The Phrase Like Dissolves Like Refers To

The principle of "like dissolves like" is a cornerstone concept in chemistry, guiding our understanding of solubility and miscibility. It elegantly encapsulates the observation that substances with similar chemical structures and intermolecular forces are more likely to dissolve in each other. This seemingly simple phrase unlocks a deeper understanding of how different substances interact at the molecular level, influencing a vast array of applications, from everyday cooking to advanced industrial processes.

Delving into the Meaning of "Like Dissolves Like"

At its core, "like dissolves like" means that polar solvents are best at dissolving polar solutes, and nonpolar solvents are best at dissolving nonpolar solutes. The "like" refers to the similarity in polarity, which dictates the types of intermolecular forces present within the substances.

Polarity: A molecule is considered polar if it has an uneven distribution of electron density, resulting in a partial positive charge (δ+) on one side and a partial negative charge (δ-) on the other. This charge separation creates a dipole moment.
Intermolecular Forces (IMFs): These are the attractive or repulsive forces that exist between molecules. The types of IMFs present significantly influence a substance's physical properties, including its solubility.

Understanding Polarity

Polarity arises due to differences in electronegativity between atoms within a molecule. Electronegativity is the ability of an atom to attract electrons in a chemical bond. When two atoms with significantly different electronegativities form a bond, the more electronegative atom pulls the electron density towards itself, creating a polar bond.

For example, in a water molecule (H₂O), oxygen is more electronegative than hydrogen. This leads to the oxygen atom having a partial negative charge and each hydrogen atom having a partial positive charge. The bent shape of the water molecule further contributes to its overall polarity, as the dipole moments of the two O-H bonds do not cancel each other out.

Exploring Intermolecular Forces

The primary intermolecular forces (IMFs) that govern solubility are:

Hydrogen Bonding: This is a strong type of dipole-dipole interaction that occurs when a hydrogen atom bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) is attracted to another electronegative atom in a different molecule. Hydrogen bonding is crucial for the solubility of many polar compounds in water.
Dipole-Dipole Interactions: These occur between polar molecules due to the attraction between the partial positive end of one molecule and the partial negative end of another. These interactions are weaker than hydrogen bonds but still contribute significantly to the solubility of polar substances in polar solvents.
London Dispersion Forces (LDFs): These are temporary, weak attractive forces that arise from instantaneous fluctuations in electron distribution within molecules. LDFs are present in all substances, both polar and nonpolar, but they are the dominant IMF in nonpolar substances. The strength of LDFs increases with increasing molecular size and surface area.

The Science Behind Solubility

Solubility is the ability of a substance (solute) to dissolve in a solvent to form a homogeneous mixture. The process of dissolution involves breaking the intermolecular forces holding the solute molecules together and the intermolecular forces holding the solvent molecules together. New intermolecular forces then form between the solute and solvent molecules.

For a solute to dissolve, the energy required to break the solute-solute and solvent-solvent interactions must be less than or equal to the energy released when new solute-solvent interactions are formed. This is where the principle of "like dissolves like" comes into play.

Polar Solvents & Polar Solutes: Polar solvents like water (H₂O) and ethanol (C₂H₅OH) have strong dipole-dipole interactions and hydrogen bonding capabilities. They readily dissolve polar solutes such as salt (NaCl) and sugar (C₁₂H₂₂O₁₁) because the solute-solvent interactions (ion-dipole or dipole-dipole) are strong enough to overcome the solute-solute (ionic or dipole-dipole) and solvent-solvent (hydrogen bonding) interactions.
Nonpolar Solvents & Nonpolar Solutes: Nonpolar solvents like hexane (C₆H₁₄) and toluene (C₇H₈) primarily exhibit London dispersion forces. They effectively dissolve nonpolar solutes like oil and grease because the solute-solvent interactions (LDFs) are comparable in strength to the solute-solute and solvent-solvent interactions.
Polar Solvents & Nonpolar Solutes: When a polar solvent attempts to dissolve a nonpolar solute, the strong intermolecular forces of the solvent (e.g., hydrogen bonds in water) must be disrupted to make room for the solute molecules. However, the weak London dispersion forces between the nonpolar solute and the polar solvent are not strong enough to compensate for the energy required to break the solvent-solvent interactions. As a result, the nonpolar solute remains undissolved. Similarly, nonpolar solvents struggle to dissolve polar solutes.

Practical Examples of "Like Dissolves Like"

The "like dissolves like" principle has numerous practical applications in everyday life, chemistry labs, and various industries.

In the Kitchen

Sugar in Water: Sugar (sucrose) is a polar molecule due to the presence of numerous hydroxyl (-OH) groups that can form hydrogen bonds with water molecules. Therefore, sugar readily dissolves in water.
Oil and Vinegar Salad Dressing: Oil is nonpolar, while vinegar is primarily water (polar). As a result, oil and vinegar do not mix. They form separate layers because the intermolecular forces between oil molecules are significantly different from those between water molecules. To create a stable salad dressing, an emulsifier like mustard (which contains both polar and nonpolar components) is often added to bridge the gap between the oil and vinegar.
Removing Grease: Water alone is often ineffective at removing grease from dishes or clothing because grease is nonpolar. Soap, however, contains both a polar (hydrophilic) head and a nonpolar (hydrophobic) tail. The hydrophobic tail interacts with the grease, while the hydrophilic head interacts with the water, allowing the grease to be washed away.

In the Chemistry Lab

Extraction: In chemical separations, "like dissolves like" is used to selectively extract compounds from mixtures. For example, if you want to extract a nonpolar compound from an aqueous solution, you would use a nonpolar solvent like diethyl ether or hexane. The nonpolar compound will preferentially dissolve in the nonpolar solvent, allowing it to be separated from the polar components of the mixture.
Chromatography: Various chromatographic techniques, such as thin-layer chromatography (TLC) and column chromatography, rely on the principle of "like dissolves like" to separate compounds based on their polarity. The stationary phase in chromatography can be polar or nonpolar, and compounds will separate based on their affinity for the stationary phase and the mobile phase (solvent).
Recrystallization: This purification technique leverages solubility differences to purify solid compounds. A solid is dissolved in a hot solvent, and as the solution cools, the solid crystallizes out. Impurities with different polarities remain dissolved in the solvent. Choosing the right solvent is crucial and guided by the "like dissolves like" principle.

In Industry

Pharmaceuticals: The solubility of drugs is a critical factor in their effectiveness. Drug developers must consider the solubility of a drug in various solvents, including water (for intravenous administration) and lipids (for absorption through cell membranes). Understanding "like dissolves like" helps in formulating drugs that can be effectively delivered and absorbed by the body.
Paints and Coatings: The selection of solvents for paints and coatings is guided by the "like dissolves like" principle. The solvent must be able to dissolve the resin and other components of the paint but also evaporate at a controlled rate to allow the paint to dry properly.
Dry Cleaning: Dry cleaning utilizes nonpolar solvents like perchloroethylene to remove stains from fabrics that are difficult to clean with water. The nonpolar solvent effectively dissolves grease, oil, and other nonpolar stains without damaging delicate fabrics.

Beyond Simple Polarity: Limitations of the Rule

While "like dissolves like" is a useful guideline, it's important to recognize its limitations. Solubility is a complex phenomenon influenced by factors beyond just polarity.

Temperature: Temperature significantly affects solubility. In general, the solubility of solids in liquids increases with increasing temperature, while the solubility of gases in liquids decreases with increasing temperature.
Pressure: Pressure has a negligible effect on the solubility of solids and liquids but significantly affects the solubility of gases in liquids. Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.
Molecular Size and Shape: Larger molecules tend to be less soluble than smaller molecules due to increased intermolecular forces and steric hindrance. The shape of a molecule can also affect its solubility, with more symmetrical molecules generally being more soluble.
Hydrogen Bonding Donors and Acceptors: The ability of a molecule to donate or accept hydrogen bonds can significantly influence its solubility, especially in water. Molecules with both hydrogen bond donors and acceptors tend to be more soluble in water than molecules with only one or the other.
Salting Out: The addition of a salt to a solution can sometimes decrease the solubility of a nonpolar solute, a phenomenon known as "salting out." This occurs because the ions of the salt interact strongly with the polar solvent molecules, reducing their ability to interact with the nonpolar solute.
Complex Formation: Some substances can form complexes with solvents, which can increase their solubility beyond what would be predicted by the "like dissolves like" principle. For example, iodine (I₂) is only slightly soluble in water, but its solubility increases significantly in the presence of iodide ions (I⁻) due to the formation of the triiodide ion (I₃⁻).

The Importance of Entropy

While intermolecular forces are crucial, it is also important to consider the role of entropy in the dissolution process. Entropy is a measure of disorder or randomness. Dissolving a solute in a solvent generally increases the entropy of the system because the solute and solvent molecules are more randomly distributed in the solution than they were in the separate solid and liquid phases. This increase in entropy contributes to the spontaneity of the dissolution process.

For a dissolution process to be spontaneous, the change in Gibbs free energy (ΔG) must be negative. The Gibbs free energy is related to the enthalpy change (ΔH) and the entropy change (ΔS) by the equation:

ΔG = ΔH - TΔS

Where T is the temperature in Kelvin.

A negative ΔH (exothermic process) and a positive ΔS (increase in entropy) favor a negative ΔG and thus spontaneous dissolution. Even if ΔH is positive (endothermic process), a sufficiently large positive ΔS can still result in a negative ΔG and spontaneous dissolution. This explains why some substances dissolve even though the process is endothermic.

"Like Dissolves Like": A Fundamental Principle

The phrase "like dissolves like" provides a powerful and intuitive framework for understanding solubility. By considering the polarity and intermolecular forces of both the solute and solvent, one can predict with reasonable accuracy whether a substance will dissolve. While other factors, such as temperature, pressure, and entropy, also play a role, the "like dissolves like" principle remains a cornerstone concept in chemistry, guiding countless applications in diverse fields. Its enduring relevance underscores its fundamental importance in understanding the behavior of matter at the molecular level.