Generally, most molecular compounds are insoluble or only very sparingly soluble in water. However, some molecular compounds do dissolve, typically because they can react with water or form hydrogen bonds with water molecules.
Understanding Molecular Solubility
Water is a polar solvent, meaning it has a slightly positive end and a slightly negative end. Solubility often depends on how well a solute (the substance being dissolved) can interact with the solvent (water). Molecular compounds are formed when atoms share electrons (covalent bonds). Unlike ionic compounds which break apart into charged ions in water, molecular compounds usually remain intact molecules when dissolved.
The General Rule for Molecular Compounds
As stated in the reference: "Most molecular substances are insoluble (or only very sparingly soluble) in water". This is because many molecular compounds are nonpolar or have limited polarity, and they don't have strong enough attractions to overcome the strong attractions between water molecules themselves. Think of oil and water – oil is a molecular compound, but it doesn't mix because its molecules aren't attracted to water molecules.
Exceptions: When Molecular Compounds Do Dissolve
The reference clarifies the conditions under which molecular substances dissolve: "Those which do dissolve often react with the water, or else are capable of forming hydrogen bonds with the water."
- Hydrogen Bonding: Water molecules can form special, strong attractions called hydrogen bonds with other molecules that have O-H, N-H, or sometimes F-H bonds, or oxygen/nitrogen atoms with lone pairs of electrons. If a molecular compound can form hydrogen bonds with water, these new attractions can be strong enough to pull the molecular compound into solution.
- Examples: Sugar (like glucose, C₆H₁₂O₆) and ethanol (drinking alcohol, C₂H₅OH) are molecular compounds that dissolve readily in water because they can form numerous hydrogen bonds with water molecules.
- Reaction with Water: Some molecular compounds dissolve because they undergo a chemical reaction with water, often breaking apart or changing their structure.
- Example: Although technically a gas, hydrogen chloride (HCl) is a molecular compound. When it dissolves in water, it reacts to form hydrochloric acid by ionizing into H⁺ and Cl⁻ ions.
Solubility Overview
Here's a simple summary based on the reference:
| Type of Molecular Compound | Solubility in Water | Primary Reason for Solubility (Based on Reference) |
|---|---|---|
| Most Molecular Substances | Insoluble or Very Sparingly Soluble | Generally lack the ability to form strong interactions (like hydrogen bonds) or react significantly with water. |
| Specific Molecular Substances | Soluble | Capable of forming hydrogen bonds with water OR react with water. |
Key Points to Remember
- Solubility of molecular compounds in water is not a universal YES.
- The majority are not soluble or only slightly soluble.
- Solubility happens for molecular compounds that can hydrogen bond or react with water.
Understanding these interactions helps explain why some molecular substances like sugar mix perfectly with water, while others like oil do not.
