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Sorry if this question has a simple answer, but I've been unable to find it elsewhere.

Using a diffraction grating, burning ethanol (99.9%) will produce a rainbow of colours. However, when burning a mixture of alcohols (methanol 76.6%, ethanol 21.4%, propanol 0.3%) the rainbow pattern is missing. Could someone explain?

If methanol only emits a small range of wavelengths, why wouldn't there be a dimmer ethanol rainbow?

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    $\begingroup$ Please clarify "However, when burning a mixture of alcohols (methanol 76.6%, ethanol 21.4%, propanol 0.3%) the rainbow pattern is missing". What do you see instead, and how is ethanol being burnt? Is there a wick? $\endgroup$ – M. Farooq Jun 18 '19 at 1:28
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    $\begingroup$ Is this the X-ray emission spectrum of ethanol you are asking about? $\endgroup$ – Mathew Mahindaratne Jun 18 '19 at 3:20
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    $\begingroup$ This is hard to believe. $\endgroup$ – Alchimista Jun 18 '19 at 10:23
  • $\begingroup$ Sorry, to clarify. I'm working in an elementary school. We are simply holding up a diffraction grating between the flame and the naked eye. With ethanol, the students would see 8 elongated rainbow patterns. But with the mixture (bought as fuel for spirit lamps), there would simply be 8 flames (blue/orange) without the elongation or rainbow pattern. There is a possibility that the rainbow is present but simply too faint to be detected by the naked eye. When the students are looking at the flames, they are in spirit lamps. However, when I was testing, I poured some in a petri dish and ignited it $\endgroup$ – David Beggs Jun 19 '19 at 0:20
  • $\begingroup$ Do exactly what you will do in class. There might be a difference in flame temperature but I would expect to see more or less the same. Also avoid the term rainbow if not specifying other characteristics and taking it as synonymous of dispersed light. Otherwise they might be confused between refraction reflection diffraction. ... $\endgroup$ – Alchimista Jun 21 '19 at 13:52

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