Universal Solvent? Water?

Water’s Ability to Dissolve (What Water Can Dissolve)

Water is known as the “universal solvent” because it dissolves a wide range of substances, particularly those that are ionic or polar. However, not all substances are easily soluble in water.

What Water Can Dissolve:

• Ionic Compounds (Salts): Water can dissolve most ionic compounds, such as sodium chloride (NaCl), potassium nitrate (KNO₃), and magnesium sulfate (MgSO₄). These dissolve because water molecules surround the ions and break apart the ionic lattice.
• Ammonium Compounds: These are generally soluble in water, like ammonium chloride (NH₄Cl), due to their polarity.
• Nitrates and Acetates: Nitrates and acetates (e.g., potassium acetate) are highly soluble in water.
• Chlorides, Bromides, and Iodides: Most of these are soluble in water, except for a few, such as silver chloride (AgCl), lead chloride (PbCl₂), and mercury(II) chloride (HgCl₂).
• Sulfates: Water dissolves most sulfates, but some, like barium sulfate (BaSO₄) and calcium sulfate (CaSO₄), are only sparingly soluble. Lead sulfate (PbSO₄) also has low solubility in water.
• Alcohols: Simple alcohols, like methanol and ethanol, are soluble because they have a hydroxyl group (-OH) that can form hydrogen bonds with water.
• Sugars: Water readily dissolves many simple sugars, such as glucose, fructose, and sucrose, due to hydrogen bonding.
• Amino Acids: Many amino acids are soluble in water because they contain both hydrophilic (polar) and hydrophobic (non-polar) parts.
• Some Gases: Water can dissolve certain gases, like oxygen (O₂) and carbon dioxide (CO₂), although their solubility depends on temperature and pressure.

Clarifications:

• Hydrogen Peroxide (H₂O₂) is a relatively weak solute in water, but it is soluble.
• Ammonia (NH₃) is highly soluble in water and forms ammonium hydroxide (NH₄OH), making it a base.
• Sulfuric Acid (H₂SO₄), Nitric Acid (HNO₃), and Hydrochloric Acid (HCl) are all highly soluble in water.

Water’s Limited Ability to Dissolve (What Water Cannot Dissolve)

While water dissolves many substances, it’s not effective at dissolving all compounds, especially those that are non-polar or hydrophobic.

What Water Cannot Dissolve:

• Oils and Fats: These are non-polar substances that do not interact well with water, which is polar. Therefore, oils (like vegetable oil) and fats do not dissolve in water. However, they can form emulsions with the help of surfactants.
• Petroleum Products: Like oils, petroleum is a mixture of hydrocarbons and does not dissolve in water.
• Plastics: Most plastics (like polystyrene and polyethylene) are non-polar and insoluble in water.
• Paraffin Wax: Like oils, paraffin wax is hydrophobic and doesn’t dissolve in water.
• Sand and Chalk: Sand (mainly silicon dioxide) and chalk (calcium carbonate) are insoluble in water due to their crystalline structure. Calcium carbonate can dissolve in acidic water (like rainwater containing CO₂) but not in pure water.
• Most Molecular Substances (Non-Polar Compounds): Compounds like oxygen (O₂) and nitrogen (N₂) are gases and do dissolve to some extent in water, but they are not considered “soluble” in the same sense as salts or polar molecules.
• Many Hydroxides: While sodium hydroxide (NaOH) and potassium hydroxide (KOH) are highly soluble in water, most hydroxides (like iron(III) hydroxide or calcium hydroxide) are insoluble or only slightly soluble.
• Most Carbonates: Sodium carbonate (Na₂CO₃) and potassium carbonate (K₂CO₃) are soluble in water, but many metal carbonates (like calcium carbonate (CaCO₃) and barium carbonate (BaCO₃)) are insoluble.
• Phosphates: Most phosphates (like calcium phosphate or magnesium phosphate) are insoluble in water, but some (like sodium phosphate) are soluble.
• Sulfides: Most metal sulfides (like lead sulfide (PbS) or zinc sulfide (ZnS)) are insoluble, but ammonium sulfide (NH₄)₂S is soluble.
• Some Chlorides: While most chlorides dissolve in water, some, like silver chloride (AgCl) and lead chloride (PbCl₂), have low solubility.
• Chromates and Fluorides: Chromates like lead chromate (PbCrO₄) and barium chromate (BaCrO₄) are insoluble, and some fluorides, like barium fluoride (BaF₂), are also poorly soluble in water.

Clarifications:

• Polystyrene is a polymer and is insoluble in water, but it can dissolve in non-polar solvents like toluene or acetone.
• Sulfur compounds like sulfur (S₈) and sulfur dioxide (SO₂) are only sparingly soluble in water.
• Sodium bicarbonate (NaHCO₃) and calcium chloride (CaCl₂) are soluble in water, but calcium sulfate (CaSO₄) is slightly soluble and requires specific conditions to dissolve.

General Notes:

• Solubility is influenced by factors like temperature, pressure, and the nature of the substance. For instance, temperature generally increases the solubility of many solid solutes, while gases tend to become less soluble in water as temperature increases.
• Water-soluble vitamins (like vitamin C and B-vitamins) dissolve in water, which is why we need to regularly consume them in our diet.

Examples of different types of water dissolving substances:

1.

Soft water contains few or no dissolved minerals. When it dissolves table salt (NaCl), the process can be represented as:

H2O + NaCl → Na+ + Cl- + H2O

The water molecules surround the sodium and chloride ions, forming hydration shells. The salt dissociates into its constituent ions in the water.

2.

Hard water, which contains dissolved calcium and magnesium ions, can further dissolve limestone (CaCO3) in the presence of dissolved carbon dioxide. This process, known as carbonate dissolution, can be represented as:

CaCO3 + H2O + CO2 → Ca2+ + 2HCO3-

The limestone dissolves, increasing the calcium ion concentration and forming bicarbonate ions in the water.

3.

Sulphurous water contains dissolved hydrogen sulfide (H2S). When it comes into contact with iron (Fe), it can dissolve it through a redox reaction:

Fe + H2S → FeS + H2

The iron is oxidized to iron sulfide (FeS), which dissolves in the water, while hydrogen gas is produced. This reaction is one of the reasons why sulphurous water can be corrosive to metals.