Question

What do we mean when we say that "like dissolves like"? Do two molecules have to be identical to be able to form a solution in one another?

Short answer

"Like dissolves like" is a chemistry concept referring to the tendency of similar substances, mainly based on polarity, to dissolve and form solutions with one another. For example, polar substances dissolve in polar solvents, while nonpolar substances dissolve in nonpolar solvents. Two molecules do not have to be identical to form a solution; they just need to have similar properties, such as polarity, which enables them to interact and form a homogeneous mixture.

Step-by-step solution

Introduction

"Like dissolves like" is a concept in chemistry that describes the tendency of similar substances to dissolve and form solutions with one another. It mainly refers to the polarity of the molecules, meaning that polar substances tend to dissolve in polar solvents, and nonpolar substances tend to dissolve in nonpolar solvents.

Polar vs. Nonpolar Molecules

To better understand this concept, let's briefly review the difference between polar and nonpolar molecules. Polar molecules have an unequal distribution of charges due to electronegativity differences between the atoms. For example, water is a polar molecule because oxygen is more electronegative than hydrogen and pulls the electrons toward itself, resulting in a slightly negative charge on the oxygen atom and slightly positive charges on the hydrogen atoms. Nonpolar molecules, such as oil, have a more equal distribution of charges, so there is no overall separation of charges within the molecule.

Solubility Principles

Solubility occurs when solvent molecules interact with solute molecules through intermolecular forces, which depend on the polarity of the molecules. Polar solvents, like water, can dissolve polar solutes because the similar charges create strong intermolecular interactions known as hydrogen bonding, dipole-dipole interactions, or ion-dipole interactions. Nonpolar solvents can dissolve nonpolar solutes through weaker dispersion forces.

Examples

Some examples of "like dissolves like": 1. Polar solvents and solutes: Salt (polar) dissolves in water (polar) because the positive ions of the salt are attracted to the negatively charged oxygen atoms in the water molecules, and the negative ions of the salt are attracted to the positively charged hydrogen atoms in the water molecules. 2. Nonpolar solvents and solutes: Oil (nonpolar) does not dissolve in water (polar) but does dissolve in gasoline (nonpolar). The nonpolar oil molecules are not attracted to the polar water molecules and prefer to stay together due to the dispersion forces. However, they can mix with the nonpolar gasoline molecules through weak van der Waals forces.

Do molecules have to be identical to form a solution?

Two molecules do not have to be identical to form a solution in one another. As long as they have similar properties, specifically polarity, they should be able to dissolve in each other and form a solution. It is the similarity in their molecular properties and interactions that allow them to associate and form a homogeneous mixture, even if they have different chemical compositions. For example, ethanol (alcohol) is a polar molecule that can dissolve in water, even though they are not identical. The important factor is that both molecules have similar polarity and can form hydrogen bonds with each other.

Key concepts

Polarity and Its Role in Chemistry

Understanding the concept of polarity is key to grasping why certain substances dissolve in others. Polarity refers to how electrons are distributed in a molecule. In a polar molecule, the electrons are not shared equally between the atoms due to differences in electronegativity. This results in one end of the molecule having a slight positive charge while the other end has a slight negative charge. A common example of a polar molecule is water, where the oxygen atom has a slightly negative charge, and the hydrogen atoms have a slightly positive charge.
This charge imbalance creates a dipole moment, which is an essential feature of polar molecules.
Conversely, nonpolar molecules share electrons more equally, resulting in no permanent charge separation. Oil is an example of a nonpolar substance, where the charge distribution is even, and thus there are no poles. In summary, knowing whether a molecule is polar or nonpolar helps predict its solubility properties, as well as the types of intermolecular forces it can participate in.

Solubility Principles Explained

Solubility principles revolve around the interactions between solvent and solute molecules. Essentially, solubility is the capability of a solute to dissolve in a solvent, forming a homogeneous solution. This process is driven by intermolecular interactions, which need to be strong enough to separate solute molecules from each other and integrate them into the solvent.

• Polar Molecules: These generally dissolve well in polar solvents. This is because similar charges attract, creating strong intermolecular forces such as hydrogen bonds, dipole-dipole interactions, or ion-dipole interactions.
• Nonpolar Molecules: These tend to dissolve well in nonpolar solvents. Here, the forces might be weaker, known as dispersion forces or London dispersion forces, which still allow for solubility due to their proportionality with the size and mass of the molecules.

Ultimately, the principle "like dissolves like" is a simplified way of describing that the closer the match in polarity between a solute and a solvent, the more likely they are to form a solution. However, molecules don't need to be identical; they must simply have similar polarity and engage in compatible intermolecular forces.

Intermolecular Forces and Their Impact

Intermolecular forces are the interactions that hold molecules together and play a critical role in the process of solubility. These forces vary in strength and type, depending on the polarity of the molecules involved.
There are several types of intermolecular forces:

• Hydrogen Bonds: Strong interactions that occur between polar molecules that have hydrogen atoms bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Water molecules extensively use hydrogen bonding.
• Dipole-Dipole Interactions: These occur between the positive end of one polar molecule and the negative end of another. This type of force is essential for the solubility of many substances in polar solvents.
• Ion-Dipole Forces: Occur between ions and polar molecules, which are particularly relevant in solutions like salt dissolved in water.
• Dispersion Forces: Also known as London forces, these forces are found in nonpolar molecules and arise due to temporary fluctuations in electron density. Despite being relatively weak, they are significant in larger or heavier molecules.

Understanding these forces helps explain why certain solute-solvent combinations work that might not initially seem compatible based solely on their chemical identities. By recognizing these force interactions, students can better predict and understand solubility behaviors in numerous contexts.