Ocean water. Homogeneous or Heterogenous

[temporalExile]

So I an exam I recently took, there was a question that asked the question in the title. I said it was heterogeneous, on account of the varying salinity, and the pressence of dust and plankton in the water. Apparently, whoever wrote the test did take this into account, and I got the question wrong.

What do you guys think? Homogeneous or heterogeneous? Please explain your answer.

Edited May 22, 2015 by temporalExile

[LN400]

I'm no expert on chemistry but it seems to me it comes down to the question of whether ideal sea water, not actual sea water, is one or the other. Ideal in the sense that all pollution, dust, plankton etc is left out of the equation and all they look at is the water-salt mixture. If you take 2 spoons of this ideal sea water you won't be able to tell the difference. Take 2 spoons of (ideal) muddy water (water and lumps of dirt) and there would be a difference in the mixture. So, sea water: Homogeneous. Mud: Heterogeneous.

[stoani96]

water = Homogeneous; put a bit salt into water = Homogeneous; put too much salt into water = Heterogeneous (because a part of the salt will settle at the ground)

It is just about the fact, if you can tell a difference between single components.

I think the question was not meant so exactly as you put it, cause you're right: algae on top of water would result in a heterogenous mixture!

I think the question was more about the saltwater, that saltwater (although it is a mixture of salt and water) is homogeneous

[Wesreidau]

Heterogenous. However you have the correct answer for incorrect reasons, which justifies missing the question. It is not heterogenous because of floating debris but because the oceans are seperated into layers due to thermal layering. I had an AP Bio test back in the day regarding lake water thermal layering that we'd not studied in the slightest. My entire class was griping about it, except for me. I BS'd that essay so hard... I mean, I posited a theory based on known fluid dynamics to account for the ecological impact of thermal columns in an aquatic environment. In the aftermath review it turned out I was close enough for horse shoes and hand grenades.

http://en.wikipedia.org/wiki/Thermocline

[lajoswinkler]

Heterogeneous, absolutely no doubt about that. If you can see discrete pieces of stuff in a sample using a light microscope, even using ultramicroscopy, the sample is heterogeneous. There is all kids of tiny stuff, even visible to a human eye, in a sample of ocean water.

Whoever says differently needs to get their facts straight. This is really not something any expert doubts.

[temporalExile]

Heterogenous. However you have the correct answer for incorrect reasons, which justifies missing the question. It is not heterogenous because of floating debris but because the oceans are seperated into layers due to thermal layering. I had an AP Bio test back in the day regarding lake water thermal layering that we'd not studied in the slightest. My entire class was griping about it, except for me. I BS'd that essay so hard... I mean, I posited a theory based on known fluid dynamics to account for the ecological impact of thermal columns in an aquatic environment. In the aftermath review it turned out I was close enough for horse shoes and hand grenades.

http://en.wikipedia.org/wiki/Thermocline

Yeah, I guess thats another reason, but thermal layering was not brought up a single time in the entire course, so even more full of BS if they wanted that to be taken into account. And yes, I am aware of varying compositions at different depths, I just went with simpler reasons first. Another reason that I didn't bother mentioning was how ocean water near the poles would have a lower salt content, or how areas around volcano's will have all sorts of different things.

[LN400]

Whoever say sea water is heterogeneous and that experts say it is, can we have a link to a site where the experts say such a thing? I have found various sites where they say sea water is indeed homogeneous.

One such site

http://cnx.org/contents/3253f5f7-4f85-4035-be8a-1a5a5bf3e861@2/Classification_of_matter:_Mixt

I can't vouch for them as being experts but still it seems to me they have it covered.

[razark]

It's a question of scale, and how the question is worded could make a difference.

If a sample of water is taken from an ocean near where a large freshwater river empties into it, and a sample is taken from another location in the middle of the ocean, there will be a difference in salinity. If two samples are taken from the mouths of two different rivers, there will be differences based on what minerals each river carries. Seawater as a whole is heterogeneous.

However, if two separate samples are taken from a single location, they will be indistinguishable enough to be called homogenous.

- - - Updated - - -

I have found various sites where they say sea water is indeed homogeneous.

One such site

http://cnx.org/contents/3253f5f7-4f85-4035-be8a-1a5a5bf3e861@2/Classification_of_matter:_Mixt

That says only that seawater is a mixture, and that saltwater is homogenous.

[LN400]

It's a fair point and I'm more than ready to stand corrected but do they say it's a heterogeneous mixture?

EDIT: I stand corrected. A chemistry 101 paper is good enough for me. Heterogeneous it is.

Edited May 22, 2015 by LN400

[Fel]

So I an exam I recently took, there was a question that asked the question in the title. I said it was heterogeneous, on account of the varying salinity, and the pressence of dust and plankton in the water. Apparently, whoever wrote the test did take this into account, and I got the question wrong.

What do you guys think? Homogeneous or heterogeneous? Please explain your answer.

Can I just rant here? Some 100% of the time, exam questions are 100% complete fluff.

Jimmy throws a ball at tommy who is 50m away, applying 50N of force to the 3kg ball. How long does it take the ball to reach Tommy?

Assumption 1)

Jimmy and Tommy are both of infinite height (The ball never touches the ground)

Assumption 2)

The ball is thrown horizontal, even though the problem never said so

Assumption 3)

The ball is perfectly "solid" (some property of matter that I don't remember) and absorbs none of the force applied to it

Assumption 4)

The ball has uniformly distributed mass.

Assumption 5)

There is a clear path between Jimmy and Tommy (for all you know, there could be a glass window between the two)

Assumption 6)

Jimmy and Tommy are in a vacuum and effects of air resistance do not factor into play

Assumption 7)

Jimmy and Tommy have no mass, and gravitational discrepancies do not factor in.

Assumption 8)

Jimmy and Tommy are not moving close to the speed of light, thus having special relativity and apparent mass come into play

Assumption 9)

Okay this is getting absurd

It's the problem with those type of problems; they make absolutely no sense outside of the context they're suppose to be taken in.

Hell, there is NO SUCH THING as homogeneous matter in the strict definition. We can't even create 100% pure silicon, how are we going to equally distribute molecules to create homogeneous matter? Of course, at some ppm we'll accept matter to be "like" homogeneous and simplify calculations by treating it as such.

Heck, food for thought, it is possible to have solid water molecules and gaseous water molecules in a container of room temperature water; average temperature of the whole drastically can differ from the temperature of individual molecules. (Re: entropy) So much for that homogeneous cup of water ;p

[stoani96]

I think with "sea water" the question meant "salt water", and salt water(with the same percentage of salt than sea water) itself is with no doubt homogenous!

I guess it was a multiple choice test? This is why I hate these tests! In normal test you could wirte: heterogenous because of this and that.

(If it was a normal written answer test and you did not wirte down the reasons for your answer, its your own fault! )

[lajoswinkler]

If you took 10 litres of water and keep removing only the salty water until you got to 10 millilitres, sample of that would look like this.

The sample actually starts looking like mud, and you can see various stuff swimming and floating in it. Phytoplankton, zooplankton, pollen, quartz grains, etc.

Try to tell that seawater is homogeneous to a marine biologist and they'll look at you like you're insane.

Seawater is not just salty water.

[Starwhip]

This reminds me of a story my grandfather told me. He said that an exam once asked "If a rifle is fired with a 0 degree inclination at the same time a ball is dropped, will both the bullet and the ball hit the ground at the same time?"

He answered no, because of the curvature of the Earth, which is completely correct. But the professor didn't like that answer.

I say it is heterogenous, for all the reasons listed above.

[Xannari Ferrows]

Take a look at the Sargasso Sea. Purest, most beautiful water I have ever seen. Off topic of course, but I felt the need to point it out. Very high salinity as well.

[WinkAllKerb'']

fascinating ; ) found this to illustrate as well. Thks xannari

[stoani96]

The sample actually starts looking like mud, and you can see various stuff swimming and floating in it. Phytoplankton, zooplankton, pollen, quartz grains, etc.

Try to tell that seawater is homogeneous to a marine biologist and they'll look at you like you're insane.

Seawater is not just salty water.

when a teacher says seawater he actually means salty water (at least in my expirience...)

but back to topic: if you overstress the term of homogeneous that much, tell me one natural thing, that is actually homogeneous. In every sample of anything there is something else than the acutal sample. even in desert sand you will find something different. not even a piece of metal is really homogeneous. in my opinion homogeneous is subjectiv

[Camacha]

Can I just rant here? Some 100% of the time, exam questions are 100% complete fluff.

That is physics for you. Where do you think the joke about the spherical cow comes from?

[GoSlash27]

Heterogenous.

Like air masses, sea water masses at different temperatures resist mixing and tend to stratify. This is why you have effects like thermoclines/ ocean currents, etc.

Best,

-Slashy

[Red Iron Crown]

Clearly heterogenous. Every time I hear of an erroneous question/answer combination in a school, I think of this.

I know that image is a fake, but the fact it's believable speaks volumes.

[Starwhip]

Clearly heterogenous. Every time I hear of an erroneous question/answer combination in a school, I think of this.

I know that image is a fake, but the fact it's believable speaks volumes.

Took me a moment, had to read it three times before I figured out what was wrong.

Have you ever read something backwards? Yeah, that's what happened.

[temporalExile]

Clearly heterogenous. Every time I hear of an erroneous question/answer combination in a school, I think of this.

I know that image is a fake, but the fact it's believable speaks volumes.

If that happened to my kid I'd be like "Yeah, I think I'll discipline him/her myself... I think going out for ice cream should be enough"

[Camacha]

If that happened to my kid I'd be like "Yeah, I think I'll discipline him/her myself... I think going out for ice cream should be enough"

I that were real you could not laud the child enough. Knowing something for a fact and holding firm when someone tries to challenge you purely based on position is an honourable trait.

People will sometimes feel hurt if you do, but that does little to diminish it.

[stoani96]

If that happened to my kid I'd be like "Yeah, I think I'll discipline him/her myself... I think going out for ice cream should be enough"

reminds of interstellar