The solvent does the dissolving.
Types of Solutions
Gas
If the solvent is a gas, only gases are dissolved under all given set of conditions. An example for a gaseous solution is air (oxygen and other gases dissolved in nitrogen). Since interactions between molecules play almost no role, dilute gases form rather trivial solutions. In part of the literature, they are not even classified as solutions, but addressed as mixtures.
Liquid
If the solvent is a liquid, then gases, liquids, and solids can be dissolved. Examples are:
- Gas in liquid:
- Oxygen in water.
- Carbon dioxide in water is a less simple example, because the solution is accompanied by a chemical reaction (formation of ions). Note also that the visible bubbles in carbonated water are not the dissolved gas, but only an effervescence; the dissolved gas itself is not visible since it is dissolved on a molecular level.
- Liquid in liquid:
- The mixing of two or more substances of the same chemistry but different concentrations to form a constant.(Homogenization of solutions)
- Alcoholic beverages are basically solutions of ethanol in water.
- Solid in liquid:
- Sucrose (table sugar) in water
- Sodium chloride or any other salt in water forms an electrolyte: When dissolving, salt dissociates into ions.
Counterexamples are provided by liquid mixtures that are not homogeneous: colloids, suspensions, emulsions are not considered solutions.
Body fluids are examples for complex liquid solutions, containing many different solutes. They are electrolytes since they contain solute ions (e.g. potassium and sodium). Furthermore, they contain solute molecules like sugar and urea. Oxygen and carbon dioxide are also essential components of blood chemistry, where significant changes in their concentrations can be a sign of illness or injury.
Solid
If the solvent is a solid, then gases, liquids, and solids can be dissolved.
- Gas in solid:
- Hydrogen dissolves rather well in metals, especially in palladium; this is studied as a means of hydrogen storage.
- Liquid in solid:
- mercury in gold, forming an amalgam
- Hexane in paraffin wax
- Solid in solid
- Steel, basically a solution of carbon atoms in a crystalline matrix of iron atoms.
- Alloys like bronze and many others.
- Polymers containing plasticizers.
Properties of Solutions
Pure liquids have a set of characteristic physical properties (melting point, vapor pressure at a given temperature, etc.). Solutions in a solvent exhibit these same properties, but the values differ from those of the pure solvent because of the presence of the solute. Moreover, the change observed in these properties in going from the pure solvent to a solution is dependent only upon the number of solute molecules; these properties are called colligative properties. The properties of a solvent that show a predictable change upon the addition of a solute are melting point, boiling point, vapor pressure, and osmotic pressure.
1. Melting and boiling points
Solutions exhibit higher boiling points and lower melting points than the parent solvent. The increase in boiling point and decrease in melting point is dependent upon the number of solute particles in the solution; the greater the number of solute particles (i.e., the concentration), the greater the boiling point elevation and melting point depression. A common application of this effect in some parts of the world is in the use of antifreeze solutions in the cooling systems of automobiles in cold climates. "Antifreeze" compounds are usually organic liquids that are miscible with water so that large freezing point effects can be attained.
2. Vapor pressure
All liquids exhibit a vapor pressure, the magnitude of which depends on the temperature of the liquid. For example, water boils at 100°C, which means that at 100°C the vapor pressure of water is equal to the atmospheric pressure allowing bubbles of gaseous water (steam) to escape from the liquid state. However, the vapor pressure of a solution (at any temperature) is less than that of the solvent. Thus, boiling water ceases to boil upon the addition of salt because the salt solution has a lower vapor pressure than pure water. The salt solution will eventually boil when the temperature of the solution increases bringing about an increase in vapor pressure sufficient to again form bubbles.
3. Osmosis
This property of solutions is perhaps the least familiar of the colligative properties, but in a sense it is more important than those already mentioned. In 1748 French clergyman and physicist Jean-Antoine Nollet observed that certain animal membranes are selectively permeable to different molecules. Since then, many examples of semipermeable membranes have been discovered, including animal bladder or gut tissues, eggshell lining, and certain vegetable tissues. A semipermeable membrane may be defined as a material that allows molecules of one kind to pass through it but prevents the passage of other kinds of molecules or allows the passage of different kinds of molecules at different rates.
Solubility --- property of solute to be dissolved in a given quantity of solvent. The composition of many solutions cannot be varied continuously because there are certain fixed limits imposed by the nature of the substances involved.
Conditions That Affect Solubility
1. Pressure
Changes in pressure have little effect on the solubility of solid or liquid solutes in a liquid solvent, but pressure has a much greater influence on the solubility of a gaseous solute.
2. Temperature
In general, a change in temperature affects the solubility of gaseous solutes differently than it does the solubility of solid solutes, because the solubility of a gas in a liquid solvent decreases with increasing temperature.
3. The nature of solute and solvent
Crystalline substances consist of a regular arrangement of atoms, molecules, or ions; in the latter case, the forces that hold the crystal together are electrostatic in nature.
