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Much of the world around us is composed of solutions. Air, oceans, gasoline, and soda pop are just a few examples of common substances that exist as solutions. A solution is nothing more than a homogeneous mixture in which the particles range in size from 0.2 to 2.0 nm. There are other homogeneous mixtures with smaller particle size. These are known as colloids and suspensions.
When considering solutions, it helps to think in terms of one substance dissolved in another substance. The simplest solutions consist of two components. The component in the greater amount is referred to as the solvent and the component in the lesser amount is called the solute. Therefore a solution can be thought of as a homogeneous mixture containing a solute dissolved in a solvent.


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Solute + Solvent = Solution

"A solution is a homogeneous mixture of two or more components."
The components in a solution can be separated by purely physical processes such as distillation, evaporation, freezing or diffusion. Although there can be solutions of any phase in any other phase, solutions involving liquids are of the most interest.


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Solutions may exist in any of the three states of matter, that is they may be gases, liquids or solids. The terms solute and solvent refer to the components of a solution. The solute in the case of a solution of a gas or solid dissolved in a liquid, is the gas or solid; in other cases, the solute is the component in smaller amount. The solvent in a solution of a gas or solid dissolved in a liquid is the liquid, in other cases the solvent is the component in greater amount. 

Some of the examples of solutions are listed below.

Solute Solvent Example
Solid Liquid mineral water
Solid Solid  alloys such as brass, which is a solution of zinc in copper
Liquid Solid dental amalgam, mercury in silver
Liquid Liquid  antifreeze in radiator, ethylene glycol in water
Gas Solid hydrogen gas in palladium metal
Gas Liquid carbonated beverage, CO2 in water
GasGas air in the atmosphere, many gases in nitrogen


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It is the quantity of solute that will dissolve in a given amount of solvent or solution at a specific temperature. Solubilities are normally measured in grams of solute per 100 grams of solvent. In most instances the solvent is water.
In most of the case it is concerned with substances dissolved in water. The substance that is dissolved is called the solute and the water is called the solvent. Sugar is one of the substance have a very high solubility in water. The solubility of one substance in another can be explained in terms of two factors. One is the natural tendency of substances to mix. This is also referred to as natural tendency toward disorder. The solubility of a solute in a solvent depends on a balance between the natural tendency for the solute and solvent species to mix and the tendency for a system to have the lowest energy possible.


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The characteristics of solutions by knowing the components of solutions by which is dissolved in what.
  • For a solution of a gas or a solid in a liquid, the liquid is the solvent and the gas or solid is the solute.
  • For a solution of one liquid in another liquid, the choice of the term solvent or solute is arbitrary but generally the liquid taken in larger amount is considered to be the solvent.
  • When solids or gases are dissolved in liquid solvents there is a limit to the amount of solute that can dissolve in a fixed amount of solvent at a fixed temperature.
  • The amount of solute tat can be dissolved in a given amount of a saturated solution at a fixed temperature is the solubility of the solute in the solvent.


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Vapor pressure

For ideal solutions of non electrolytes the vapor pressure of the solution follows Raoult's law.


Where PA is the vapor pressure of the solution, PoA is the vapor pressure of the pure solvent and XA is the mole fraction of solvent. This relationship states that the vapor pressure of the solution is proportional to the number of molecules of solvent in the solution. 

Boiling point

The boiling point is reached at the temperature where the vapor pressure of the liquid matches the pressure of the gas at the liquid gas boundary. Because the presence of solute particles reduces the vapor pressure of the solution, it also increases the boiling point of the solution. It is expressed by the formula.

$\Delta$ T = iKbM


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Various way of expressing concentration are mole fraction, mass percent, molarity, molality, normality, mole fraction and parts per million. Concentration expressions are defined and expressed in equation form in the below table.

Concentration unit Definition Equation
Mass percentage (mass%)
The percentage of mass of a component of a solution in a given mass of the solution.
mass % = $\frac{mass\ of\ the\ component}{mass\ of\ solution} \times 100$
Parts per million (ppm)
The grams of a solute in a million (106) grams of solution. This is equivalent to 1 mg of solute per kg of solution.
ppm = $\frac{mass\ of\ solute}{mass\ of\ solution} \times 10^{6}$
Mole fraction (X)
Ratio of the number of moles of a component in a mixture to total number of moles of all components in solution.
XA = $\frac{moles\ of\ component\ A}{total\ moles\ of\ all\ components}$
Molarity (M)
Number of moles of solute in a litre of solution.
M = $\frac{moles\ solute}{volume\ of\ solution\ in\ litres}$
Molality (m)
Number of moles of solute in a kilogram of solvent.
m = $\frac{moles\ of\ solute}{mass\ of\ solvent\ in\ kilograms}$
Normality (N)
Number of solute equivalents per litre of a solution.
N = $\frac{number\ of\ solute\ equivalents}{volume\ of solution\ in litres}$

Aqueous Solution

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In aqueous solution ions are stabilized by their interaction with the solvent; they become hydrated and this state is indicated in equations by the (aq) symbolism. In aqueous solution the individual ions have an interaction with the solvent that is associated with their ionic radius, but the resulting hydrated ion is difficult to quantify.

"Aqueous solutions are the solutions in which water is the solvent (dissolving substance). Water ability to dissolve many substance is considered its most valuable property." Even some alcohols, such as ethanol and ethylene glycol (used in cars in the antifreeze coolant solution) are soluble in water. Sucrose solution (sugar dissolved in water), salt solution (salt dissolved in water), carbonated water (carbon dioxide dissoved in water) and alcohol solution (ethanol dissolved in water) are examples of aqueous solutions. In these examples sugar, salt, carbon dioxide and ethanol are the solutes and water is the solvent.

Saturated Solution

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A saturated solution is a solution that contains just as much chemical or solute as can be dissolved in the solvent at the temperature of the solution. A saturated solution should be in contact with undissolved solute. How much of the solute is needed for a saturated solution depends on the individual solute and how soluble it is in the solvent, and how the solubility varies with temperature.

A saturated solution is prepared by adding an excess of solute to a solvent with stirring and allowing time for as much solute to dissolve as possible. Any excess or spare solid solute will sink to the bottom of the container and the solution above this is then saturated at any particular temperature. If the solution is warmed it will normally be able to dissolve more of the solute, so saturated solutions must be kept at the correct temperatures.

Unsaturated Solution

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An solution contains as less solute as it can dissolve at that temperature. That is it can dissolve more. At every step, before obtaining a saturated solution a solution is always unsaturated with respect the solute. An unsaturated solution contains less solute than it can dissolve at that temperature. It becomes more unsaturated with the rise of temperature.

An unsaturated solution has the ability to dissolve additional solute and therefore does not contain precipitate. To determine whether a solution is saturated or unsaturated add a small quantity of solute. If it dissolves, the solution is unsaturated. Several factors can cause an unsaturated solution to change into a saturated solution. This usually results in the formation of precipitate.