Reef Chemistry Question of the Day #15

Randy Holmes-Farley

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Reef Chemistry Question of the Day #15

At typical coral reef aquarium temperatures, the specific gravity of seawater is:


A. Higher than its density
B. Lower than its density
C. The same as its density
D. Either higher or lower than its density, depending on the barometric pressure

Good luck!
 

cheezybuda

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That's kind of a hard question to answer. SG is the density compared to a reference material ( usually water).
Numerically the density is higher in standard kg/m3,but it will always directly correlate to SG.
In the end I guess you would say B....
 

tyler1503

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Hmmm I thought SG was a measurement of the salt content based on the waters density. Hence why hydrometers float at the certain point, like it's too dense to sink further without any additional force applied to it. Try putting a floating glass hydrometer in thick custard. With that said, I'll guess C.
My explanation above suggests why I didn't choose A or B. And I didn't answer D because to my (very very very limited) knowledge of barometric pressure, it is only an issue in bodies of water much deeper than a typical aquarium, so it's irrelevant in this hobby. In nature it would be a factor, but not in a 2ft deep tank.
 

jt17

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Specific gravity = density of tank water/density of water. If the density of water is 1 then they're equal. If the density of water is less than 1 then SG is greater and if the density of water is greater than 1 then SG would be less. Water has a density of 1 at 4C or 39F so it stands to reason that water at temps greater than 39F will be less dense therefore 78 degree water will have a density less than 1. So I pick A
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Nice explanation, jt17! That is exactly right. The answer is A.

Specific gravity is defined as the ratio of the density of a liquid divided by the density of water.

At reef aquarium temperatures, and in fact, at any temperature above 4 deg C (39 deg F), the density of water is less than 1 g/cm3 (= kg/m3/1000).


So, if the density of fresh water is below 1, even if only a little below, the specific gravity must be higher than the density.

The density of fresh water at 77 deg F is about 0.9970479 g/cm3.

That seems very close to 1. Maybe close enough to ignore the difference from 1?

No!

The density of 35 ppt seawater is about 1.0234 g/cm3 at 77 deg F, so dividing 1.024 g/cm3 by 0.9970479 g/cm3 gives a specific gravity of 1.0264.

FWIW, I think some early aquarists may have confused density and specific gravity, and hence recommended 1.023 - 1.024 as matching natural seawater, and didn't realize that the specific gravity (which is what people actually measure with a hydrometer, typically) was not the same as density. That, I believe, is part of the reason for many older recommendations to reef aquarists to keep "specific gravity" in the 1.023-1.024 range. At least one salt mix still makes this error on their salt bucket itself, claiming that 36.7 ppt seawater has a "specific gravity" of only 1.025 (Fluval says this).

The barometric pressure won't noticeably impact the density of either seawater or fresh water within the ranges present on the earth's surface. :)

See below for more on the density of fresh water:

http://docs.engineeringtoolbox.com/documents/595/water_temperature_density.png

water_temperature_density.png
 
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hart24601

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Specific gravity = density of tank water/density of water. If the density of water is 1 then they're equal. If the density of water is less than 1 then SG is greater and if the density of water is greater than 1 then SG would be less. Water has a density of 1 at 4C or 39F so it stands to reason that water at temps greater than 39F will be less dense therefore 78 degree water will have a density less than 1. So I pick A

Nice!
 

jt17

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Nice explanation, jt17! That is exactly right. The answer is A.

Specific gravity is defined as the ratio of the density of a liquid divided by the density of water.

At reef aquarium temperatures, and in fact, at any temperature above 4 deg C (39 deg F), the density of water is less than 1 g/cm3 (= kg/m3/1000).


So, if the density of fresh water is below 1, even if only a little below, the specific gravity must be higher than the density.

The density of fresh water at 77 deg F is about 0.9970479 g/cm3.

That seems very close to 1. Maybe close enough to ignore the difference from 1?

No!

The density of 35 ppt seawater is about 1.0234 g/cm3 at 77 deg F, so dividing 1.024 g/cm3 by 0.9970479 g/cm3 gives a specific gravity of 1.0264.

FWIW, I think some early aquarists may have confused density and specific gravity, and hence recommended 1.023 - 1.024 as matching natural seawater, and didn't realize that the specific gravity (which is what people actually measure with a hydrometer, typically) was not the same as density. That, I believe, is part of the reason for many older recommendations to reef aquarists to keep "specific gravity" in the 1.023-1.024 range. At least one salt mix still makes this error on their salt bucket itself, claiming that 36.7 ppt seawater has a "specific gravity" of only 1.025 (Fluval says this).

The barometric pressure won't noticeably impact the density of either seawater or fresh water within the ranges present on the earth's surface. :)

See below for more on the density of fresh water:

http://docs.engineeringtoolbox.com/documents/595/water_temperature_density.png

water_temperature_density.png


Thanks
 
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