A crystal is a solid material where the constituent atoms, molecules, or ions are arranged in a highly ordered, repeating pattern called a crystal lattice. This precise internal architecture gives crystals their characteristic geometric shapes. Solubility is the chemical property describing the ability of a substance (the solute) to dissolve in a liquid (the solvent) to form a uniform solution. Dissolution occurs when the attractive forces between the solvent molecules and the solute particles are strong enough to overcome the forces holding the crystal lattice together.
Why Water Is a Universal Solvent
Water’s ability to dissolve numerous substances is linked to its unique molecular structure. A single water molecule (H₂O) consists of two hydrogen atoms and one oxygen atom. The shared electrons are not distributed evenly; the oxygen atom pulls them closer, creating a slight negative charge on the oxygen side and slight positive charges on the two hydrogen sides.
This separation of charge makes water a polar molecule, acting like a tiny magnet with distinct positive and negative poles. When a crystal is placed in water, the charged ends of the water molecules are attracted to the oppositely charged particles within the crystal lattice. For an ionic crystal, such as salt, the negative oxygen ends of the water molecules surround the positive ions, while the positive hydrogen ends surround the negative ions.
The collective force of many water molecules pulling on the crystal’s ions is powerful enough to break the ionic bonds holding the crystal together. Once separated, the individual ions are encased by a shell of water molecules, a process called hydration, which keeps them dispersed throughout the liquid. This mechanism explains why water, a highly polar solvent, effectively dissolves other polar substances or ionic compounds.
Examples of Crystals That Dissolve and Those That Do Not
Crystals that dissolve in water are typically those with ionic bonds or highly polar molecular structures. Common table salt (sodium chloride) is a primary example of a soluble ionic crystal, where water molecules easily pull apart the positively charged sodium ions and negatively charged chloride ions. Epsom salts (magnesium sulfate) are another example of a highly soluble ionic compound.
Sugar (sucrose) is a common example of a crystal that dissolves easily despite not being ionic. Sugar molecules are highly polar because they contain many hydroxyl (-OH) groups, allowing them to form strong hydrogen bonds with water molecules. This strong attraction overcomes the weaker intermolecular forces holding the sugar crystal together, causing it to dissolve.
Conversely, crystals that do not dissolve in water possess bonds too strong for water’s polarity to overcome, or they are non-polar themselves. Quartz, a mineral crystal made of silicon dioxide, is insoluble because its atoms are held together by an extensive network of strong covalent bonds. Water molecules are unable to break these bonds, causing the quartz to remain solid.
Diamond, which is pure carbon, is another example of an insoluble crystal due to its rigid, three-dimensional network of covalent bonds. Certain minerals and most non-polar crystalline solids, such as waxes, also remain undissolved. This occurs because water molecules are more attracted to each other than they are to the uncharged surfaces of the non-polar crystal, favoring the crystal remaining intact.
How Temperature and Other Factors Affect Solubility
Temperature is a primary factor influencing the dissolution of crystalline solids in water. For most solids, increasing the water’s temperature provides the solvent molecules with greater kinetic energy. This causes them to move faster and collide with the crystal surface more frequently, allowing water molecules to break apart the crystal lattice more effectively. This generally results in a greater maximum amount of solute that can be dissolved.
While temperature affects the final amount that can dissolve, other factors primarily influence the rate of dissolution. Reducing the particle size of the crystal, such as by crushing it, increases its surface area exposed to the water. Since dissolution is a surface-level phenomenon, maximizing the contact area between the crystal and the solvent speeds up the process.
Agitation, or stirring the solution, also increases the dissolution rate by continuously moving fresh, unsaturated water molecules to the crystal’s surface. Without stirring, the water immediately surrounding the crystal becomes saturated quickly, slowing the overall process. Therefore, a combination of heat, fine particles, and stirring dissolves a crystal much faster than an unheated, stagnant solution.