Solutions, Solutes, and Aqueous Solvents

A solution is a homogeneous mixture in which a solute is dissolved in a solvent, and an aqueous solution is one in which the solvent is water. In brief: when 5.85 g of table salt (the solute) dissolves in enough water (the solvent) to make 1.00 L of solution, the result is a 0.100 M aqueous solution of sodium chloride.

Most chemistry that matters for life, medicine and industry takes place in solution. Blood, seawater, cleaning products and the cytoplasm of every cell are all solutions. This article explains the terms, why water dissolves so many substances, the main types of solutions and the way dissolving works at the particle level.

What are a solution, a solute and a solvent?

A solution is a mixture that is uniform on the molecular scale. Its composition and properties are the same everywhere, and the dissolved particles do not settle out or scatter visible light in a noticeable way. The component present in the smaller amount is usually called the solute, and the component present in the larger amount, which determines the physical state of the solution, is the solvent.

Solutions are different from suspensions, such as muddy water, in which particles are large enough to settle, and from colloids, such as milk, in which particles are larger than molecules but remain dispersed. A solution can be filtered without leaving a residue on the filter paper.

What types of solutions exist?

Solutions can form in any state of matter. The state of the solution is normally that of the solvent.

Why is water such a good solvent?

A water molecule is bent, with an H–O–H angle of about 104.5°. Oxygen is much more electronegative than hydrogen, so the molecule has a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. This makes water a polar molecule with a large dipole moment of 1.85 D.

Water molecules are attracted to ions and to other polar molecules. They can also form hydrogen bonds with solutes that contain O–H or N–H groups, such as alcohols, sugars and amines. In addition, water has a high dielectric constant of about 78 at 25 °C, which reduces the attraction between oppositely charged ions in solution by a factor of nearly 80 compared with a vacuum.

How does a solid dissolve in water?

The dissolving of an ionic solid such as sodium chloride takes place in three steps:

1. Water molecules collide with the surface of the crystal and orient themselves with the negative oxygen end toward Na⁺ and the positive hydrogen end toward Cl⁻.
2. The attraction between the ions and the water molecules pulls ions away from the crystal lattice.
3. Each ion becomes surrounded by a shell of water molecules, called a hydration shell, which prevents the ions from recombining.

The overall equation is NaCl(s) → Na⁺(aq) + Cl⁻(aq). The symbol (aq) means that the species is dissolved in water. A molecular compound such as sucrose dissolves without forming ions: its molecules are separated from one another and hydrogen-bonded to water.

Worked example: preparing a solution

How many grams of NaCl are needed for 250 mL of a 0.100 M solution? The molar mass of NaCl is 58.44 g/mol.

1. n = M × V = 0.100 mol/L × 0.250 L = 0.0250 mol.
2. m = n × M = 0.0250 mol × 58.44 g/mol = 1.46 g.

The salt is dissolved in some water in a 250 mL volumetric flask, and then water is added up to the mark. The solution is not made by adding 250 mL of water to the salt, since the volume of the final solution is not the sum of the separate volumes.

What does “like dissolves like” mean?

Polar and ionic substances tend to dissolve in polar solvents such as water. Nonpolar substances tend to dissolve in nonpolar solvents such as hexane. This is the reason why oil and water do not mix, and why salt does not dissolve in gasoline. Substances with both polar and nonpolar parts, such as soap molecules, can form structures that bridge both worlds.

The rule is a useful rule of thumb that is based on intermolecular forces: a substance dissolves when the attractions between solute and solvent are comparable with or stronger than the attractions within the pure solute and the pure solvent.

What affects how fast a solute dissolves?

• Surface area: powdered sugar dissolves faster than a sugar cube because more surface is exposed.
• Stirring: it brings fresh solvent into contact with the solid.
• Temperature: most solids dissolve faster, and in larger amounts, in hot water. Gases show the opposite trend.

The rate of dissolving is a different matter from the amount of solute that can dissolve, which is the solubility.

How did the understanding of solutions develop?

For a long time it was unclear whether dissolving was a physical mixing or a chemical reaction. In 1884, the Swedish chemist Svante Arrhenius proposed in his doctoral thesis that salts split into charged particles in water, which explained why such solutions conduct electricity. His ideas were at first received with skepticism, but he was awarded the Nobel Prize in Chemistry in 1903. Jacobus van ‘t Hoff showed in 1886 that dilute solutions behave in some ways like gases, a result that led to the study of colligative properties.

Which mistakes are common?

• Confusing solute and solvent: the solvent is the component in excess, not necessarily a liquid.
• Adding the volumes: 500 mL of ethanol and 500 mL of water give about 965 mL of solution, not 1000 mL.
• Calling every clear liquid a solution: pure water and many mixtures such as colloids can also look clear.
• Equating dissolving with melting: a dissolved solid has been separated by the solvent, but it has not changed its state by heating.
• Assuming that “aqueous” means that the substance is soluble: it only means that the solvent is water.