Water dissolves many materials. When a substance dissolves, its particles spread evenly within the water, forming a uniform mixture called a solution. Dissolution occurs because water molecules interact with a substance’s particles, pulling them apart and dispersing them. Water’s effectiveness as a solvent leads to its common reference as the “universal solvent,” though it does not dissolve everything.
Understanding How Things Dissolve
Water’s dissolving power stems from its molecular structure. A water molecule, with two hydrogen and one oxygen atom, has a bent shape. This results in an uneven electrical charge distribution, with the oxygen side having a slight negative charge and hydrogen sides carrying slight positive charges. This makes water a “polar” molecule, acting like a tiny magnet with distinct positive and negative ends.
This polarity allows water molecules to attract and interact with other molecules having charged regions or particles, such as ions. The principle “like dissolves like” explains this: polar solvents like water dissolve other polar substances and ionic compounds. When a soluble substance enters water, water molecules surround its particles, positive ends attracting negative parts of the solute and negative ends attracting positive parts.
This attraction helps water molecules pull solute particles away and disperse them throughout the liquid. For ionic compounds, this process, called dissociation, involves water molecules separating positive and negative ions. For polar molecular compounds, water forms new bonds, such as hydrogen bonds, with solute molecules, integrating them into the solution.
Common Substances That Dissolve in Water
Many everyday substances dissolve in water. Ionic compounds, such as table salt (sodium chloride), are a prime example. When salt is added to water, polar water molecules surround the charged sodium and chloride ions, pulling them apart and distributing them evenly to form a clear solution. This dissociation process is why salt seems to disappear in water.
Sugars, like sucrose and glucose, are highly soluble in water. These polar molecular compounds form hydrogen bonds with water molecules. The network of hydrogen bonds between sugar and water molecules allows sugar to dissolve completely, creating a sweet solution.
Certain gases dissolve in water, although their solubility is generally less visible. Carbon dioxide, for instance, dissolves in water to create carbonated beverages. Ammonia is another gas that dissolves in water, forming ammonium hydroxide. Water’s ability to dissolve gases is crucial for aquatic life, as dissolved oxygen is necessary for fish and other organisms to breathe.
Common Substances That Do Not Dissolve in Water
While water is an excellent solvent, some substances do not dissolve in it. Nonpolar compounds, such as oils and fats, are examples. These substances lack the charged regions or ionic structure water molecules are attracted to. Instead, oil molecules are more attracted to other oil molecules than to water, causing them to separate and form distinct layers.
Certain ionic compounds, despite being composed of charged particles, exhibit very low solubility in water. Minerals and rocks like sand, primarily silicon dioxide, do not dissolve because the bonds holding their atomic structures together are too strong for water molecules to overcome. The attractive forces within the sand particles are stronger than water’s ability to pull them apart.
Polymers, such as plastics, generally do not dissolve in water. Their complex, long-chain molecular structures and nonpolar nature mean there are insufficient attractive forces between water molecules and polymer chains to break them apart and disperse them. These materials typically remain intact when exposed to water.
Factors Affecting Dissolution
Several factors can influence how quickly or how much of a substance dissolves in water. Temperature plays a significant role; for most solid substances, increasing the water’s temperature generally increases their solubility and the rate at which they dissolve. For example, sugar dissolves more quickly and in larger quantities in hot tea compared to cold tea.
Conversely, the solubility of gases in water typically decreases as temperature rises. This is why a warm soda goes flat faster than a cold one, as carbon dioxide gas escapes more readily from warmer water. The increased kinetic energy of gas molecules at higher temperatures makes it harder for them to remain dissolved.
Surface area also impacts the rate of dissolution. Substances with a larger surface area exposed to the water dissolve faster. Crushing a sugar cube into granulated sugar, for instance, increases its surface area, allowing it to dissolve more rapidly. Agitation, or stirring, is another factor that speeds up dissolution by bringing fresh solvent into contact with the solute particles, helping to disperse them quickly.