Predict whether each of the following is soluble in water, hexane, or both. Justify your answer. (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) (b) \(\mathrm{Li}_{3} \mathrm{PO}_{4}\)

Understanding how polarity affects solubility is crucial when predicting if a substance will dissolve in a particular solvent. Water, a 'universal solvent', is polar, meaning it has a slight electrical charge due to uneven distribution of electrons in its molecule. This polarity allows water to interact more strongly with other polar molecules and ions.

Polar solvents like water are effective at dissolving ionic compounds and other polar substances due to the attraction between the partial charges of the solvent and the solute. These attractions are called ion-dipole interactions in the case of ionic solutes, and hydrogen bonds when the solute is capable of forming them, such as alcohols or ammonia. Substances with similar levels of polarity will dissolve each other, according to the principle 'like dissolves like'.

Conversely, nonpolar solvents such as hexane, which consist of molecules with a more uniform distribution of electrical charge, are better suited for dissolving nonpolar molecules, like oils and fats. In the exercise provided, we consider the polarity of given compounds to deduce their solubility in different solvents.

Nonpolar hydrocarbons such as heptane, the compound in question (a), comprise a chain of carbon atoms surrounded by hydrogen atoms. These molecules lack polar bonds due to the small difference in electronegativity between carbon and hydrogen. Consequently, the electrons are shared fairly equally, making the molecule nonpolar.

Given their nonpolar nature, hydrocarbons do not interact well with polar solvents like water. They lack the ability to form the necessary intermolecular forces, such as ion-dipole interactions or hydrogen bonds, that would allow them to dissolve in such solvents. Instead, they mix readily with other nonpolar substances, such as hexane, a similar hydrocarbon solvent. This concept is applied in the solution when we predict that heptane will not be soluble in water but will be soluble in hexane.

Relevance to Everyday Life

Understanding solubility rules for nonpolar hydrocarbons can explain phenomena like oil spills on water, where oil doesn't mix with the water but rather floats on the surface due to difference in polarity and density.

Ionic compounds, exemplified by lithium phosphate (Li₃ PO₄) in the exercise (b), consist of positively charged cations and negatively charged anions held together by strong electrostatic forces known as ionic bonds. When introduced to polar solvents such as water, these ionic compounds can dissociate into their individual ions.

The water molecules surround the ions and the partial charges of the water interact with the full charges on the ions, effectively 'pulling' them into solution through ion-dipole interactions. This process is energetically favorable and leads to the dissolution of the ionic compound in water. However, in nonpolar solvents like hexane, these interactions do not occur, as hexane does not have the polar characteristics required to disrupt the ionic lattice of a compound like lithium phosphate.

The solubility of ionic compounds in water is also influenced by the size and charge of the ions and the temperature of the solution. In the given exercise, we see this principle at work as lithium phosphate is predicted to be soluble in water but not in hexane.