Question

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

Short answer

(a) Immiscible, (b) Miscible.

Step-by-step solution

Understanding Miscibility

Miscibility refers to the ability of two substances to mix and form a homogeneous solution. Generally, 'like dissolves like,' meaning that polar substances tend to be miscible with other polar substances, and nonpolar substances are generally miscible with other nonpolar substances.

Assessing Solubility of Polar and Nonpolar

For a combination of a nonpolar solute and a polar solvent, the two are usually immiscible. This is because the nonpolar solute does not dissolve well in the polar solvent due to the differences in their intermolecular forces.

Evaluating Nonpolar Combinations

For a combination of a nonpolar solute and a nonpolar solvent, these are generally miscible. Because both the solute and solvent are nonpolar, their intermolecular forces are similar, allowing them to mix well together.

Key concepts

Polar and Nonpolar Interactions

In the world of chemistry, knowing how substances interact based on their polarity is crucial. Polar substances have molecules with a partial electric charge due to an uneven distribution of electrons, resulting in a dipole moment. These molecules are like little magnets with a positive and a negative side.
Nonpolar substances, however, have molecules that are electrically balanced because electrons are evenly distributed. When polar and nonpolar substances meet, they tend to not mix well. The polar molecules form strong interactions with one another, due to their charge separation, which nonpolar molecules cannot break. This is why a nonpolar solute often remains separate from a polar solvent—a common example being oil (nonpolar) and water (polar).

• Polar molecules: Uneven electron sharing, leads to charges.
• Nonpolar molecules: Even electron sharing, no charge separation.

Understanding these interactions helps us predict whether substances will mix or separate.

Solubility

Solubility is all about whether and how much of a solute can dissolve in a solvent to form a solution. It is highly dependent on the nature of the solute and solvent: whether they are polar or nonpolar.
Think of solubility like fitting puzzle pieces together. A perfect fit results in the pieces locking together smoothly, just as polar solutes dissolve well in polar solvents and nonpolar solutes in nonpolar solvents. The term 'miscible' refers to two liquids being able to mix in all proportions, forming a homogeneous solution. Water and alcohol, both polar, mix well, demonstrating miscibility. In contrast, "immiscible" refers to liquids that do not mix, like oil and water. This is due to the "like dissolves like" rule, which basically states that solubility occurs better when the solute and solvent share similar polarities.

• Miscible: Two substances that mix completely.
• Immiscible: Two substances that do not mix.

Solubility isn't just about whether substances mix; it's an examination of their molecular compatibility.

Intermolecular Forces

Intermolecular forces are the 'glue' that hold molecules close to each other, playing a significant role in determining the components' miscibility.
Different types of these forces account for interactions within and between substances, influencing whether they will mix. Key Types of Intermolecular Forces:

• Van der Waals forces: Weak attractions important in nonpolar interactions.
• Dipole-dipole forces: Occur in polar molecules, stronger than van der Waals forces.
• Hydrogen bonds: A special strong type of dipole-dipole interaction, crucial for the properties of water.

When mixing substances, the prevailing intermolecular forces determine the outcome. Nonpolar substances rely heavily on Van der Waals forces, enough to allow mixing amongst themselves. Meanwhile, polar substances rely on stronger forces like hydrogen bonds, which nonpolar molecules cannot participate in.
Hence, the similarity in the types of intermolecular forces explains why similar types of substances mix well, embodying the principle of "like dissolves like." Understanding these forces offers insights into why some substances are miscible and others are not.