Which Pair Of Compounds Is Soluble In Water

Water, often dubbed the "universal solvent," possesses a unique ability to dissolve a wide range of substances. Understanding which compounds are soluble in water is crucial in various fields, including chemistry, biology, and environmental science. Solubility hinges on the interplay of intermolecular forces between the solute (the substance being dissolved) and the solvent (water). This article delves into the factors governing solubility, identifies specific pairs of compounds and analyzes why one is water-soluble while the other is not, and provides a comprehensive understanding of this fundamental chemical principle.

Understanding Solubility

Solubility refers to the ability of a substance (solute) to dissolve in a solvent to form a homogeneous solution. The extent of solubility is determined by the interactions between the solute and solvent molecules. Water's exceptional dissolving power stems from its polar nature.

Polarity: Water molecules are polar, meaning they have a slightly negative charge (δ-) on the oxygen atom and slightly positive charges (δ+) on the hydrogen atoms. This polarity arises from the difference in electronegativity between oxygen and hydrogen.
Hydrogen Bonding: Water molecules can form hydrogen bonds with each other and with other polar molecules. These bonds are relatively strong intermolecular forces.
"Like Dissolves Like": A general rule of thumb is that "like dissolves like." Polar solvents like water tend to dissolve polar solutes and ionic compounds, while nonpolar solvents tend to dissolve nonpolar solutes.

Factors Affecting Solubility

Several factors influence the solubility of a compound in water:

Intermolecular Forces: The strength of intermolecular forces between the solute and solvent molecules plays a critical role. If the solute-solvent interactions are stronger than the solute-solute and solvent-solvent interactions, the solute will dissolve.
Polarity: Polar compounds and ionic compounds tend to be more soluble in water due to favorable interactions between the polar water molecules and the charged or polar solute molecules.
Hydrogen Bonding: Compounds that can form hydrogen bonds with water are generally more soluble. Alcohols, amines, and carboxylic acids are examples of compounds that can form hydrogen bonds with water.
Size and Shape: Smaller molecules tend to be more soluble than larger molecules. The shape of the molecule also affects solubility; more symmetrical molecules may pack more efficiently and be less soluble.
Temperature: The solubility of most solids in water increases with increasing temperature. However, the solubility of gases in water typically decreases with increasing temperature.
Pressure: Pressure has a significant effect on the solubility of gases in water but has little effect on the solubility of solids or liquids.

Case Studies: Pairs of Compounds and Their Water Solubility

To illustrate the principles of solubility, let's examine several pairs of compounds and analyze why one is water-soluble while the other is not.

1. Ethanol (C₂H₅OH) vs. Diethyl Ether (C₂H₅OC₂H₅)

Ethanol: Ethanol is an alcohol with a small hydrocarbon chain and a hydroxyl (OH) group. The hydroxyl group makes ethanol polar and allows it to form hydrogen bonds with water. The small hydrocarbon chain does not significantly hinder its solubility.
Diethyl Ether: Diethyl ether has two ethyl groups attached to an oxygen atom. While it has a polar C-O bond, it cannot form hydrogen bonds with water as it lacks a hydrogen atom bonded to the oxygen. The larger nonpolar ethyl groups reduce its overall polarity and solubility in water.

Explanation: Ethanol is highly soluble in water due to its ability to form hydrogen bonds and its relatively small nonpolar region. Diethyl ether, lacking the ability to form hydrogen bonds and having larger nonpolar regions, is only sparingly soluble in water.

2. Sodium Chloride (NaCl) vs. Silver Chloride (AgCl)

Sodium Chloride: Sodium chloride is an ionic compound formed from Na+ and Cl- ions. It is highly soluble in water because water molecules can effectively solvate the ions, forming strong ion-dipole interactions.
Silver Chloride: Silver chloride is also an ionic compound, but it is virtually insoluble in water. The attraction between Ag+ and Cl- ions is much stronger than the attraction between the ions and water molecules, leading to a low degree of solvation.

Explanation: Sodium chloride dissolves readily in water because the hydration energy (the energy released when ions are hydrated by water molecules) is sufficient to overcome the lattice energy (the energy required to separate the ions in the crystal lattice). In contrast, silver chloride has a much higher lattice energy and lower hydration energy, making it insoluble.

3. Glucose (C₆H₁₂O₆) vs. Cyclohexane (C₆H₁₂)

Glucose: Glucose is a simple sugar with multiple hydroxyl (OH) groups. These hydroxyl groups make glucose highly polar and capable of forming extensive hydrogen bonds with water.
Cyclohexane: Cyclohexane is a cyclic hydrocarbon with no polar groups. It is entirely nonpolar and cannot form hydrogen bonds with water.

Explanation: Glucose is very soluble in water due to its numerous hydroxyl groups, which allow for strong hydrogen bonding. Cyclohexane, being nonpolar, is virtually insoluble in water, as there are no favorable interactions between cyclohexane and water molecules.

4. Acetic Acid (CH₃COOH) vs. Octanoic Acid (CH₃(CH₂)₆COOH)

Acetic Acid: Acetic acid is a carboxylic acid with a short hydrocarbon chain and a carboxyl (COOH) group. The carboxyl group makes acetic acid polar and capable of forming hydrogen bonds with water.
Octanoic Acid: Octanoic acid is also a carboxylic acid, but it has a much longer hydrocarbon chain. The long nonpolar chain reduces the overall polarity and solubility in water.

Explanation: Acetic acid is miscible with water (soluble in all proportions) because the polar carboxyl group dominates the behavior of the molecule. Octanoic acid, with its long nonpolar chain, is only sparingly soluble in water, as the nonpolar interactions outweigh the polar interactions.

5. Ammonia (NH₃) vs. Methane (CH₄)

Ammonia: Ammonia is a polar molecule with a lone pair of electrons on the nitrogen atom. It can form hydrogen bonds with water, making it highly soluble.
Methane: Methane is a nonpolar molecule consisting of carbon and hydrogen atoms. It cannot form hydrogen bonds with water.

Explanation: Ammonia is highly soluble in water due to its polarity and ability to form hydrogen bonds. Methane, being nonpolar, is practically insoluble in water, as there are no attractive forces between methane and water molecules.

6. Sucrose (C₁₂H₂₂O₁₁) vs. Palmitic Acid (CH₃(CH₂)₁₄COOH)

Sucrose: Sucrose, or table sugar, is a disaccharide composed of glucose and fructose. It has numerous hydroxyl groups that can form strong hydrogen bonds with water.
Palmitic Acid: Palmitic acid is a saturated fatty acid with a long hydrocarbon chain and a carboxyl group. The long nonpolar chain dominates its behavior, making it insoluble in water.

Explanation: Sucrose is highly soluble in water due to its abundance of hydroxyl groups that can form hydrogen bonds. Palmitic acid, with its extensive nonpolar chain, is insoluble in water because the nonpolar interactions outweigh the polar interactions.

7. Formaldehyde (HCHO) vs. Ethane (C₂H₆)

Formaldehyde: Formaldehyde is a simple aldehyde with a polar carbonyl (C=O) group. The carbonyl group can interact with water molecules through dipole-dipole interactions and hydrogen bonding (as an acceptor).
Ethane: Ethane is a simple alkane with only carbon-hydrogen bonds. It is nonpolar and cannot form hydrogen bonds with water.

Explanation: Formaldehyde is soluble in water due to its polar carbonyl group, which allows it to interact with water molecules. Ethane, being nonpolar, is insoluble in water because there are no favorable interactions between ethane and water molecules.

8. Potassium Iodide (KI) vs. Lead Iodide (PbI₂)

Potassium Iodide: Potassium iodide is an ionic compound formed from K+ and I- ions. It is highly soluble in water due to the relatively weak attraction between the ions and the favorable hydration of the ions by water molecules.
Lead Iodide: Lead iodide is also an ionic compound, but it is sparingly soluble in water. The attraction between Pb2+ and I- ions is stronger, and the hydration energy is insufficient to overcome the lattice energy.

Explanation: Potassium iodide dissolves readily in water because the hydration energy is sufficient to overcome the lattice energy. Lead iodide, however, has a higher lattice energy and lower hydration energy, leading to its low solubility.

9. Glycerol (C₃H₈O₃) vs. Decane (C₁₀H₂₂)

Glycerol: Glycerol is a triol, meaning it has three hydroxyl (OH) groups. These hydroxyl groups make glycerol highly polar and capable of forming extensive hydrogen bonds with water.
Decane: Decane is a straight-chain alkane with ten carbon atoms. It is entirely nonpolar and cannot form hydrogen bonds with water.

Explanation: Glycerol is miscible with water due to its numerous hydroxyl groups, which allow for strong hydrogen bonding. Decane, being nonpolar, is virtually insoluble in water, as there are no favorable interactions between decane and water molecules.

10. Dimethyl Sulfoxide (DMSO) vs. Heptane (C₇H₁₆)

Dimethyl Sulfoxide: DMSO is a polar aprotic solvent with a highly polar sulfoxide (S=O) group. It can interact strongly with water molecules through dipole-dipole interactions.
Heptane: Heptane is a straight-chain alkane with seven carbon atoms. It is nonpolar and cannot form hydrogen bonds with water.

Explanation: DMSO is miscible with water due to its polar sulfoxide group, which allows it to interact strongly with water molecules. Heptane, being nonpolar, is virtually insoluble in water because there are no favorable interactions between heptane and water molecules.

The Role of Entropy in Solubility

While enthalpy (the change in heat) is a significant factor in determining solubility, entropy (the measure of disorder) also plays a crucial role. Dissolving a solute in a solvent generally increases the entropy of the system, as the solute and solvent molecules become more disordered in the solution. This increase in entropy favors dissolution.

However, if the solute-solvent interactions are very weak, the entropy increase may not be sufficient to overcome the unfavorable enthalpy change (the energy required to break the solute-solute and solvent-solvent interactions). In such cases, the solute will be insoluble.

Applications of Solubility Principles

Understanding solubility is crucial in various fields:

Chemistry: Solubility is fundamental in chemical reactions, separations, and purifications. It is used in titrations, extractions, and recrystallizations.
Biology: Solubility is essential for the transport of nutrients and waste products in biological systems. The solubility of oxygen and carbon dioxide in blood is critical for respiration.
Pharmaceuticals: The solubility of drugs affects their absorption, distribution, metabolism, and excretion in the body.
Environmental Science: Solubility determines the fate and transport of pollutants in water and soil.
Food Science: Solubility influences the texture, stability, and flavor of food products.

Predicting Solubility: Rules and Guidelines

While a comprehensive understanding of intermolecular forces is essential, several rules and guidelines can help predict the solubility of compounds in water:

"Like Dissolves Like": Polar solvents dissolve polar solutes and ionic compounds, while nonpolar solvents dissolve nonpolar solutes.
Small Molecules are More Soluble: Smaller molecules tend to be more soluble than larger molecules.
Hydrogen Bonding: Compounds that can form hydrogen bonds with water are generally more soluble.
Ionic Compounds: Most alkali metal (Group 1) salts and ammonium salts are soluble. Nitrates, acetates, and perchlorates are generally soluble. Halides (chlorides, bromides, iodides) are soluble, except for those of silver, lead, and mercury. Sulfates are soluble, except for those of barium, strontium, and lead. Carbonates, phosphates, sulfides, and hydroxides are generally insoluble, except for those of alkali metals and ammonium.

Conclusion

Solubility in water is a complex phenomenon governed by intermolecular forces, polarity, hydrogen bonding, size, shape, temperature, and pressure. Understanding these factors allows us to predict which compounds are soluble in water and why. By examining pairs of compounds, we can appreciate the interplay of these factors and their influence on solubility. The principles of solubility are fundamental in various fields, including chemistry, biology, pharmaceuticals, environmental science, and food science. A solid grasp of these principles is essential for scientists and engineers working in these areas.