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Two Models of Lishui History in Nanjing
(1) Because H2O is the most chemical substance in seawater, followed by NaCl,

So the answer is: Na+

(2) Because the main chemical composition of algae is C106H263o116p, the number ratio of C and N atoms in the composition is = 106: 16 = 53: 8. Nitrogen and phosphorus

(3) Because the solubility of sodium chloride is less affected by temperature, it should be crystallized by evaporation, so B should be selected;

The process of preparing refined salt from crude salt is as follows: dissolving crude salt and filtering to obtain a mixed solution of NaCl, MgCl2 and CaCl2 (filtrate A), and adding excessive NaOH solution (solution A) to it. After filtration, Mg(OH)2, Ca(OH)2 precipitate, NaCl solution and remaining NaOH solution (filtrate B) were obtained, and excessive hydrochloric acid (solution B) was added to them.

So choose B? Filter evaporation

(4) Observing the figure given in the question, we can see that the polymer membrane allows water molecules to pass through, while other ions cannot.

So the answer is: polymer membrane allows water molecules to pass through, but other ions can't.