaline) react fast when they meet in water, releasing carbon dioxide gas. The bubbles are the part that holds the kid’s attention; the rainbow paint that lifts out of the melting ice is the souvenir.
What goes in the kitchen:
- A fish-shaped silicone ice cube tray. Star, heart, or any fun-shape tray works too; fish keep the Rainbow Fish theme intact. Mini muffin tins also give a usable shape.
- White vinegar. About half a cup is enough for one tray.
- Food coloring in five or six colors. Liquid gel coloring gives the brightest fish. A few drops of acrylic paint per cavity is the fallback if you run out of food coloring partway through (we did, twice; paint fish melt slower than dye fish, which became a useful side note).
- Baking soda. The cheap supermarket kind. About one cup spread across a tray.
- A shallow tray with a rim, big enough to hold the baking soda in a layer and the fish on top. A baking sheet with sides works.
- A small jug of water and an extra splash of vinegar for the second round of the reaction.
- A few small kitchen utensils for the kid: a teaspoon, a small whisk, a fork.
Freezing fish in five colors
Pour vinegar into the ice cube tray, filling each fish cavity about three-quarters full. Place the tray in the freezer for about thirty minutes, until the vinegar is partly frozen (slushy on top, solid underneath). Take the tray out and drop a few drops of food coloring into each cavity. Stir gently with a toothpick to mix the color into the slush. Return the tray to the freezer for another two to three hours until fully solid.
Two practical notes from when we did this. First, food coloring will float to the top of fully-liquid vinegar, which is why the slush stage matters: the slush traps the color uniformly through each fish. Second, do not leave the frozen fish at the bottom of the freezer for days before you use them. Vinegar has a strong smell, and ours flavored the rest of the freezer’s contents. Use within twenty-four hours.
Bubbles in a baking-soda sea
Spread the baking soda across the tray in an even layer, about one centimeter thick. Lay the frozen fish on top in whatever pattern the kid likes (we always did a row, then a school, then a rainbow arc). Let the kid drip water from the small jug onto the fish.
The reaction starts as soon as the water hits the vinegar inside the melting fish. Bubbles rise from the baking soda around each fish. The melting drags color across the soda; by the time the fish are fully gone, the tray is a watercolor wash of pink, blue, green, yellow.
If the kid wants a louder version, pour a splash more vinegar straight from the bottle onto the half-melted fish. The reaction visibly speeds up. Foam comes up where the colored puddles meet undisturbed baking soda. The whole thing fits within a single tray and is over in about fifteen minutes.
For more painted-character book activities, our Penguin Story Stones use the same kid-meets-book-character logic but with painted pebbles instead of frozen ice. And for paper versions of the underwater world the rainbow fish lives in, our mermaid coloring pages cover fish, sea plants, and mermaid characters in printable line-art for a calmer afternoon.
Cleanup is one big rinse. The baking soda and the colored vinegar puddles all flush down the drain (the same chemistry takes care of any greasy buildup in the pipes, a small bonus). The silicone fish tray washes by hand; the bigger tray gets a quick wipe.
We’ve done a few variations since. A pirate sea version uses just blue and black. A rainbow-storm version uses purple and white. The book stays on the shelf next to the kitchen and comes out before the activity, which makes the connection clear to the kid: the fish in the book turned into the fish on the tray.