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Name: Jennifer J.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 12/9/2004

I teach 8th grade science, and many experiments in the book call for the use of ethanol. Can rubbing alcohol be used? If not, where can I get ethanol?


Pure ethanol is wonderfully versatile: you can use it for flame heating, cleaning and degreasing, sterilizing, drying out wet things, dissolving some chemicals water cannot, boiling at a lower temperature than water, and more other things than I can think up right now. That is probably why they featured it in your textbook. One simple, low-toxicity reagent to use for so many things. You can get it as technical-grade or reagent-grade 100% ethanol from chemical distributors. I'm not sure of the rules, but your being a teacher may make them willing to send it to you.

Any given thing ethanol does can be substituted by something. But there are a few different somethings, for different uses. Learning a little about solvent properties, you can probably pick the right ones for each experiment.

For burning, standard rubbing alcohol is very marginal, because it's 30% water, only 70% isopropanol. You can find 91% isopropanol rubbing alcohol, and that will burn well and be adequate for some other things. It's still 9% water, so it won't dry wet things as well as 100% of some solvent: (ethanol or isopropanol or acetone). The vapor flammability hazards of 91% isopropyl are a bit less than those of 100% ethyl alcohol, because the vapor pressure is lower. 70% isopropyl, much lower.

Isopropanol has a higher boiling-point / lower vapor-pressure than ethanol, methanol, or acetone so it might not work well in a "drinking bird" thermal apparatus. But you could try it. It also does not cool things as aggressively when it evaporates.

You can find denatured ethanols and/or 100% isopropanol in solvent cans at some paint or hardware stores. It's 100% solvent and should burn great and extract water thoroughly. Denaturant additives (~10% concentration) are often like acetone, which can dissolve many plastics. There are many plastic things you will not want to wipe down with them or store them in. It can ruin a shiny finish, or gradually weaken some plastic containers. The fumes are a little more unpleasant, too. The flammability risk is a little higher. The price/volume is low in gallon cans.

Anything mixed might not have a single, simple boiling point. 70% rubbing alcohol does. It is an azeotrope: the right percentage mixture so the water and alcohol boil together at the same time. So you cannot distill them apart, which is why 70% is the standard concentration. It's a whole 'nuther job for the manufacturer to go to higher percentage. There is only one azeotropic mix for each pair of solvents, so the 91% grade of rubbing alcohol will not have a single boiling point. Instead it will try to evaporate isopropanol vapors and leave 70/30 liquid behind in the vessel.

30% water is substantial for solvent solubility properties. There are some greases or waxes that will not dissolve as well in rubbing alcohol as in pure ethanol. And it can take a little longer for rubbing alcohol to dry than pure ethanol.

People do use ultra-distilled vodka (ethanol with as little as 10% water) for some things. More often, I use rubbing alcohol, 70% or 91% as needed.

Jim Swenson

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