Water as a Universal Solvent

Water is called the universal solvent because it dissolves more substances than any other common liquid, especially ionic compounds and polar molecules, although it does not dissolve everything. In brief: salt, sugar and many gases dissolve in water, but oil, wax and plastic do not; the rule “like dissolves like” explains why.

The ability of water to dissolve so many substances is essential for life. Nutrients dissolve in blood, minerals dissolve in rivers and chemical reactions in cells take place in solution. This article explains how water dissolves ionic and polar substances, why oil does not mix, compares substances in a table, works through an example with ions and describes the importance of dissolved substances in nature.

Why does water dissolve so many substances?

The water molecule is polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. These charged ends attract other charged or polar particles. When a solute is added, water molecules gather around its particles and pull them into the solution.

The guideline “like dissolves like” summarizes the pattern: polar and ionic substances dissolve in polar solvents such as water, while nonpolar substances dissolve in nonpolar solvents such as hexane or oil.

How does water dissolve ionic compounds?

An ionic crystal such as sodium chloride consists of positive sodium ions and negative chloride ions held by electrical attraction. In water, the oxygen ends of water molecules point toward the sodium ions, and the hydrogen ends point toward the chloride ions. The attraction between the water molecules and ions pulls the ions away from the crystal, one by one. Each ion becomes surrounded by a shell of water molecules, which is called hydration.

The dissolved ions can move freely, which is why salt solutions conduct electricity. The process of splitting into ions is called dissociation: NaCl(s) → Na⁺(aq) + Cl⁻(aq).

How does water dissolve polar molecules?

Molecules such as sugar, ethanol and ammonia contain polar parts, often with oxygen or nitrogen atoms bonded to hydrogen. These parts form hydrogen bonds with water molecules. The solute molecules stay intact, and they are surrounded by water, but they do not form ions. A sugar solution does not conduct electricity for this reason.

Why do some substances not dissolve in water?

Nonpolar substances such as oil, wax and gasoline have no charged regions, so they do not attract water molecules. Water molecules attract each other more than they attract the nonpolar molecules, and they exclude the oil from their network of hydrogen bonds. As a result, the oil forms a separate layer. Large networks of tightly bonded atoms, such as sand (silicon dioxide), are also insoluble because the water cannot break their strong bonds.

What dissolves in water and what does not?

Worked example: dissociation of salts

Write the dissociation of two salts and count the ions in a solution.

1. Sodium chloride: NaCl → Na⁺ + Cl⁻. One formula unit gives 2 ions.
2. Calcium chloride: CaCl₂ → Ca²⁺ + 2Cl⁻. One formula unit gives 3 ions.
3. Moles of ions. For 0.10 mol of CaCl₂ dissolved in water: 0.10 mol of Ca²⁺ and 0.20 mol of Cl⁻, giving 0.30 mol of ions.
4. Comparison. 0.10 mol of NaCl gives 0.20 mol of ions, so the calcium chloride solution has 1.5 times as many dissolved particles from the same number of moles of salt.

The number of particles is important for properties such as freezing-point lowering. This is why calcium chloride is a more efficient de-icing salt than sodium chloride on a per-mole basis.

Why is dissolving in water important?

• Living organisms: blood and plant sap carry dissolved nutrients and waste.
• Oceans: seawater contains dissolved salts, with sodium and chloride ions being the most abundant.
• Geology: water dissolves minerals, forming caves in limestone and supplying minerals to groundwater.
• Gases: dissolved oxygen enables fish to breathe.
• Pollution: water also dissolves harmful substances, so pollutants spread easily through it.

How did scientists describe the universal solvent?

Alchemists searched for an “alkahest”, an imagined universal solvent that would dissolve all substances. If one existed, it would dissolve any container as well, so the idea could never be realized. Water gained the name because it dissolves more substances than any other common liquid, and the modern explanation came with the idea of ions in solution proposed by Svante Arrhenius in 1884 and the later understanding of polar molecules and hydrogen bonding in the 20th century.

Which mistakes are common?

Mistake 1: Believing that water dissolves everything

It does not dissolve oil, wax or sand. The name is an exaggeration.

Mistake 2: Thinking that dissolving always forms ions

Ionic compounds dissociate into ions, but sugar dissolves as intact molecules.

Mistake 3: Saying that “like dissolves like” refers to color or appearance

The rule refers to the type of attraction between particles, such as polar or nonpolar.

Mistake 4: Forgetting that the dissolved substance is still there

Dissolved salt is not destroyed. It can be recovered by evaporating the water.