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And the answer is...
If I were to add one billion of each of the following ions to a liter of seawater (as, say, the chloride salts), which of them will show the largest increase in the actual specific gravity (i.e., not necessarily what some particular device might measure, but the true specific gravity)?
E. Strontium
Strontium is by far the heaviest ion in the list. It turns out that while heavier ions are also generally larger, they get heavier faster than they get larger, so the heaviest ions are likely to have the biggest impact on specific gravity.
Another effect is that cations with higher positive charge pull their electrons in more tightly than those with a lower charge and similar mass. For example, Mg++ with a molecular weight of 24.3 g/mole has an ionic radius of 86 pm, while sodium with a similar molecular weight of 23 g/mole has a much larger ionic radius of 116 pm; in fact, magnesium even has a smaller ionic radius than the much lower mass lithium ion. Thus, having +2 charge (magnesium, calcium, strontium on our list) makes an ion much smaller than having a +1 charge (sodium and potassium on our list), and so can make them more "dense" and have a higher impact on specific gravity.
Additionally, to add a billion ions would likely imply the need to add counterions unless the water acquired a static charge. While the question doesn't specify what the counterion might be, strontium, calcium, and magnesium (all +2 charges) would necessarily need more counterions than sodium and potassium (+1 charge).
As Jim suggests, if you look up how much effect on specific gravity you have for 1 M solutions of the chloride salts of these ions, you get:
A. Sodium (sg ~1.040)
B. Potassium (sg ~ 1.046)
C. Calcium (sg ~ 1.090)
D. Magnesium (sg ~ 1.070)
E. Strontium (sg ~ 1.136)
Happy Reefing.
If I were to add one billion of each of the following ions to a liter of seawater (as, say, the chloride salts), which of them will show the largest increase in the actual specific gravity (i.e., not necessarily what some particular device might measure, but the true specific gravity)?
E. Strontium
Strontium is by far the heaviest ion in the list. It turns out that while heavier ions are also generally larger, they get heavier faster than they get larger, so the heaviest ions are likely to have the biggest impact on specific gravity.
Another effect is that cations with higher positive charge pull their electrons in more tightly than those with a lower charge and similar mass. For example, Mg++ with a molecular weight of 24.3 g/mole has an ionic radius of 86 pm, while sodium with a similar molecular weight of 23 g/mole has a much larger ionic radius of 116 pm; in fact, magnesium even has a smaller ionic radius than the much lower mass lithium ion. Thus, having +2 charge (magnesium, calcium, strontium on our list) makes an ion much smaller than having a +1 charge (sodium and potassium on our list), and so can make them more "dense" and have a higher impact on specific gravity.
Additionally, to add a billion ions would likely imply the need to add counterions unless the water acquired a static charge. While the question doesn't specify what the counterion might be, strontium, calcium, and magnesium (all +2 charges) would necessarily need more counterions than sodium and potassium (+1 charge).
As Jim suggests, if you look up how much effect on specific gravity you have for 1 M solutions of the chloride salts of these ions, you get:
A. Sodium (sg ~1.040)
B. Potassium (sg ~ 1.046)
C. Calcium (sg ~ 1.090)
D. Magnesium (sg ~ 1.070)
E. Strontium (sg ~ 1.136)
Happy Reefing.
