Solubility in water is primarily determined by the polarity of the substance being dissolved and its interaction with water's polar nature.
The key principle governing what dissolves in water is often summarized by the phrase "like dissolves like." Since water (H₂O) is a polar molecule, it has a positive charge on one side and a negative charge on the other. This polarity allows it to effectively interact with and dissolve other polar substances and many ionic compounds.
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Whether a solid is soluble in water depends on its polarity. Since water is a polar molecule, it will only dissolve polar solids, and many ionic compounds which dissociate in water. Water does not dissolve nonpolar molecular compounds, and does not dissolve all ionic compounds.
Understanding Polarity and Solubility
Water's unique structure, with oxygen being more electronegative than hydrogen, creates a dipole moment, making it polar. For another substance to dissolve well in water, it needs to be able to form attractive forces with the water molecules.
Polar Substances
- How they dissolve: Polar molecules have charged ends similar to water. When a polar substance is added to water, the positive end of a water molecule is attracted to the negative end of the solute molecule, and vice versa. These attractions (specifically, hydrogen bonds or dipole-dipole interactions) are strong enough to pull the solute molecules apart and surround them, effectively dissolving the substance.
- Examples: Sugar (sucrose), ethanol (drinking alcohol).
Ionic Compounds
- How they dissolve: Ionic compounds are made of positively and negatively charged ions held together by strong electrostatic forces. When many ionic compounds are placed in water, the polar water molecules are strongly attracted to these ions. The positive ends of water molecules surround the negative ions, and the negative ends surround the positive ions. This process, called hydration, pulls the ions away from the crystal lattice and disperses them throughout the water.
- Examples: Table salt (sodium chloride, NaCl), potassium chloride (KCl).
- Important Note: As mentioned in the reference, water does not dissolve all ionic compounds. Some ionic compounds have very strong internal forces that the hydration energy from water cannot overcome, making them insoluble (e.g., silver chloride, AgCl; barium sulfate, BaSO₄).
Nonpolar Substances
- Why they don't dissolve: Nonpolar molecules do not have significant positive or negative ends. They primarily interact via weak forces (van der Waals forces). Water molecules are much more attracted to each other (due to strong hydrogen bonds) than they are to nonpolar molecules. When a nonpolar substance is mixed with water, the water molecules push the nonpolar molecules away, preferring to stay together. This causes the nonpolar substance to separate, forming layers or droplets instead of dissolving.
- Examples: Oil, grease, wax, hexane. Water does not dissolve nonpolar molecular compounds.
Factors Influencing Solubility
While polarity is the primary determinant, other factors can influence the degree of solubility:
- Temperature: For most solids, solubility in water increases with temperature. For gases, solubility usually decreases with increasing temperature.
- Pressure: This primarily affects the solubility of gases; higher pressure leads to higher gas solubility (Henry's Law).
- Surface Area: For solids, a larger surface area (e.g., a powder vs. a large crystal) can speed up the rate of dissolution, but it does not change the total amount that can dissolve (solubility).
Summary Table
| Substance Type | Polarity | Interaction with Water | Solubility in Water |
|---|---|---|---|
| Water (Solvent) | Polar | Attracts polar/ionic | N/A (is the solvent) |
| Polar Solute | Polar | Strong attraction | Generally soluble |
| Ionic Compound | Charged Ions | Strong attraction | Many are soluble |
| Nonpolar Molecular Comp. | Nonpolar | Weak interaction | Generally not soluble |
Understanding these relationships is crucial in many scientific fields, from chemistry and biology to environmental science.
