Pythons

[Bignose]

No, the limescale on the kettles is because by boiling off water, you have left so much calcium in the water, that the water is oversaturated. The calcium comes out of solution.

This exact same concept can be observed if you sit some saltwater out on a plate in the sun. The sun causes evaporation -- which is overhwlemingly pure water -- and what is left is some very pretty salt crystals. Classic 3rd grade science experiment.

 

[Underwurlde]

Yeeeees (I'm having a thicko moment here, so bear with me)....

Evaporation totally removes the water from (what ever) and is left behind in the plate. Yep junior stuff, we've all done it.

When boiling water, that same water still has the same calcium 'in it' in effect, albeit now a solid lump stuck to the side of the kettle or floating around as heavy flakes. You then pour the water out, bosh, water then removed from calcuim (deposits remain stuck to the side of the kettle).... This is where I'm coming from, surely the KH of that previously boiled water will be less when you pour it out?

I am not arguing (little or no knowledge), just trying to get a free chem lesson...

Cheers again BN,

Andy

 

[Bignose]

No, the water can dissolve a certain amount, called the saturation point.

Let me give just a simple example.

Take water that is at 80% saturated with minerals. Lets just call it 8mg of minerals in 1 L of water. In other words, 100% saturation is 10 mg/L.

Pretty much all the minerals stay in the water when boiled, so what happens when we boil off 200 mL, leaving 8 mg of minerals in 800 mL. Well, now saturation is at 100%.

Remember that our measures of hardness are just concentrations. ppm is pretty darn close to a mg/L. A degree of hardness is like 17.9 ppm. All concentrations.

In the example I just gave, the concentration just went from 8 ppm to 10 ppm. So the hardness has increased.

Now, what happens if we boil off another 200mL? leaving 8 mg in 600 mL. Or 125% saturated.

What happens is that 6 mg of the minerals stay in the water, leaving water still at 100% saturation. The other 2 mg fall out of solution. The water cannot hold that much minerals.

This is the limescale on the walls of the kettle. By boiling water away, you keep depositing the milligrams of material that the water cannot hold in solution.

So, end results, you have 2 mg of minerals on your kettle, but still 100% mineral-saturated water. That water will still have all the buffering capabilities that any other 100% mineral saturated water would have.

 

[Underwurlde]

This is still bugging my tiny brain this one.

I was saying that by simply bringing water to the boil, you can reduce the carbonate hardness of the water KH or temporary hardness (doing so will not however effect the General Hardness or permanent hardness – thus a way differentiating the two).

I was not implying that you boil the water for a long time, boiling off a reasonable percentage of the water and in doing so passing the saturation point. I do not think is has anything to do with the saturation point. I mean, in a kettle, you bring the water to the boil and it switches itself off – takes what? 3 minutes. In this time the amount of water will probably be 99.99% of what you started from, i.e. the saturation of KH is the same as before the water was boiled – so where does the lime scale come from?

Do you see what I mean?

I think this has something to do with boiling out the CO2… Wiki to the rescue:
http/en.wikipedia.org/wiki/Hard_water

Andy

 

[Bignose]

I just gave the 80% saturated number as an example. The truth is, if you have hard water, your water is probably very close to 100% saturated. So any boiling/evaporation will leave some limescale. The amount you boil off is probably more than you think, 0.01% is not right, even if you take the water to just boiling and stop, it has probably lost 2,3,even 5%. Also, you only deposit a tiny amount of limescale mass each time, it adds up. You don't get the huge amount of limescaling after the first use. There is a tiny amount -- but you cannot see it. A big culprit is also people leaving water in the kettle. Water evaporates slowly over time, which again always leaves a little bit of minerals. All this adds up.

Yes, you can boil off the CO2, but the CO2 comes back. Rather quickly, too, especially in a fishtank where the water is circulated. That same circulation that provides good oxygen exchange at the surface also provides good CO2 exchange. Just letting the water cool after boiling will also bring the CO2 back in. So, unless you did a very large water change, like 75% or so, with water that was just boiled and not allowed to sit out, the KH will be back to it normal standard pretty darn quickly. Even then, the circulation of the water in the tank will be restoring the KH. At the beginning, when the water has very little CO2, the CO2 will enter the water very rapidly, it is probably only a matter of a few minutes before at least half the KH is back, maybe more. I agree the water will lose KH temporarily, but it will get back to its normal level rather quickly. That water retains is buffering capability once the CO2 comes back into it.

 

[Underwurlde]

So, does my statement stand in respect to the fact that I should have said recently boiled water has less buffering capacity...

Andy

 

[Bignose]

Recently boiled water temporarily has less buffering capacity.

Your first statement all the way back in post #6, "Boiled water has no buffering capacity as the calcium which forms the buffers has been 'boiled' out. Over time it could lead to low KH levels in the tank and eventual pH crash." is definately wrong. You were clearly talking about long-term here, as the acidic end-products of the cycle build up. But, the water will have recaptured the CO2 and thus regained is buffering capability much more quickly than the acidic pH crash could occur.

But yes, for a few hours, recently boiled water will have less buffering capacity.

 

[TerraDreamer]

I already do 2-3 30% water changes a week...

 

[Underwurlde]

@Bignose, got it now (at last ).

Thank you for taking the time & effort to explain this to me. It is very much appreciated.

Cheers,

Andy