A solution is a homogenous mixture of two or more substances in which the particles are completely dispersed. A colloidal solution is one in which some of the components are present as small particles that have not dissolved. The term solution comes from the Latin solvere, to dissolve or to release.
A solution may contain a liquid or a solid evenly dispersed through it. This may occur by the material being totally dispersed throughout the solvent or it may be that the solid is held in suspension. For a solution to be truly regarded as a solution the solute must be evenly dispersed (the mixture must be homogenous) and there must be no tendency for the solute to settle out of solution. A colloidal solution has the same properties as described. The physical properties of a solution vary depending on the proportions of the components of the system. Solutions can be formed from a solid in a liquid, a gas in a liquid, a solid in a solid, a liquid in a solid, and a gas in solid. The air we breathe is a solution of several gases. Brass is a solid solution of zinc in copper. Sea water is a solution of several solids dissolved in water. Each of the substances in a solution is a component of the solution. The solvent is the component that is present in the greatest quantity. The other components are solutes.
The amount of substance dissolved in another substance is the concentration, or strength, of the solution. This is generally given in terms of grams per liter, or as a molar fraction. A solution with only a small amount of material dissolved in it is said to be weak or dilute. A solution with a high proportion of material dissolved in it is said to be strong or concentrated. If a solution has the maximum amount of material dissolved in it, it is said to be saturated. The maximum amount that can be dissolved into a solution is termed solubility. As the temperature increases the solubility of a solid in a liquid generally increases. The solubility of a gas in a liquid decreases as the temperature increases. It is possible to dissolve more material into a liquid than the saturated solution should hold. The solution is produced by heating the liquid and adding excess solute. This will then dissolve and by gradual cooling a supersaturated solution will be produced. A supersaturated solution is unstable and the addition of a small piece of the solute to a supersaturated solution will cause the excess dissolved solute to precipitate out.
A solution will form when the various intermolecular attractive forces between the particles of solute and solvent are similar in size to the forces between either the solute molecules themselves or between the solvent molecules. With sodium chloride and water, the saturated condition is rapidly reached because the attraction between the sodium chloride ions and the polar water molecules is so strong. The attractive force experienced is easily sufficient to break the lattice structure that sodium chloride is normally encountered in. Hydrogen bonding plays a strong part in this particular reaction. The separate ions are actually surrounded by water molecules. This is a process known as hydration. The more general term is solvation, hydration refers only to the situation where water is the solvent. To overcome the bonds that are present between the sodium and chloride ions and the bonds that are present between the water molecules, energy is needed. Consequently energy is drawn into the system, however energy is also given up by the system as new bonds form. The overall net effect may be of an endothermic or exothermic reaction. Sodium hydroxide dissolved into water is an exothermic process, ammonium nitrate dissolved in water is an endothermic process.
When ionic compounds form a solution they are capable of conducting electricity. How well they are able to do this depends on how complete the disassociation into ions is in a solution. For example, methanol in solution in water does not form ions so electricity cannot be passed. Mercury chloride partially disassociates so it can pass electricity poorly. Sodium chloride in water disassociates completely and consequently it is a good conductor of electricity. A substance that passes electricity by this manner is known as an electrolyte.
The solute can be reclaimed from a solution by a process of evaporation. Sea water, which is a solution containing water and sodium chloride as its main components, can undergo this process. If sea water is allowed to evaporate a white solid is left behind in the container. This is sodium chloride (common table salt). If the evaporation is carried out slowly then the crystals of sodium chloride produced are large in size. A quick evaporation will yield small crystals. The water is driven off as a vapor, this can be collected and condensed to give pure solute.
A solution has different properties when compared to any of the components of the system. Salt when dissolved in water lowers the freezing point and raises the boiling point. Exactly how much effect this has depends on the concentration of solute. The increase in boiling point and the decrease in freezing point are directly proportional to the number of solute molecules per mole of solvent molecules. This characteristic is commonly encountered in everyday life. Salt is added to water to make it boil at a higher temperature that is used in cooking. Calcium chloride can be added to ice on roads to reduce the freezing point, the practical effect being that the ice is no longer at a sufficient temperature to remain as cold so the ice melts.
If a solution and solvent or two solutions of different strength are separated by a semi permeable membrane, osmosis can occur. Osmosis is the passage of water from a weak solution to a strong solution through a semi permeable membrane. The net effect of osmosis is to try to average out the concentration between the two solutions. A solution can have the solute removed, it can be purified, by reverse osmosis. Reverse osmosis is used to prepare drinking water from sea water.
If large particles of material are placed into a liquid they may not dissolve. For example fine clay in water will merely settle to the bottom of the container due to the action of gravity. The dispersed particles of a solution are of molecular size. Between these two extremes there are particles which are larger than molecules but not so large that they will settle out due to the action of gravity. The solution containing particles of this size is termed a colloidal solution or often just a colloid. Many examples of colloids using different phases are commonly encountered and many of their properties are similar to those of normal, molecular solutions.
A solution of two gases generally has each gas acting as if the other component were not present. This is due to the large intermolecular distances encountered between gas molecules. This is most obviously encountered when the pressure of the gas solution is looked at. This is covered by Dalton's law of partial pressure.
A solution is a mixture of a solute and a solvent.
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