At first, what is a solution?! It is a homogenous mixture of complex composition, whose components can be separated with physical means. The solution may consist of two or more pure substances, then its mix, it leads to the formation of the product having different properties than the properties of all its components. For example, if you add sugar to water, you will get solution, which hardens at a certain temperature, which is below the temperature required for crystallization of water and, respectively, for sugar. In each solution presents solvent and solute. The solvent is usually the substance that is taken in larger quantities. Sometimes possible to know that the solvent - it is the substance does not change its aggregate state (for example, water remains a liquid state with dissolving in it other substances like a salts).
Solution is formed when the molecules or ions of dissolved substances are distributed among the molecules of solvent to the total displacement (diffusion). In this case, the substances are called mixed, otherwise - immiscible.
Absolutely insoluble substances - does not exist! The maximum amount of dissolved substances that can dissolve in a specific amount of solvent at a some temperature is called solubility. If the substance can not dissolve in the solvent (at a some temperature), such a solution is called saturated solution. To determine, how much substance dissolves at a certain temperature in a solvent, you can use the special graphs of solubility: Graphs of the solubility of salt in water.
If the saturated solution to slowly cool, then it will have more the solute than it can hold at a some temperature, and the solution is saturated. If in such solution to place a crystal of the dissolved substance or shake the solution, then part of the solute precipitates. By the way, saturated solution salt is widely used for growing crystals. If increas temperature, then solubility of solids substances up, but for gaseous substances - on the contrary! - the solubility of gases increases then is decreasing temperature! As for the pressure - for gases - the more pressure, the better the solubility. For solids substances the pressure has not affect!
As already noted, insoluble substances do not exist. Even solid substances that we know like a insoluble, is also partially dissolve, forming tiny particles in the solvent - micelles. These particles pass through the filter. They are so small and light that do not precipitate. These particles (micelles) are called colloid, and solution - colloid solutions. Colloid particles you can see in the solution with means of a beam of light rays are reflected and refracted, then colloidal particles become visible to the naked eye. (This phenomenon is called the Tyndall effect). In the colloid solutions particles have a size from 0.1 to 0.001 microns.
Another common name of colloid solutions - sols. During long-term storage Zoli pass into the gel (structure of gel differs from solswith position of the particle (the micelle), they are grouped in the gels). But if gel to heat, it will once again go into a Sol.
Zols, which pass into the gel, but the reverse transformation is excluded - are called suspensions. If gel turns to Sol - it is emulsion. Examples of colloid solutions can be milk, glue, ink, mayonnaise, emulsion of films (the silver solution). The clouds are also colloidal solutions, where the air is solvent, and drops of water - colloidal particles.
If you add the sand to a water and shake it the sand very quickly separates from water and will precipitate on the bottom. If you mix the dust with water, it will precipitate too, but much slower. In these cases cannot speak about of a solution as the particles of dissolve substances are visible by naked eye or with a magnifying glass or microscope. This mixture is heterogeneous. Mixtures of this type, in which the components can be separated from each other with simple filtration, also called suspensions.