# Specific Gravities of liquids

**URL:** <https://boards.straightdope.com/t/specific-gravities-of-liquids/855568>\
**Category:** Factual Questions\
**Created:** [June 12, 2020, 10:09pm UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568 "2020-06-12T22:09:38Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![DrForrester](https://avatars.discourse-cdn.com/v4/letter/d/8edcca/32.png) [@DrForrester](https://boards.straightdope.com/u/DrForrester)\
**Post date:** [June 12, 2020, 10:09pm UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568/1 "2020-06-12T22:09:38Z")

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Does anyone know if the Specific Gravities of liquids are linearly additive for mixtures?

I know that volumes are not. I am hopeful that Specific Gravities are, because my company’s accounting software thinks that they are… but, I have not been able to establish this as a fact.

Thanks for any information!

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**Author:** ![octopus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/octopus/32/3716_2.png) [@octopus](https://boards.straightdope.com/u/octopus)\
**Post date:** [June 12, 2020, 10:16pm UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568/2 "2020-06-12T22:16:20Z")

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If volumes are not it seems specific gravity wouldn’t be since if volume changes wouldn’t density?

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [June 12, 2020, 11:22pm UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568/3 "2020-06-12T23:22:40Z")

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It would be immensely surprising if specific gravities were additive. That would imply that if you took equal parts of DHMO (specific gravity 1) and hydroxic acid (specific gravity 1), you’d get a mixture with a specific gravity of 2.

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**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [June 12, 2020, 11:39pm UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568/4 "2020-06-12T23:39:50Z")

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> [@DrForrester](#):
>
> I know that volumes are not. I am hopeful that Specific Gravities are …

If volumes are not, then it implies that specific gravity doesn’t scale either.

If the volume is additive (and many times it is) then the equation is :

Dmix = 1/{(xa/Da)+ (xb / Db)}

Dmix = density of mixture

xa and xb = masss fraction of component a and b, xa + xb = 1

Da = Density of liquid a  
Db = Density of liquid b

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<div class="post-metadata">

**Author:** ![DrForrester](https://avatars.discourse-cdn.com/v4/letter/d/8edcca/32.png) [@DrForrester](https://boards.straightdope.com/u/DrForrester)\
**Post date:** [June 13, 2020, 1:08am UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568/5 "2020-06-13T01:08:59Z")

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> [@am77494](#):
>
> If volumes are not, then it implies that specific gravity doesn’t scale either.

Assuming this is the case, is there a way to calculate it?

> [@am77494](#):
>
> If the volume is additive (and many times it is)

This is immensely reassuring. 🙂 I hope that it is often “within a reasonable margin of error”.

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<div class="post-metadata">

**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [June 13, 2020, 1:57am UTC](https://boards.straightdope.com/t/specific-gravities-of-liquids/855568/6 "2020-06-13T01:57:59Z")

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> [@DrForrester](#):
>
> Assuming this is the case, is there a way to calculate it?

Yes there is. But it is specific to the liquids, the quantities being mixed and the temperature.

> [@DrForrester](#):
>
> This is immensely reassuring. 🙂 I hope that it is often “within a reasonable margin of error”.

It really depends on what you are mixing and how much. A 10% solution of alcohol is okay, 65% is not. Gasoline and Diesel, is always okay. (Okay means the density can be scaled per the equation above)