Question

Arrange the following compounds in order of increasing solubility in water: \(\mathrm{O}_{2}, \mathrm{LiCl}, \mathrm{Br}_{2},\) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\).

Short answer

Order: \(\mathrm{Br}_{2}\), \(\mathrm{O}_{2}\), methanol, \(\mathrm{LiCl}\).

Step-by-step solution

Understand the types of compounds

Know the nature of each compound given. \(\mathrm{O}_{2}\) is a nonpolar molecule, \(\mathrm{LiCl}\) is an ionic compound, \(\mathrm{Br}_{2}\) is also a nonpolar molecule, and methanol \((\mathrm{CH}_{3}\mathrm{OH})\) is a polar molecule with the ability to hydrogen bond.

Assess solubility rules

Recall that compounds dissolve in water based on the principle 'like dissolves like'. Polar and ionic substances, such as \(\mathrm{LiCl}\) and methanol, are generally more soluble in water than nonpolar substances like \(\mathrm{O}_{2}\) and \(\mathrm{Br}_{2}\). Water, being polar, interacts better with polar and ionic substances.

Compare polarities and capabilities for hydrogen bonding

Water, a polar solvent, significantly dissolves substances that are polar and can form hydrogen bonds. Methanol, being polar and able to hydrogen bond, will therefore be very soluble in water.\(\mathrm{LiCl}\), being ionic, dissociates in water and is highly soluble.

Compare the nonpolar substances

Between \(\mathrm{O}_{2}\) and \(\mathrm{Br}_{2}\), solubility can be affected by molecular size and interactions with water. Both are nonpolar, but \(\mathrm{Br}_{2}\), being larger, may have slightly more interaction due to induced dipole interactions. However, generally, both are poorly soluble in water, with \(\mathrm{O}_{2}\) slightly more soluble than \(\mathrm{Br}_{2}\) due to slight solubility of gases.

Arrange the molecules

Based on the discussions above, the order of increasing solubility in water is: \(\mathrm{Br}_{2}\), \(\mathrm{O}_{2}\), methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)), \(\mathrm{LiCl}\).

Key concepts

Solubility Rules

Solubility rules help us predict which substances can dissolve in water. The saying "like dissolves like" is central here. This means that polar substances tend to dissolve in polar solvents like water, while nonpolar substances dissolve in nonpolar solvents.
In an aqueous solution, polar molecules and ionic compounds interact well with water because of their complementary polarities. Polar molecules have dipole moments that attract and align with water's own polar structure, facilitating dissolution.
Ionic compounds dissociate into ions, which readily interact with water molecules. Understanding the chemical nature of substances using solubility rules helps us anticipate their solubility behavior in various solvents.

Polar and Nonpolar Molecules

Molecules come in two main types based on their electric charge distribution: polar and nonpolar. Polar molecules have an uneven distribution of charge, meaning there is a positive end and a negative end. Water is a classic example of a polar molecule.
Nonpolar molecules, on the other hand, have a more even charge distribution. This means they lack distinct poles and don't interact as well with polar substances like water.

• Polar molecules tend to dissolve well in water.
• Nonpolar substances dissolve better in solvents that don't have a distinct charge.

Understanding the polarity of a molecule allows us to predict its behavior when mixed with different solvents, like water.

Hydrogen Bonding

Hydrogen bonding is a special attractive force acting among molecules, making it crucial for solubility. It occurs when a hydrogen atom, bonded to a highly electronegative atom like oxygen or nitrogen, interacts with another electronegative atom in a different molecule.
This kind of interaction is significant in water solubility. Methanol, for example, which has -OH groups, can form hydrogen bonds with water molecules, enhancing its solubility significantly.

• Hydrogen bonds are stronger than ordinary dipole interactions.
• They significantly affect the boiling and melting points of compounds.

Molecules that can hydrogen bond will generally have higher solubility in polar solvents like water.

Ionic Compounds

Ionic compounds consist of positive and negative ions held together by electrostatic forces. Common table salt, NaCl, is a typical ionic compound. When placed in water, these compounds dissociate into their respective ions (e.g., Na extsuperscript{+} and Cl extsuperscript{-}), which then interact strongly with water molecules.
The interaction stems from the attraction between the charged ions and polar water molecules.

• Ionic compounds generally have high solubility in water.
• They contribute to water's ability to conduct electricity.

This dissociation and interaction make ionic compounds like LiCl very soluble in water, often even more than polar, covalent molecules.