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Making a saline water solution by dissolving table salt (NaCl) in water. The salt is the solute and the water the solvent.

Background

Many important chemical reactions occur in solution. A reactant in solution is known as a solute. The medium in which it is dissolved is known as a solvent. A solute dissolved in water as solvent constitutes an aqueous solution. Water is a very important solvent. For instance, the life of all organisms, including human beings, is totally dependent on chemical reactions that take place in water. However, there are a variety of circumstances in which water cannot be used as a solvent. In those cases a non-aqueous solvent may be used. A non-aqueous solvent is any solvent other than water.

Limitations of water as a solvent

Solubility. Many substances have a very low solubility in water. Some organic compounds are said to be hydrophobic on account of their very low solubility in water. Groups such as alkyl groups and aryl groups confer hydophobic character on the molecules to which they are attached. Many hydrophobic compounds dissolve in common organic solvents.

Another class of insoluble compounds is exemplified by silver iodide, AgI. This compound dissolves in liquid ammonia; it is insoluble in water because of the weak interaction of silver ions with water molecules (weak solvation) compared to the high lattice energy of the solid, but it dissolves in liquid ammonia with the formation af an ammine complex.

Redox reactions. Water can be oxidized by strong oxidizing reagents or reduced by strong reducing agents.

oxidation: H₂O - 2e^- → O₂↑ + 2 H⁺

reduction: 2 H₂O + 2e^- → H₂↑ + 2 OH^-

The Pourbaix diagram shows that that the region of stability of water is somewhat pH dependent, with oxidation being easier at high pH and reduction being easier at low pH. Alkali metals, such as sodium reduce water, but are dissolve in liquid ammonia.

Acid/base reaction. Water can act as a Brønsted–Lowry acid or base.

reaction with base, B: B + H₂O ⇌ BH⁺ + OH^-

reaction with acid, AH: AH + H₂O ⇌ A^- + H₃O⁺

These reactions are also pH dependent. A strong acid, such as nitric acid is one that is fully dissociated in water, except at very high concentrations.

Hydrolysis. A hydrolysis reaction is one in which a solute reacts chemically with the solvent water. A typical reaction is the hydrolysis of silicon tetrachloride.

SiCl₄ + 2 H₂O → SiO₂ + 4 HCl

Many substances cannot exist in aqueous solution becuase of hydrolysis reactions.

Classification of solvents

Any liquid other than water can act as a non-aqueous solvent. This encompasses a very wide range of substances.