Why are Water and Ethanol densities specified at different temperatures?

[Whisky Distiller]

Why is the standard density of water given at 4°C while the density of ethanol is generally given at 20°C?

 

[.Scott]

At 1 atmosphere of pressure, water is most dense at about 4C.
Here is an article showing this: UTK Phases of Water Lesson

 

[.Scott]

 

[Whisky Distiller]

Hi @.Scott thank you.

Would you know why then the ethanol density is usually given at 20°C although it is most dense at negative temperatures?

 

[jbriggs444]

Hi @.Scott thank you.

Would you know why then the ethanol density is usually given at 20°C although it is most dense at negative temperatures?

The point of using 4 degrees for water is that temperature variations have a small effect on density when it is near the point of maximum density.

As far as I know, and as Google seems to confirm, there is no similar region of near-constant density for alcohol. So why not make life easy and measure it at a more comfortable temperature?

 

[.Scott]

There are two common temperature/pressure conditions used for reporting properties such as a material density:
Standard Temperature and Pressure (STP): 0°C, 100kPa
Normal Temperature and Pressure (NTP): 20°C, 101.325kPa

Unless you keep your work area very cold, NTP is more representative of the conditions where you will be using various materials.

Some sites claim that NTP was introduced by the National Institute of Standards and Technology (NIST). But I have not found anything on nist.gov confirming that.

The density of water is often specified for 4°C (actually 3.98°C) for two reasons:
1) Water is very unusual in that it has a "local maximum density" that does not occur at absolute 0 or at extreme pressures.
2) It is part of the standard relating linear measurements to mass.

 

[JT Smith]

1) Water is very unusual in that it's "minimum density" does not occur at absolute 0.

I think you mean maximum. And maybe not just very unusual but unique? Are there any other substances that have this property?

 

[jbriggs444]

I think you mean maximum. And maybe not just very unusual but unique? Are there any other substances that have this property?

Google was kind enough to auto-complete "what substances expand when they freeze".

https://xtronics.com/wiki/Expands_on_freezing.html

 

[JT Smith]

Yes, but do any of them have a maximum as a liquid, aside from at the freezing point. Water freezes from the top down. Does silicon?

I see that silicon does have a maximum as a liquid when supercooled.

 

[rcgldr]

Certain "phases" of ice (water frozen at higher pressure) are more dense than liquid water.

https://en.wikipedia.org/wiki/Ice#Phases

 

[.Scott]

I think you mean maximum.

Thanks. I have corrected my post.

And maybe not just very unusual but unique? Are there any other substances that have this property?

I wasn't sure, so I stuck with "unusual".

Certain "phases" of ice (water frozen at higher pressure) are more dense than liquid water.

I will correct to "local maximum".