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Sugar rainbow in a straw, a density science activity

With a few glasses, a clear straw, sugar or salt, a tablespoon, water, and some food coloring, kids (and adults) can create this hands-on density science activity at home. Oregon Museum of Science and Industry (OMSI) educator Alexe leads this Sugar Rainbow science lesson, a four-minute introduction to density.

Density describes how much ‘stuff’ or matter is packed into a particular space or volume. In this case, the more sugar added, the higher the density of the solution.

adding sugar
Density also helps explain why two cubes that are the same size might have different weights, and why one floats while the other sinks. The object that’s floating is less dense than the liquid it’s floating in.

density demonstration
What happens if you turn your straw upside down or stack the colors in a different order? Some additional information from Steve Spangler Science:

So, density explains why the solutions stack on top of each other inside the straw, but what keeps the solutions in the straw? You might expect the solutions to just fall out of the straw as you lift the straw from a solution. However, because of cohesion (similar molecules attracting each other) and adhesion (different molecules attracting each other), there is surface tension sealing the water at the bottom of the straw. The surface tension is strong enough to help hold the solutions in the straw as long as air pressure inside the straw is lower than all the air pressure outside the straw. Gravity tugs the solutions downward which creates a slight vacuum in the empty part of the straw. That lowers the air pressure inside the straw which is why you need your thumb to cap the straw. This prevents air pressure from equalizing in the straw. If you remove your thumb, the air pressure equalizes, and gravity simply moves the colored solutions out.

sugar rainbow straw demonstration
The educators and creators at OMSI in Portland have made a bunch of smart science education videos. Watch them all here on OMSI’s YouTube Channel.

Follow this lesson with these related videos about density, surface tension, and floating:
• How to make an Amazing 9 Layer Density Tower
• Aluminum foil boat floating on a sulfur hexafluoride sea
• Hot & Cold Water Science Experiment
• Cardboard Boats, a DIY engineering activity
• Watch this 3,500 ton combat ship get launched sideways

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