The phrase “like dissolves like” is a fundamental principle in chemistry that explains why certain substances readily mix to form solutions while others do not. This statement provides a rule of thumb for predicting solubility, which is the ability of one substance to dissolve into another.
Understanding Polarity
Molecular polarity refers to the uneven distribution of electrical charge within a molecule. This occurs when atoms within a molecule do not share electrons equally, creating regions with slight positive and slight negative charges, known as a dipole. Electronegativity, the ability of atoms to attract electrons, determines the extent of this uneven sharing. If there is a significant difference in electronegativity between bonded atoms, the bond itself becomes polar.
Molecular geometry also influences overall polarity. A molecule with polar bonds can still be nonpolar if its symmetrical structure causes these individual bond dipoles to cancel each other out. For instance, carbon dioxide has polar carbon-oxygen bonds, but its linear shape means the dipoles cancel, making the molecule nonpolar. In contrast, water is a polar molecule because its bent shape prevents the dipoles from canceling, resulting in a net positive charge on the hydrogen side and a net negative charge on the oxygen side.
Nonpolar molecules have an even charge distribution, either due to similar electronegativity or symmetrical arrangement of polar bonds. Examples of nonpolar molecules include oxygen gas and hydrocarbons like propane.
The Mechanics of Dissolving
Dissolving depends on the intermolecular forces (IMFs) between molecules. For a solute to dissolve in a solvent, attractions between solute and solvent must be comparable to or stronger than forces holding solute molecules together and solvent molecules together.
Polar substances, with uneven charge distributions, form strong attractions like dipole-dipole interactions or hydrogen bonds. These forces allow solvent molecules to surround and separate solute molecules, integrating them into the solution. Nonpolar substances interact through weaker London dispersion forces. When nonpolar solutes and solvents mix, their weak attractions allow them to combine. The energy to separate solute and solvent molecules is compensated by new, comparable weak attractions.
Mixing polar and nonpolar substances is challenging. Polar molecules, with strong attractions, cluster together, excluding nonpolar molecules. The energy to disrupt strong polar attractions for weak polar-nonpolar interactions is not offset. Polar solvent molecules prefer to interact with each other, separating the nonpolar substance.
Practical Examples of “Like Dissolves Like”
The principle of “like dissolves like” appears in many everyday scenarios. Sugar dissolving in water is a common example. Both water and sugar are polar molecules, forming hydrogen bonds. These attractions allow water to disperse sugar molecules, forming a homogeneous solution.
Table salt, an ionic compound, readily dissolves in water. Water’s polarity allows it to attract the salt’s positive sodium ions and negative chloride ions. These ion-dipole interactions overcome ionic bonds, separating ions into solution.
Oil and water do not mix. Water is highly polar, while most oils are nonpolar hydrocarbons. Water molecules prefer to associate with each other, as do oil molecules. This polarity difference causes separation into layers, with oil floating on top.
Soap allows oil and water to mix. Soap molecules are amphiphilic, possessing a polar (hydrophilic) end and a nonpolar (hydrophobic) end. When added to oily water, soap’s nonpolar tails embed in grease, while polar heads remain exposed to water. This forms micelles, encapsulating oil droplets for dispersal and washing away.
Nail polish remover, typically acetone, is another application. Nail polish is primarily nonpolar organic polymers and dyes. Acetone has both polar and nonpolar characteristics, allowing it to dissolve nail polish by disrupting its forces.
Paint thinner selection also illustrates this principle. Oil-based paints, with nonpolar resins, use nonpolar solvents like mineral spirits. Water-based (latex) paints use water as their primary solvent. Choosing the correct thinner based on paint polarity ensures proper application and cleaning.