Re: Why do dyes travel at different rates on flower experiment

Hi Malcolm,

First, let’s go over how the water gets from the glass into the flower. There are several small openings in the flower, too small to see without a microscope, called stomata. Water inside the flower petals evaporates out of those openings, and creates kind of a suction that pulls more water into the petals from the stem. In turn, that pulls water from the glass into the stem. It may be strange to think of pulling water, but this is possible because water molecules like to stick to one another. You may have heard of surface tension, which also results from water molecules tending to stick to one another. So evaporation of water from the petals literally pulls water into the stem. The food dye molecules don’t really get pulled to the same degree, they’re just along for the ride.

Each dye is a different molecule with its own characteristics. The most obvious difference between them is the color that they appear, and that is due to the fact that they absorb different colors of light. Not so obvious is that each food dye will have a different molecular weight. It’s also possible that they each have a different affinity for water. In other words, some of them might stick to water molecules a little more than others.

If a food dye is a small molecule, and it tends to stick to water molecules, then it will quickly be drawn into the flower as the plant takes up water from the glass. If a food dye is a larger molecule, it won’t get taken along for the ride quite as fast as the small molecules.

It’s analogous to rocks in a stream. Very small rocks can easily be pushed along by the current, but larger rocks will be much harder to move.

Dr. Alex Brands