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capillary action with colors and variables

Postby jameslang » Wed Dec 09, 2015 3:19 am

My 3rd grader is using capillary action with variables such as sugar, salt, and vinegar with food coloring added to track. Question? Why did the vinegar (after moving through towel) come out clear while others still kept color. Can she use this in her project

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Re: capillary action with colors and variables

Postby lynnsamuelson » Wed Dec 09, 2015 12:02 pm


That is an interesting result and to determine whether it is useful to your daughter's project depends on what she is looking to report on. In short, the answer is in the chemical properties of vinegar and possible the towel used. To break it down more, I'm going to assume the following procedure.

1) She made solutions of sugar, water and food coloring; salt, water and food coloring; and vinegar, water and food coloring.
2) She used some type of towel (or other material) to examine capillary action. That towel sucked up the solution and dripped it out into a different container.

From this, she observed that the liquid coming out of the towel was colored for the sugar and salt solutions, but not for the vinegar.

With that observation, then the loss of color is likely due to the pH of the vinegar (it's acidic) and the affinity of the food coloring to the towel. In an acidic solution, the molecule (dye) used in the food coloring is changed. This changes the properties of the dye. Based on your observation, the property change makes the dye stickier the towel. In more technical terms, the excess protons in the vinegar solution are interacting with the dye molecules (this is called protonation). The protonated molecule has a higher affinity to the material in the towel then the unprotonated.

In terms of whether she can use this, it depends on how much detail she wants to go into. She could research material type/solution pH on capillary action.

Just in case you are wondering, the loss of color should not affect the capillary action or rate. If you wanted to test this, run the 3 solutions without any food coloring.

Thank you for sharing this cool observations and Good Luck with the project.


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