Question

State whether the solutes and solvents in each of the following combinations are miscible or immiscible. (a) polar solute \(+\) polar solvent (b) polar solute \(+\) nonpolar solvent

Short answer

(a) Miscible; (b) Immiscible.

Step-by-step solution

Understanding Polarity

To determine miscibility, we first need to understand the concept of polarity. A polar substance has molecules with a partial positive charge on one end and a partial negative charge on the other. Polar substances are soluble in other polar substances due to their similar intermolecular forces.

Analyzing (a) Polar Solute + Polar Solvent

When a polar solute is mixed with a polar solvent, the similar polarity means that the solute and solvent can interact strongly through dipole-dipole interactions or hydrogen bonds. These strong interactions generally result in the solute dissolving in the solvent, making them miscible.

Analyzing (b) Polar Solute + Nonpolar Solvent

For a polar solute in a nonpolar solvent, the solute and solvent do not share the same type of intermolecular forces. The polar solute cannot establish strong interactions with the nonpolar solvent, resulting in the solute being immiscible in the nonpolar solvent because they won't dissolve well.

Key concepts

Polarity

Polarity is a key concept in chemistry that helps determine how substances interact with each other. It refers to the distribution of electrical charges around a molecule. If a molecule has a positive charge on one side and a negative charge on the other, it is considered polar.
This uneven distribution happens because certain atoms attract electrons more strongly due to their electronegativity. For example, in a water molecule, the oxygen atom is more electronegative and pulls electrons closer, making the area around the oxygen slightly negative and the hydrogen slightly positive.
Understanding the polarity of a substance is crucial when predicting how substances mix or dissolve in each other.

Miscibility

Miscibility refers to the ability of two liquids to mix and form a homogeneous solution. When discussing miscibility, 'like dissolves like' is an essential guideline in chemistry.
This principle means that polar substances are more likely to mix well (or be miscible) with other polar substances. They both have similar charges and can form strong interactions through hydrogen bonds or dipole-dipole interactions. For example, alcohols and water are both polar, so they can dissolve in each other.
On the other hand, polar substances are generally immiscible with nonpolar substances. This happens because their differing polarity types lead to weak or no attractive forces between them, so they do not mix well.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between molecules, crucial for determining a substance's properties like boiling points, melting points, and solubility.
There are several types of intermolecular forces:

• **Dipole-dipole forces:** Occur between two polar molecules. Because they have permanent dipoles, these molecules attract each other easily.
• **Hydrogen bonds:** A stronger type of dipole-dipole force where hydrogen is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine.
• **London dispersion forces:** Present in all molecules, but are the only forces present in nonpolar molecules.

Understanding these forces is critical when predicting how substances will interact. For instance, a polar solute will dissolve well in a polar solvent due to strong dipole-dipole or hydrogen bonding interactions, making them miscible. Conversely, polar solutes do not dissolve well in nonpolar solvents due to the lack of strong intermolecular force compatibility.