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21386Re: Specific Gravity

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  • Harry
    Aug 12, 2006
      Ok, I thought I may be able to give a simple answer to a simple
      question, being as how this is new_distillers & we don't want to
      confuse people with highly technical stuff that really ain't
      necessary to hobby distilling.

      However, it appears I was hoping for too much. So everyone wants
      this thing to be accurate to the nth degree (why I don't know. It
      don't matter a hill o' beans in our small setups).

      First of all, the Specific Gravity of Ethanol or Water is variable,
      depending on the prevailing temperature of the liquids at the time
      of measurement. So any declaration of the 'SG' of these substances
      should really be made with an attached reference to the temperature
      of the samples. For instance, the density of water at 4°C is 1 g/ml.

      The specific gravity of pure ethanol is 0.7939 and that of
      pure water is 1.000.

      So here's a problem for you to ponder...

      Determine the expected density of 70% v/v ethanol in water, if the
      volumes were additive. Compare the specific gravity obtained to
      actual specific gravity of 0.89. Does the solution contract or
      expand on mixing? Consider 1 ml of the 70% ethanol mixture. It
      contains 0.7 ml ethanol and o.3 ml water if the volume on mixing is
      assumed additive.

      Mass of the 0.7 ml ethanol = Volume Density = 0.7 x 0.7939 = 0.5557 g
      Mass of the 0.3 ml water = 0.3 g
      Total mass = 0.5557 + 0.3 = 0.8557 g
      Density = Mass/Volume = 0.8557 g/1 ml = 0.8557 g/ml
      Since the actual density is greater than the expected density, the
      mixture contracts.

      For all intents & purposes, the calculations that Gert Strand
      provided on the link I gave, are adequate for our hobby needs. If
      you want to split hairs, or get laboratory-grade calculations &
      results, then by all means, do the math to the nth degree. The
      margin of error over the calcs that Gert & I gave are so small as to
      be irrelevant.

      We are looking for simple calcs (at least the original
      poster/questioner was). Lab-grade 3 decimal point correctness is
      not warranted in this situation. But of course there will always be
      those who demand to know that if a thing is white, then just what
      shade of white is it? And there will always be some who wish to
      take issue with my generalisations. Oh well...life is short.

      regards Harry
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