When a substance is mixed with water, it either dissolves to form a uniform solution or remains separate. A substance is soluble if its particles can be completely surrounded and dispersed by water molecules. Conversely, a substance is insoluble if its internal forces are too strong or its molecular structure is incompatible with water, preventing dissolution. Water is often referred to as the universal solvent because it can dissolve a greater variety of substances than almost any other liquid.
The Fundamental Principle of Insolubility
The ability of a substance to dissolve in water is determined by the chemical principle known as “like dissolves like.” Water molecules are structured with a distinct separation of electrical charge, making water a polar solvent. This polarity means water readily attracts and breaks apart other molecules that are also polar or possess an electrical charge, such as ionic salts.
Insolubility occurs when a substance is non-polar, lacking this internal charge separation, or when its internal bonds are too strong for water molecules to break. Water molecules are strongly attracted to each other through hydrogen bonds, forming a tight, structured network. When a non-polar substance is introduced, water’s molecules prefer to maintain their strong attractions to one another rather than form weaker attractions with the non-polar substance. The non-polar substance is essentially squeezed out of the water’s attractive network.
Everyday Examples of Insoluble Solids
Many common solid materials fail to dissolve in water due to a lack of polarity or the sheer strength of their internal chemical bonds. Sand, primarily silicon dioxide, is an example of the latter, as its atoms are held together by a giant network of extremely strong covalent bonds. Water molecules do not possess enough energy to break this vast, tightly bound crystalline structure and pull the individual units away.
Metals like iron, copper, and aluminum are also insoluble because of their metallic bonding. These substances exist as a lattice of positive ions surrounded by a “sea” of delocalized electrons, held together by powerful metallic bonds. Water molecules cannot penetrate this structure or overcome the strong forces holding the atoms together to achieve dissolution.
The majority of plastics, such as polyethylene, are insoluble due to their chemical composition as apolar polymers. These plastics are composed of long chains of non-polar carbon and hydrogen atoms, making them hydrophobic, or water-fearing. This non-polar nature prevents interaction with the water’s hydrogen-bonded network.
Everyday Examples of Insoluble Liquids and Gasses
Insoluble liquids are characterized by their non-polar molecular structure, which prevents them from mixing with polar water. Common cooking oils, such as vegetable oil, are long-chain triglycerides that are entirely non-polar hydrocarbons. When oil and water are mixed, the water molecules strongly cohere to each other, forcing the oil molecules into a separate layer, a phenomenon known as immiscibility. Petroleum products like gasoline and motor oil are complex mixtures of non-polar hydrocarbons that remain separate from water.
Waxes and fats also share this non-polar characteristic, composed of large hydrocarbon segments, making them chemically incompatible with the polar water solvent. In the gaseous state, several common elements and compounds exhibit very low solubility in water, primarily due to their lack of polarity.
Gases like methane, nitrogen, and oxygen are non-polar molecules with very weak intermolecular forces. While trace amounts can dissolve, they do not interact strongly with the polar water molecules. The water molecules maintain their strong hydrogen-bonded network, and the gas molecules are unable to integrate effectively, resulting in low solubility.