Kids, let"s watch the ways in which food coloring can move through different liquids. You"ll need 3 clear plastic cups, water, 4 teaspoons of salt, seltzer water, and food coloring. 

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

Fill two of the cups 2/3 full with water. Add 1 drop of food coloring to the first cup and immediately observe what happens. You can use a worksheet to draw your results. To make the worksheet, first draw three simple cup shapes on the paper, indicate the liquid level by drawing a wavy line 2/3 of the way up, and label them plain water, salt water, and fizz water. Add the salt to the second cup and stir until the salt dissolves. Add one drop of food coloring to the this cup and immediately observe, and again draw what you saw. Now fill the third cup 2/3 full with seltzer water, add 1 drop of food coloring, observe, and draw.

What happened? In plain water the drop slowly swirls and moves throughout. In salt water, the drop starts to sink and then rises. In the fizz water, the drop quickly disperses and evenly colors the liquid. Why? Putting food coloring in plain water does not have a dramatic effect other than that the color becomes more pale (diluted). The gas bubbles in the fizz water act to speed things up, like an invisible stirring spoon. The drop of food coloring is quickly broken up and carried to all parts of the liquid. Salt water is more dense than plain water. This means that anything less dense will float on the top, including the food coloring (which is a drop of colored water).

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
March 1999

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Reference: Marlisa Ebeling (primary level teacher, Naperville district 203).