## 21386Re: Specific Gravity

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• 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

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.

Slainte!
regards Harry
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