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Name: Lisa
Status: student
Grade: 9-12
Location: NM
Country: USA
Date: September 2008


Question:
Can I use 70% Isopropyl Rubbing alcohol in a alcohol burner used in a chemistry kit with the same result (temperature) as I would get using ethyl alcohol or denatured alcohol?



Replies:
Lisa,

Temperatures of alcohol burners are anywhere from 600-900 deg-C. The heat produced is due to the alcohol burning and is independent of any impurities (such as water) which do not burn. Some of the water will absorb some of the heat and evaporate and this will reduce the temperature somewhat - so you may be at the lower end of that temperature range. If your experiments have to do with temperatures that have to be on the upper end of that temperature range (like melting certain metals), this might affect your experiments. However, for most of the work you will be doing, I do not think you will notice any difference.

The only problem I can foresee is that as the alcohol burns off and more of the water is retained (not all of it will evaporate with the alcohol), the liquid in the burner will have more and more water and after a while the water will start to absorb the heat produced and may even extinguish the flame. At this point the liquid in the burner is probably too rich in water and you will have to replace the fluid.

Greg (Roberto Gregorius)


70% isopropyl alcohol works, nut not as well. The extra water forces the flame to expend energy vaporizing water, which is not compensated by energy liberated in combustion. Often the wick will become enriched in water as the alcohol burns away, leaving excess water behind. He net result is that the burner burns feebly and may give up the ghost entirely after a while. I have learned this by sad firsthand experience.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy


Sorry, Lisa. 70% isopropyl (30% water) burns, but just barely. First the water almost chokes-off the reaction, then its steam dilutes the hot gas products.

The peak temperature near the tip of the flame (in degrees C) is inevitably a bit lower, for that fuel. The net power of the flame (in watts) is probably a lot lower, because the energy cost of evaporating the 30% water subtracts from the backwards heat flow into the wick, that would otherwise be available to evaporate more fuel into the flame. In regard to power, basically the flame is weaker or "smaller", much like the effect a shorter wick would have. So you might be able to compensate for the water by having a larger wick. This would get back the watts, but not the degreesC. The reduced peak temperature would be because of thermal dilution, the need to heat up a bunch of inert gas, namely the largish volume of steam that comes from the 30% water which evaporated previously at the base of the flame.

Reagent ethyl alcohol and denatured alcohol are both 100% fuel, < 1% water. Nothing slowing them down or diluting them like that 30% water does to rubbing alcohol. Even 90% ethyl alcohol is far better off than 70% isopropyl.

If you are playing at home with such burners, do not underestimate the danger of starting a fire if they spill, especially if trying these other fuels. The only place I feel comfortable running them is in a kitchen sink.

Jim Swenson


Usually 90% isopropanol, available at pharmacies, is usually used in place of ethanol. The flame temperature compared to ethanol is difficult to predict. It depends upon the design of the burner.

Vince Calder



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