Why Does Adding Salt Mix Increase Volume

[Bret]

Why does the water volume in my mixing container increase when I add my salt mix?

I just got my mixing station online, filled with 50 gallons of RODI, began adding my salt mix (instant Ocean) and was overflowing the container before I could get all the salt in (added about 6.5KG of salt mix). I would estimate the volume increased by about .5 gallons, and did not reduce appreciably after the salt had fully dissolved.

Results from a google search generally state there should actually be a slight decrease in the water volume.
What gives?
Are the additional components in aquarium salt causing the increase in volume?

 

[dreamcatcherr9]

Could a “lump” of undissolved salt have clogged a pipe, causing the over flow?

 

[Bret]

Could a “lump” of undissolved salt have clogged a pipe, causing the over flow?

No, this is in a static condition. Pump on or off does not change the volume.

 

[chipmunkofdoom2]

Why do you think the volume wouldn't increase? That 16lbs of salt has to go somewhere when you add it to water. After the compounds and elements are all distributed throughout the water, the result is the solution has a greater volume.

It's also worth noting that the Instant Ocean 50 lb bags really only make about 44 gallons of saltwater at 35 ppt. The mixing instructions are for salinity closer to 30 ppt.

 

[Bret]

Why do you think the volume wouldn't increase? That 16lbs of salt has to go somewhere when you add it to water. After the compounds and elements are all distributed throughout the water, the result is the solution has a greater volume.

It's also worth noting that the Instant Ocean 50 lb bags really only make about 44 gallons of saltwater at 35 ppt. The mixing instructions are for salinity closer to 30 ppt.

Theoretically the volume should not increase since as the salt dissolves it is essentially incorporated into the water molecules. This increases the density, saltwater at 35ppt has a density of around 1.026 g/cc, freshwater has a density of 1.00 g/cc

 

[Bret]

Why do you think the volume wouldn't increase? That 16lbs of salt has to go somewhere when you add it to water. After the compounds and elements are all distributed throughout the water, the result is the solution has a greater volume.

It's also worth noting that the Instant Ocean 50 lb bags really only make about 44 gallons of saltwater at 35 ppt. The mixing instructions are for salinity closer to 30 ppt.

I was shooting for 7.5kg/50gal, just ran out of room before I could get it all in.

 

[Biglew11]

Theoretically the volume should not increase since as the salt dissolves it is essentially incorporated into the water molecules. This increases the density, saltwater at 35ppt has a density of around 1.026 g/cc, freshwater has a density of 1.00 g/cc

The volume will increase. 1000 grams of saltwater is 965 grams of water 35 grams salt 35 ppt. If you take 1 gallon and mix it to 35 ppt you will have added 3.5 percent to the volume.
Salt is heavier than water, when salt is added it displaces some water. This is why saltwater is heavier than fresh.

 

[JimWelsh]

Theoretically the volume should not increase since as the salt dissolves it is essentially incorporated into the water molecules. This increases the density, saltwater at 35ppt has a density of around 1.026 g/cc, freshwater has a density of 1.00 g/cc

If your first sentence were correct, then the density at 35ppt would be 1.035 now wouldn't it? The lower density (acutally specific gravity) demonstrates that the volume has increased.

 

[Bret]

If your first sentence were correct, then the density at 35ppt would be 1.035 now wouldn't it? The lower density (acutally specific gravity) demonstrates that the volume has increased.

Yes! Thank you

 

[Bret]

Next question, is which component is causing the volume increase?
If pure NACL causes a slight volume decrease when in solution with water (according to several google sources), which component(s) of the salt mix is causing the volume to increase?

 

[ZiBiReef]

Next question, is which component is causing the volume increase?
If pure NACL causes a slight volume decrease when in solution with water (according to several google sources), which component(s) of the salt mix is causing the volume to increase?

There is more to salt mix than just pure NaCl..

 

[Bret]

There is more to salt mix than just pure NaCl..

Yes. My question is basically which of those components have the largest effect on volume.
Would Reef Crystals have a larger volume increase over Instant Ocean because it has a higher concentration of calcium or magnesium?

 

[BZOFIQ]

Anything in solid or liquid state added to the water would add volume. Basic physics.

 

[mitten_reef]

If pure NACL causes a slight volume decrease when in solution with water (according to several google sources), which component(s) of the salt mix is causing the volume to increase?

Not sure what you meant by this. can you share source of this notion? salt may appear to "absorb" water in a situation where you have a pile of salt and a drop of water. In fact in a more humid places, you can make water appears in a bowl, simply by putting salt out in open air. sometimes, also why salt gets caked up in an salt bottle after sitting unused for a while because it absorbs moisture from the air. but when water is aplenty, I don't see how salt just makes water disappear.


To add to the discussion. Let's say a bag of salt being drop into the mixing bucket (without mixing), its mass displaces some water thus creating more volume. Now remove the "bag" away from that picture, the same displacement of the exact same mass still takes place, albeit now everything is in solution.

 

[BZOFIQ]

Yes. My question is basically which of those components have the largest effect on volume.
Would Reef Crystals have a larger volume increase over Instant Ocean because it has a higher concentration of calcium or magnesium?

As far as solids go; anything with lowest material density will have highest displacement factor.

 

[AlexKintner]

Found this on the internet (written by someone smarter than me):


the absolute volume increases monotonically from pure water to halite saturation but it does not necessarily increase by the volume of the salt added to the water. At low salt concentrations the dissociation of the salt causes a contraction of water molecules around the ions; thus the reason for the lack of strict additivity. As salt concentration increases that contraction effect decreases.

If you’ve heard someone talk about the volume of a salt solution decreasing relative to pure water what was probably referred to was “specific volume” which is the inverse of density. As NaCl is added to water the density of the solution increases so the specific volume decreases.

 

[Bret]

See the discussion in this link

Why there is no change in water level when salt is added?

Let us say we have one glass of water and after that when we add one or two spoon of salt then we notice that salt dissolves in it but when we measure the water level we found that there is no diff...

chemistry.stackexchange.com

On a very basic level, I believe what they are saying is there is "room" within the water molecule for the NaCl ions, and not only is there room, but in solution, the salt ions cause a "re-ordering" of the molecule which actually slightly decreases the volume.

 

[Randy Holmes-Farley]

Why does the water volume in my mixing container increase when I add my salt mix?

I just got my mixing station online, filled with 50 gallons of RODI, began adding my salt mix (instant Ocean) and was overflowing the container before I could get all the salt in (added about 6.5KG of salt mix). I would estimate the volume increased by about .5 gallons, and did not reduce appreciably after the salt had fully dissolved.

Results from a google search generally state there should actually be a slight decrease in the water volume.
What gives?
Are the additional components in aquarium salt causing the increase in volume?

The volume rises on mixing sea salts into water, but not as much as you suggest. There are some incorrect assumptions in your analysis (for example, that there is no water in the salt mix, which is not correct), so here's a more accurate analysis.

The really interesting question for a chemist is why the volume of two things mixed together can be smaller than the sum of the two before mixing. But I'll leave that for another day.

35 ppt seawater contains 35 grams of salts and 965 g of water in 1 kg of seawater.

965 grams of water is about 965 mL.

How much volume does that seawater occupy after the salts have dissolved? We know that from the density

So that 1 kg of 35 ppt seawater has a density of about 1.0234 g/mL

So that 1 kg of seawater occupies 1,000 g/1.0234 g/mL = 977 mL

Thus the volume rose from 965 mL of pure water to 977 mL of seawater.

 

[Bret]

Not sure what you meant by this. can you share source of this notion? salt may appear to "absorb" water in a situation where you have a pile of salt and a drop of water. In fact in a more humid places, you can make water appears in a bowl, simply by putting salt out in open air. sometimes, also why salt gets caked up in an salt bottle after sitting unused for a while because it absorbs moisture from the air. but when water is aplenty, I don't see how salt just makes water disappear.


To add to the discussion. Let's say a bag of salt being drop into the mixing bucket (without mixing), its mass displaces some water thus creating more volume. Now remove the "bag" away from that picture, the same displacement of the exact same mass still takes place, albeit now everything is in solution.

Yes, if you drop a bag of salt in a bin of water it will increase the volume in the bin by approximately the volume of the bag. When you remove the bag, the volume will not immediately change (except by the small volume of the empty bag). As the remaining salt dissolves, and finds its way into the water molecules, the volume will then decrease.

 

[BZOFIQ]

See the discussion in this link

Why there is no change in water level when salt is added?

Let us say we have one glass of water and after that when we add one or two spoon of salt then we notice that salt dissolves in it but when we measure the water level we found that there is no diff...

chemistry.stackexchange.com

On a very basic level, I believe what they are saying is there is "room" within the water molecule for the NaCl ions, and not only is there room, but in solution, the salt ions cause a "re-ordering" of the molecule which actually slightly decreases the volume.

I believe this is only happening on a very small scale where only small amount of salt is added. You on the other hand, are dumping kgs of salt into your mixing bin.