evaporation

can someIan explain the science behind it.

during an oxygen exchange does it work both ways, water takes in oxygen and moisture leaves the tank absorbed in to the air. ok heat would be a factor but not the only factor. would the higher the surface aggitation result in more evaporation

edited: ok let me rephrase that. can "someone" explain the science behind it as i do value all your opinions

dar wrote:

can someIan explain the science behind it.

during an oxygen exchange does it work both ways, water takes in oxygen and moisture leaves the tank absorbed in to the air. ok heat would be a factor but not the only factor. would the higher the surface aggitation result in more evaporation

Do you really want the science?

Lets start with the obvious......
when we take water to its boiling point, it boils and you see loads of evaporation (steam)
The more energy, the lower the mass of water, the lower the air pressure and the higher the surface area....the faster all the water will evaporate.

Compounds dissolved in the water will also affect the rate of evaporation (and other things) within this evaporation process.

The amount of energy required to raise one gram of water through one degree C is known as the specific heat capacity......so to get to boiling point, each gram of water has to have enough energy supplied to make it raise degree by degree. BUT, that doesn't mean it will boil.....another factor needs to come into play: this is the latent heat of evaporation. For water, the latent heat of evaporation is very high ie you need a lot of energy to make water evaporate (or boil).

OK....but your tank is not at 100 C....so what happens?

When you measure the temperature you are effectively only measuring an average energy of the water.
BUT, any body of water has billions of molecules of water (about 5 with 23 zero behind the 5 molecules per every 3 teaspoons....roughly).
The amount of energy each molecule has varies (with most of the molecules having an energy relating to the temperature measured).....some molecules will enough energy to exceed the latent heat of evaporation.

If a molecule with high energy is at the surface and that energy equals or exceeds the latent heat of evaporation then that molecule and all others with similar energy will evaporate (even at low temperature).

If the pressure above the water is lowered, then it is easier for molecules to evaporate.

With agitation at the surface, not only will the agitation give enough energy for many molecules to evaporate but the agitation will increase the surface area for molecules to escape.
Hence, agitation increases the rate of evaporation (if all other things are equal).

If we go any deeper into the science of depression or elevation of boiling point or into the realms of energy exchange when oxygen enters or leaves water then we are getting into more complex science.

Does that sort of answer the question?

Ian

great stuff Ian cheers. i like others to benefit from wat's going on in my brain and more importantly wat's going on in yours

dar wrote:

great stuff Ian cheers. i like others to benefit from wat's going on in my brain and more importantly wat's going on in yours

Cheers Dar.

Water is one of the most magical of all chemicals.
It is a record breaker and is probably the most bizarre chemical in the known universe.
If I were to re-invent life on earth, then I'd use water as the cradle of that life: but mother nature already patented that idea.

ian

You are a Kind Man Mr Millichip.

While I'm at it, your Disambiguation is most welcome.
Kev.

i will keep this going with another question

ok i have not delved too far into the chemistry side of things of fish keeping, or took the route of fish that require harder water....soooo could the waters hardness be manipulated with evaporation to keep such fish or would the hardness along with other values fluctuate so much that the whole concept would be rendered unviable?

boiling water will reduce or remove carbonate hardness and/or the waste water from a dehumidifier could be filtered through carbon and used. but neither are really practical except for small scale breeding projects. the water in small killiefish tanks with heavy evaporation can become very hard (gh) but also acid.
allowing hardness to build up and then large water changes can help trigger difficult to spawn species.
but the science is all a mystery to me. cue ian!

dar wrote:

i will keep this going with another question

ok i have not delved too far into the chemistry side of things of fish keeping, or took the route of fish that require harder water....soooo could the waters hardness be manipulated with evaporation to keep such fish or would the hardness along with other values fluctuate so much that the whole concept would be rendered unviable?

That is a good question....and not just meaning it a 'polictical' sense of having no answer.

Yes, evaporation would be means to increase hardness.
But, depending upon exactly which chemicals are involved in other water parameters, many other water parameters would be affected (and to a detrimental effect).

pH would be the most difficult one to give a defo answer on as pH evolves around the concentration of non-volatile compounds and volatile compounds. The pH of water is balanced by, amongst other things, carbon dioxide exchange and at any given temperature and pressure it will equilibrate with what is in the air above the water. But, acids such as citric acid or tannins will simply become more and more concentrated.
Similarly, components making water have an alkaline pH may also become more concentrated.

pH is NOT a measure of the amount of acid in the water, but is relative to the concentration of acids.

Nitrates and nitrates will tend to concentrate up when water evaporates....and, again, we do not measure how much nitrate is in the water (that is irrelevant) but the concentration of nitrate is measured.

Sodium chloride and similar salts that do not contribute to hardness will also become concentrated. And, yet again, it is concentration and not amount of salt that is important.

'Hard-water' lakes are not really 'hard' because of evaporation processes: generally it is by virtue of the components of hard water being brought into the lack from the waters feeding the lake.

So....if all that is pretty much defo, then why is the question a good question (in my opinion)?

Simple, it addresses a major problem in water chemistry focus.
Often people will focus on getting one particular water parameter correct.....and may ignore all the other things that are of equal or even greater importance.

Although I will talk about fish being 'hard-water' fish, what I would be more concerned with is the conductivity and TDS of the water (I personally class fish water on the expected conductivity and RedOx balance, but only use Acid/Soft terminology to make things simple).
For example, tanganyikan cichlids would be more prone to problems from low conductivity and low oxidising RedOx than to low hardness; black wing hatchet fish would be more concerned with low conductivity and an overly reducing RedOX than with simply having a low pH (if that low pH meant having loads of chemicals that increase conductivity).

The answer to the question is evaporation would affect too many other important parameters; and that even though the amount of compound in the water may not change, it will become more and more concentrated: and it is concentration that is the important thing.

ian

derek wrote:

boiling water will reduce or remove carbonate hardness and/or the waste water from a dehumidifier could be filtered through carbon and used. but neither are really practical except for small scale breeding projects. the water in small killiefish tanks with heavy evaporation can become very hard (gh) but also acid.
allowing hardness to build up and then large water changes can help trigger difficult to spawn species.
but the science is all a mystery to me. cue ian!

Derek must have been posting whilst I was typing.

The boiling of hard water where that hardness is due to bicarbonates will lower the hardness through a process of decomposition rather than evaporation.

Boiling does little to change non-alkalinity hardness (ie non-carbonate calcium and magnesium hardness).

BUT....and this is a warning to marine keepers.....calcium carbonate and many sulphates are funny fish. An increase in temperature may actually decrease solubility as opposed to things such as ammonium nitrate.
Small changes in total water concentration may make calcium and magnesium to form insoluble complexes....evaporation will decrease the concentration of water and increase the concentration of solutes. If these solutes are near their maximum solubility then that decrease in water concentration can spark a precipitation of those solutes.

Derek mentions one of little tricks in breeding some fish.....let the water get 'into a bad state'.
The rationale here could possibly be that as solutes become more concentrated then that may be signally a dry season......then when those concentration start to become low (and in some fish we would use a pretty sudden lowering by adding nice fresh cooler water) that may signal a rainy season: that could be the breeding season.
It doesn't work for all fish.....because maybe they don't use that seasonal signal....and, at worst, it could even be a risky business if you're not careful.
If, however, you are careful then even difficult to spawn fish such as the various cave-spawning tetras will breed (to have that success under your belt is quite nice).

ian

some killiefish live in glorified puddles in the tropics which are rapidly evaporating which must increase hardness and they have to spawn quickly before the pool drys out. so we recreate this to some extent in the home aquarium.
any thoughts on the dehumidifier water, ian, i have tried this before but did'nt really persist because it was messy.
also is it still common practice to add salt to killie tanks?

I wouldn't know too much about the waste water from a dehumidifier.......don't have one to see what the waste water is like, and my preference is that I live a high humid atmosphere myself.

I still use aquarium salt (sodium chloride or a marine mix) with most killies.....maybe not for all of their life span, but certainly for some stages.
Those that have acidic water are the ones I prefer to use salt with, but the ones preferring rather hard and alkaline water don't get salt in my set-ups.

Yep, as the puddles dry up the minerals would become drastically more and more concentrated......so these would be likened to water bodies where the water chemistry is grossly affected by evaporation (as opposed to lakes like tanganyika).

Also, with killies, as the rains fall again on the eggs then the eggs are going to experience a period of high concentration of minerals before being diluted somewhat by more rain falling.....there would be a period where the water might be as salty as the dead sea (even if only for the first few minutes of rainfall).

But Derek, you've thrown an unfair example of a fish into this topic with killies.......their whole life cycle is still in the realms of magic, and as we know more and more about the science of their life-cycle it becomes more and more magical.

One of the secrets that I use for killies (it's not a secret really) is the use very mature water. As youngsters, I give mine mature water to which I do a lot of small partial water changes. As the fish mature, I start to hold off on the water changes and topping up of water and let it get quite concentrated with minerals etc etc.
If I keep the constant water changes going then the fish die young (eg Nothobranchius will drop about 2 weeks after wanting to spawn even if they don't spawn at about the age of 3 to 6 months), but when I stop doing the regular partial water changes at about 6 months old then the fish live a much longer and happier life (at present, I have a male Nothobranchis rachovii who is about 18 months or so old and does not look like the typical 150 year old trout of the typical 6 month old fish).

Now, the bringing up of killies is even more unfair as there is a scientific belief that the mother passes differing genetic messages (??) to different eggs based upon the experiences that she has in life with rainfall and water conditions.

That needs a Killi thread to look at kilies.

ian