When two substances are mixed, they either blend completely into a uniform mixture or separate into distinct layers. This process is governed by solubility, which describes the ability of a substance, the solute, to dissolve in another substance, the solvent, forming a solution. The solute is typically the substance present in the smaller amount, while the solvent is the one dissolving it. Understanding the chemical nature of both the solute and the solvent is necessary to predict if a solution will form.
The Core Principle of Solubility
Water is often referred to as the “universal solvent” because of its unique molecular structure that allows it to dissolve more substances than any other liquid. A single water molecule (H₂O) has a bent shape, meaning the atoms are not arranged in a straight line. This shape, combined with the unequal sharing of electrons between the oxygen and hydrogen atoms, gives the molecule a positive end and a negative end, making it a polar molecule. The oxygen atom holds the shared electrons more tightly, resulting in a slight negative charge, while the hydrogen atoms carry slight positive charges. This charge separation creates an electrical dipole moment, allowing water molecules to attract each other through strong hydrogen bonds. These strong internal attractions are the driving force behind water’s solvent power. The fundamental rule for predicting solubility is “Like Dissolves Like,” meaning that a polar solvent, like water, readily dissolves other polar or charged substances.
Substances That Dissolve Readily
Substances that dissolve easily in water are either ionic compounds or polar covalent compounds.
Ionic Compounds
Ionic compounds, such as table salt (sodium chloride), are held together by the strong electrostatic attraction between positively and negatively charged ions. When salt crystals are placed in water, the polar water molecules surround these ions. The negative oxygen ends of the water molecules are attracted to the positive sodium ions (\(\text{Na}^+\)), and the positive hydrogen ends are attracted to the negative chloride ions (\(\text{Cl}^-\)). This attraction is strong enough to pull the ions out of the crystal lattice, causing the salt to dissociate, or split apart, into individual ions. Each separated ion becomes surrounded by a shell of water molecules, known as a hydration shell, which keeps the ions dispersed and prevents them from rejoining.
Polar Covalent Compounds
Polar covalent compounds, like common table sugar (sucrose), dissolve differently because they do not dissociate into ions. Sugar molecules contain numerous hydroxyl (\(\text{OH}\)) groups, which are highly polar and allow the sugar molecule to form hydrogen bonds directly with the surrounding water molecules. Instead of breaking apart, the intact sugar molecules are simply pulled away from each other and surrounded by water, dissolving as whole molecules. This process makes both ionic and polar molecules hydrophilic, or “water-loving,” because their charges allow them to interact favorably with the polar water molecules.
Substances That Remain Insoluble
The substances that do not dissolve in water are those that are non-polar or lack a significant electrical charge separation across their structure. These molecules are typically composed of atoms that share electrons equally, resulting in no net dipole moment. Common examples include oils, fats, and waxes, which are largely composed of long hydrocarbon chains.
When a non-polar substance is introduced to water, it is unable to form strong attractive forces like hydrogen bonds or electrostatic interactions with the water molecules. The water molecules, which are strongly attracted to each other, prioritize maintaining their own hydrogen bond network. The non-polar molecules are effectively excluded from the water network, a phenomenon known as the hydrophobic effect, meaning “water-fearing.”
Consequently, non-polar liquids like oil cluster together, minimizing their contact surface area with the water, which is why oil and water separate into distinct layers. Similarly, materials like sand or certain rocks are insoluble because they are structurally complex or have strong bonds that water molecules cannot overcome. The absence of polar or ionic groups means the water’s attraction to itself remains stronger than any attraction to the insoluble substance, leading to separation or settling.