An element is a pure substance consisting of atoms with the same number of protons; it cannot be broken down into simpler substances by ordinary chemical means. Solubility describes the maximum amount of a substance (the solute) that can dissolve in a solvent, such as water, to form a uniform solution. Most of the 118 known elements are not soluble in water in their pure, elemental state. The distinction lies between elements that genuinely dissolve, those that chemically react, and those that remain completely insoluble.
Understanding Chemical Interaction and Polarity
For any substance to dissolve in water, its particles must overcome their internal forces and form new, stable attractions with water molecules. Water is a polar solvent, meaning its molecules have a slight positive charge near the hydrogen atoms and a slight negative charge near the oxygen atom due to unequal sharing of electrons. This polarity allows water to dissolve many ionic and polar compounds.
The general principle governing solubility is “like dissolves like.” Water’s strong polarity means it readily dissolves other polar or ionic substances. Conversely, nonpolar elements lack these charge separations and generally do not form strong enough intermolecular forces with water to overcome their own internal bonds.
Elements That Dissolve as Gases
The elements that exhibit true physical solubility in water are those that exist as gases under standard conditions, such as oxygen, nitrogen, and noble gases like argon and helium. These elements are nonpolar, but they dissolve because their molecules are small enough to occupy the empty spaces within the shifting structure of liquid water. The primary force enabling this dissolution is a temporary, weak attraction known as a dipole-induced dipole interaction.
The solubility of these gaseous elements is influenced by external factors. Solubility is directly proportional to the partial pressure of the gas above the liquid. Conversely, the solubility of gases decreases as the temperature of the water increases, causing the gas molecules to escape back into the atmosphere. For instance, warmer bodies of water naturally hold less dissolved oxygen, which sustains aquatic life.
Solid Elements That Interact with Water
The interaction of solid elements with water falls into two distinct categories: those that react chemically and those that are inert. These interactions determine whether the element is consumed, remains unchanged, or forms a new soluble compound.
The solid elements known as the alkali metals, including lithium, sodium, and potassium, do not dissolve but instead react vigorously with water. This is a chemical change, not a physical process of dissolution, where the metal donates an electron to the water molecule. The reaction produces a metal hydroxide and releases hydrogen gas, which is often ignited by the heat generated by the exothermic process. Reactivity increases down this group.
Most remaining solid elements are simply insoluble and unreactive in water. Elements like gold, silver, and the transition metals are held together by strong metallic bonds that water molecules cannot break apart. The energy required to separate these atoms is far greater than the energy released by the weak attractions the water molecules could form.
Similarly, network covalent solids like carbon (diamond or graphite) and silicon possess a continuous network of powerful covalent bonds. This stable structure resists attempts by water molecules to penetrate and pull the atoms away. These elements remain solid and unchanged when immersed in water because the energy balance favors the integrity of the elemental structure.