 ## Solutions

$\require{mhchem}$

### Solute, solvent and solution

A solution is composed of a solute and a solvent. When we dissolve a solute in a solvent, we can obtain a solution. To prepare a solution, a known mass of solute (e.g. $$5.85\textrm{g}$$ of $$\ce{NaCl}$$) is dissolved in a solvent such as liquid water to give a known volume of solution (e.g. $$250.0\textrm{ml}$$ aqueous solution).

In a solution, the solvent can be liquid water or something else such as alcohol-based solvents, acetone and ethyl ether. If the solvent is liquid water, we call the solution an aqueous solution. If the solvent is not liquid water, we call it a nonaqueous solution.

#### Solubility

In general, solubility depends on whether the solute (usually a solid) is ionic or molecular and whether the solvent liquid contains polar or non-polar molecules.

##### Ionic solutes in polar solvents

Example: Solubility of sodium chloride in water.

Forces between the ions ($$\ce{Na}^{+}$$and $$\ce{Cl}^{-}$$): electrostatic interactions

Forces between solvent ($$\ce{H}_{2}\ce{O}$$) molecules: dipole-dipole interactions (electrostatic interactions)

So, the nature of the forces between solute and solvent are comparable. Hence, the strong electrostatic forces between ions in the crystal lattice can be matched by the ion-dipole electrostatic interaction between ions and the polar solvent molecules. Thus, we can expect ionic (polar) solutes to be soluble in polar solvents.

##### Molecular solutes in non-polar solvents

Example: Solubility of iodine ($$\ce{I}_{2}$$) in octane ($$\ce{C}_{8}\ce{H}_{18}$$).

Forces between solute molecules ($$\ce{I}_{2}$$) : Van der Waals forces

Forces between solvent ($$\ce{C}_{8}\ce{H}_{18}$$) molecules: Van der Waals forces

The forces between the solute and the solvent molecules are comparable. Hence we can expect solubility. In this case, both solute and the solvent are molecular, so when solute contacts solvent, the molecules freely intermingle. Molecular compounds (e.g. iodine) are therefore generally soluble in non-polar liquids (e.g. Hydrocarbons such as octane).

#### Solubility rule

“Like dissolves like.”

Polar solutes dissolve in polar solvents.

Non-polar solutes dissolve in non-polar solvents.

Compare the water solubility of hexane and glucose.

• Water is a polar liquid.

• Hexane $$\left(\ce{C}_{6}\ce{H}_{14}\right)$$ is a hydrocarbon chain. Therefore, it is a non-polar solute. So hexane is insoluble in water.

• Glucose $$\ce{C}_{6}\ce{H}_{12}\ce{O}_{6}$$ is a polar solute due to the presence of many hydroxyl groups $$\left(\ce{OH}\right)$$ in its structure. Thus, glucose is soluble in water.

##### Influence of temperature on solubility

Many solids show increased solubility in water at higher temperature. In contrast, gases display low solubility in water with increased temperature.

###### Saturated, supersaturated and unsaturated solutions

The unsaturated solution contains fewer solutes than the maximum amount that can be dissolved under given conditions.

A saturated solution is a solution that contains the maximum amount of solutes that can be dissolved under the given conditions. In a saturated solution, an undissolved solid can be observed on the bottom of the container. There is a dynamic equilibrium between dissolved solutes and undissolved solutes at the molecular level. Some of the dissolved solutes crystallise back, and some of the undissolved solutes dissolve into the solution creating a dynamic equilibrium, even though you can not observe that with the naked eye.

A supersaturated solution contains more dissolved solute molecules than a saturated solution - this is an unstable temporary condition. A supersaturated solution always tries to revert to a saturated state with time.

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