Generally speaking, carbonate salts are insoluble, unless the carbonate is paired with an alkali metal. Magnesium carbonate, as a result, is considered an insoluble salt.

Salts containing alkali metals, ammonium, or nitrate ions are considered soluble. Phosphates, on the other hand, are generally considered insoluble, unless paired with an alkali metal or ammonium ion.

The question states that base A precipitates in a solution of water. We can conclude from this information that base A must be water insoluble. Recall that compounds that are hydrophobic tend to aggregate together when placed water, forming solid precipitates in the solution; therefore, base A must be a hydrophobic base. Base B, on the other hand, is soluble in water and doesn’t precipitate; therefore, base B must be a hydrophilic base.

Solubility rules state that all hydroxide compounds are insoluble in water, except sodium and potassium hydroxide. Calcium hydroxide is water insoluble and could possibly be the identity of base A.

Hydrophilic molecules, by definition, are soluble in water. A compound’s solubility in water can be determined qualitatively using the solubility rules. If we look at the solubility rules, we will notice that there are three main cations that are always soluble in water. This means that a compound containing one of these three cations will always be soluble in water. The three cations are sodium, potassium and ammonium ions. Recall that sodium and potassium are alkali metals (column I of periodic table); therefore, these elements have one electron in its outermost shell (valence electron).

Alkali earth metals are on the second column of periodic table and have two valence electrons. Not all alkali earth metal containing compounds are water soluble (for example, calcium hydroxide).

Recall that interior of a cell membrane is highly nonpolar (due to the nonpolar tails of the cell membrane). Compounds that traverse the interior of a lipophilic membrane must be nonpolar. To solve this question, we need to figure out which compounds are nonpolar. Remember that all nitrates are soluble in water and, therefore, are polar. All alkali metal containing compounds (column I of periodic table) are soluble in water. This means that sodium chloride is water soluble because it contains sodium. Most carbonate salts are insoluble in water, including calcium carbonate; however, note that the calcium carbonate has polar groups with different electronegativities. This means that even though calcium carbonate is insoluble in water it contains polarity and therefore is not lipophilic. None of the compounds listed can traverse through a lipophilic membrane.