How can I determine if a compound is soluble in water?

Use the solubility rules: most salts of Group 1 metals (like Na+, K+) and ammonium (NH4+) are soluble. Also, nitrates (NO3-), acetates (CH3COO-), and most chlorides (Cl-), bromides (Br-), and iodides (I-) are soluble, except with silver, lead, and mercury.

What makes a compound insoluble in water?

Compounds are often insoluble if they contain carbonate (CO3^2-), phosphate (PO4^3-), or sulfide (S^2-), except when paired with Group 1 metals or ammonium. Hydroxides (OH-) are insoluble except for those of Group 1 metals, barium, and strontium.

Why is sodium chloride soluble but silver chloride insoluble?

Sodium chloride is soluble because sodium is a Group 1 metal, making all its salts soluble. Silver chloride is an exception to the general solubility of chlorides, as silver (Ag+) forms insoluble chlorides, bromides, and iodides.

How does polarity affect solubility in water?

Water is polar, so polar and ionic compounds (like salts) tend to dissolve in water, while nonpolar compounds (like oils) do not. The rule 'like dissolves like' applies: polar solutes dissolve in polar solvents.

Can you give an example of matching a compound to its solubility?

For example, calcium carbonate (CaCO3) is insoluble in water because carbonates are generally insoluble except with Group 1 metals or ammonium. In contrast, potassium nitrate (KNO3) is soluble as it contains a Group 1 metal and nitrate.

Match the Following Compounds to Their Likely Solubility in

To match compounds to their likely solubility in water, you need to consider polarity, hydrogen-bonding ability, and molecular size. The simple rule is that “like dissolves like”: polar and ionic compounds tend to dissolve in water, while nonpolar compounds do not. Sodium chloride dissolves readily because it is ionic, while vegetable oil does not because it is nonpolar.

Polarity and the “Like Dissolves Like” Rule

Water is a polar molecule with a partial negative charge on the oxygen and partial positive charges on the hydrogens. This polarity allows water to interact strongly with other polar molecules and with ions.

Ionic compounds (like sodium chloride) dissociate into positive and negative ions. Water molecules surround each ion, stabilizing them in solution.
Polar covalent compounds (like ethanol) have a permanent dipole. The oxygen in ethanol can form hydrogen bonds with water, making it miscible.
Nonpolar compounds (like hexane) lack a permanent dipole. They cannot interact strongly with water molecules and tend to separate.

A compound’s polarity can be predicted by the electronegativity difference between its atoms. If the difference is large (greater than about 1.7 on the Pauling scale), the bond is ionic. If moderate (0.4 to 1.7), the bond is polar covalent. If small (less than 0.4), the bond is nonpolar covalent. In hydrogen chloride, the electronegativity difference between H (2.20) and Cl (3.16) is 0.96, making it polar covalent. HCl gas dissolves readily in water. In contrast, the C–H bonds in methane have a difference of only 0.35, making them nonpolar, so methane is poorly soluble.

Hydrogen Bonding and Functional Groups

Even among polar compounds, the ability to form hydrogen bonds greatly increases water solubility. A hydrogen bond occurs when a hydrogen atom is bonded to a highly electronegative atom (N, O, or F) and is attracted to another electronegative atom.

Alcohols (like methanol, ethanol, and isopropanol) contain an –OH group. This group can both donate and accept hydrogen bonds with water. Small alcohols are completely miscible.
Carboxylic acids (like acetic acid) have a –COOH group that also forms strong hydrogen bonds. Acetic acid is miscible with water.
Amines (like methylamine) have an –NH₂ group that can hydrogen bond with water.

As the nonpolar carbon chain in a molecule grows longer, overall solubility decreases. Ethanol is miscible, but octanol is only slightly soluble. The polar –OH group is still present, but the large nonpolar tail dominates.

Worked Example: Matching a Set of Compounds

Consider these four compounds: sodium nitrate (NaNO₃), acetone (CH₃COCH₃), carbon tetrachloride (CCl₄), and glucose (C₆H₁₂O₆).

Sodium nitrate: An ionic compound. It dissolves readily in water (high solubility).
Acetone: A polar covalent molecule with a carbonyl (C=O) group. It can accept hydrogen bonds from water. Acetone is miscible.
Carbon tetrachloride: A nonpolar molecule. The C–Cl bonds are polar, but tetrahedral symmetry cancels the dipole. It has very low solubility in water.
Glucose: A sugar with multiple –OH groups. Each –OH group can form hydrogen bonds. Glucose is very soluble.

The likely solubility order from highest to lowest is: glucose > sodium nitrate > acetone > carbon tetrachloride.

Size and Molecular Structure Effects

Molecular size and shape also influence solubility. As a general trend, solubility decreases as the nonpolar portion of a molecule increases.

Hydrocarbon chains: For alcohols, solubility decreases as chain length increases. Methanol is miscible, but butanol is only partially miscible.
Branched chains: Branching can increase solubility because it reduces the molecule’s ability to pack tightly in a nonpolar phase. Tert-butanol is more soluble than n-butanol.
Multiple polar groups: Adding more polar groups (like –OH, –COOH, –NH₂) generally increases solubility. Glycerol (with three –OH groups) is completely miscible, while propanol (with one –OH group) is only partially miscible.

Example: Comparing Ethanol and Ethylene Glycol

Ethanol has one –OH group and is miscible with water. Ethylene glycol has two –OH groups. Because it has more hydrogen-bonding sites, ethylene glycol is also miscible and has a stronger interaction with water. This is why ethylene glycol is used as an antifreeze: it lowers the freezing point of water more effectively than ethanol per mole.

Key Takeaways

Water solubility depends primarily on polarity and hydrogen-bonding ability. Ionic and polar compounds tend to dissolve; nonpolar compounds do not.
The “like dissolves like” rule is the simplest guide: match polar solvents with polar solutes.
Hydrogen bonding between the solute and water greatly increases solubility. The presence of –OH, –COOH, and –NH₂ groups is a strong indicator of good solubility.
As the nonpolar (hydrocarbon) part of a molecule grows, solubility decreases. Branching and multiple polar groups can offset this effect.
For a quick match, identify whether a compound is ionic, polar covalent, or nonpolar covalent. Then check for functional groups that can form hydrogen bonds.

How to Match the Following Compounds to Their Likely Solubility in Water