Diffusion of alcohol through a gel

[Sjmorris513]

Hi there - I am writing a science activity for elementary students for a STEM camp focusing on the science of slime. The activity involve demonstrating diffusion. I made clear slime with borax and glue, please a small amount in a petri dish and added a drop of food coloring to the center of the dish. The campers will watch and measure the color as it diffuses over a period of time.

For kicks, I decided to repeat the experiment using some alcohol ink I have on hand. When I added a drop of the alcohol ink, it spreads a small amount very quickly and then stops. What's the chemistry behind the different behavior?

Additionally, I filled a plastic lid with slime and added several drops of different colored alcohol ink. When two drops of also ink come in contact with each other they mix quickly.

Of you can help me understand the chemisrey behind this I would certainly appreciate it!

Thank you!

[mbdowell]

Hi Stephanie,

I love your experiment! Since it works with food coloring, you've already got a winner and just want to diagnose why extending the experiment to ink didn't work. When you dilute the ink, are there particles in it? Is it cloudy and you can see the light path when you shine a light beam through it? If so, I have an idea for you.

Best regards,

Mike Dowell

[scooke]

Dear Stephanie,

One simple "rule" in chemistry is that "Like attracts like".  Apart from its other interesting features, "slime" mixtures are basically water solutions.  Thus, anything soluble in water will be soluble in slime.  Food coloring is already water soluble, so the dye diffuses fairly rapidly and evenly through the gel.  Alcohol-based colors on the other hand may only be slightly soluble in water, if at all.  So, the alcohol-based inks are not likely to diffuse as quickly or as far as the food dye colors.

Best regards,

Steven Cooke

[Sjmorris513]

Thank you for your response to my question! i just diluted some alcohol ink in some water. The ink seems to stay at the top and it looks like there are small particles floating. When I shine a beam of light through, it is diffused like the Tyndall effect.

[Sjmorris513]

Thank you for your help. When I drop the alcohol ink onto the slime, it spreads very quickly for about 1.5 centimeters and then stops spreading. The food coloring does not spread quickly but it's slow and steady... continuing for a few days until its completely diffused.

[mbdowell]

Hi Stephanie,

     It looks as though inks create problems for your diffusion experiment but you should keep them in mind if you want to cover related topics. Your ink really is an ink; it's a finely ground colored pigment dispersed in alcohol as a vehicle. The solid pigment doesn't diffuse, but the alcohol can go off fast into the slime and leave the pigment particles stranded. Dyes are better than pigments in general, for your experiment, because dyes are true solutions. I hadn't thought of Steven's point that alcohol dyes may precipitate the solute when they are placed in water. If that happens when you try it, you can tell your students about it and tell them that some chemists are "formulators", who do these experiments and come up with mixtures and procedures that are just right for a given job. But you are probably OK with most alcohol dyes to deliver a drop of color to slime; there's a good chance that the solute dissolves in PVA.

     What I really like about your experiment is that you make a gel and diffuse color in that. It's nice when a science teacher talks about diffusion! It's a pretty slow process and is somewhat slower in gels than in solutions. By making a gel, you stop several process that spread or transport color faster than diffusion does, so what your students see really is diffusion and not another transport process.. Surface tension is faster; an alcohol dye spreads outward on a water column because of that, and then dissolution brings the color down. Gravity is faster than diffusion; if you drop a mixture of aqueous food coloring and sugar into water it sinks faster than it spreads. If you heat the water from the bottom, convection does a great job of mixing. You have a very clean experiment! Of course, if you ever want to show your students other transport processes you can do that too. Spotting the dominant transport process is a great game for budding chemical engineers.

Best regards,

Mike Dowell 

[SSSeelig]

Hi Stephanie,

For added fun, you may want to add fluorescent, pearlescent and glow-in-the dark (luminol) pigments dissolved in water.  The addition of sparkle power also has a great effect.  Maybe if you play with minor changes in pH, you can use indicators to change the color of your slime.  I tried some of these when my kids were younger.  We even took a plastic model of a brain and engulfed it in slime for a scary effect. 

Best regards,

Stan Seelig