Overview

This experiment shows how certain substances can change the surface tension of water — and when they dissolve unevenly, they can actually push themselves across the surface. The result looks like tiny, fast-moving “color boats” zooming around your bowl.

Materials

You will need:

A few drops of liquid food coloring
A small plate or saucer
A knife, fork, or toothpick for scraping
A bowl filled with water

Preparation

1. Dry the Food Coloring

Place several drops of food coloring on a plate.
Leave it in a sunny or warm spot until completely dry.

It should form a thin, brittle layer you can scrape.
If it still smears, let it dry longer — it must be solid.

Procedure

1. Scrape the Coloring

Use the tip of a knife or fork to gently scrape up tiny chunks or flakes of the dried food coloring.

You only need a few pieces the size of crumbs.

2. Drop the Pieces Into Water

Place the bowl of water on a table.
Carefully drop a dried piece (or two) onto the surface.

Now watch closely.

What You’ll See

Some pieces will sink.
Some will float without moving.

But a few pieces will suddenly…

➡️ Zip across the water
➡️ Leave trails of color behind them
➡️ Change direction as they dissolve

They look just like tiny motorboats sailing around the bowl.

This effect only lasts a few seconds — be ready to observe!

The Science: Why Do They Move?

Surface Tension

Water molecules pull tightly together at the surface, forming an invisible “skin.”
This tension keeps light objects (like pepper or dried color flakes) on top.

Food Coloring Can Weaken Surface Tension

When the dried coloring touches water:

The color dissolves
The dissolved coloring reduces surface tension in the nearby area

Uneven Dissolving Creates Movement

This part is key.

The dried pieces are not perfectly smooth.
They dissolve more on one side than another.

Where more coloring dissolves, the surface tension becomes weaker.

Water with stronger surface tension pulls away from the weaker spot, dragging the piece with it.

So the piece moves because:

One side has weaker surface tension
The opposite side has stronger surface tension
The stronger side pulls the flake like a tiny invisible rope

This is very similar to how the pepper moved in the classic soap-and-pepper experiment — but here, the color flakes create the effect on their own.

Why Only Some Pieces Move

Some dissolve too slowly
Some dissolve too evenly
Some sink because they’re too heavy

Only pieces with the “right” thickness and shape move dramatically.

Extension Activities

Younger Grades

Predict which pieces will move
Try different colors
Make a chart of “sink / float / zoom”

Older Grades

Compare different brands of food coloring
Try drying the colors in thin vs. thick layers
Test whether other substances (drink mixes, soap flakes, sugar coatings) create motion

Middle School Challenge

Explore the Marangoni effect — the scientific name for liquid flow caused by differences in surface tension.

Color Boats: How Dried Food Coloring Makes Water Move