In normal life we rarely come across pure substances. Most of these are mixtures containing two or more pure substances. Their utility or importance in life depends on their composition.
For example, the properties of Brass (mixture of Cu and Zn) are quite different from those of German silver (mixture of Cu, Zn and Ni) or Bronze (mixture of Cu and Sn); 1 ppm of Fluoride ions in water prevents tooth decay, while 1.5 ppm causes the tooth to become mottled and high concentrations of fluorides ions can be poisonous (for example, sodium fluoride is used in rat poison); intravenous injections are always dissolved in water containing salts at particular ionic concentrations that match with blood plasma concentrations and so on.
Solution - What does it mean?
A true solution is defined as a perfectly homogeneous mixture of two or more substances (called as components) having uniform properties throughout and the composition of which may vary within limits. The substances by which solution is made are known as components of solution.
A solution containing two, three or four substances is generally referred as binary, ternary and quaternary solution respectively.
When solute is dissolved in solvent other than water, it is called aqueous solution. When solute is dissolved in solvent other than water, it is called non-aqueous solution.
Solvent and solute
In binary solution, the components present are solute and solvent.
Solution = Solute + Solvent
(1) Methods to classify between solvent and solute:
(I) Phase method:
If a substance A is in solid phase and B in liquid phase and the phase of resulting mixture is
(i) Solid, then solvent is A and solute is B
(ii) Liquid, then solvent is B and solute is A
In simple words, the component which has the same physical state in pure form as the solution is called solvent and the other is called solute.
(II) Amount method:
If both the components have same state as the solution the one substance which is in larger proportion by mass is solvent and which is in lesser proportion is solute.
For example, in a solid mixture of A and B, if wA > wB, then solvent is A and solute is B.
For the solution of solid in liquid, in which the liquid is in large excess over the solid, the liquid is solvent and the solid is solute. For the solution of liquids, e.g., alcohol and water, here the liquid is lesser quantity is solute.
(2) Classification of solutions based on the physical state of solute and solvent
All the three states of matter (gas, liquid or solid) may behave either as solvent or solute. In general, the physical state of a solution is the physical state of solvent
Depending on the state of solute or solvent, mainly there may be the following nine types of binary solutions.
Table: Types of Solutions
Type of solutions | State of Solute | State of Solvent | Examples |
Solid Solutions | Gas | Solid | Hydrogen in Pd |
Liquid | Solid | Amalgams i.e., Hg in Tl, FeSO4.7H2O | |
Solid | Solid | Alloys, Co in Ni, Au in Ag, PbCl2 in PbBr2 | |
Liquid Solutions | Gas | Liquid | CO2 in water, HCl in benzene |
Liquid | Liquid | Alcohols in water, bromine in CS2 or in CCl4 | |
Solid | Liquid | Sugar in water, iodine in CCl4, Pb in Hg, NaCl in water | |
Gaseous Solutions | Gas | Gas | Air, mixture of all gases |
Liquid | Gas | Water in air, bromine in chlorine, acetic acid in nitrogen | |
Solid | Gas | Naphthalene in methane, iodine in air, camphor in air |
Note: Solid in solid, solid in gas, liquid in solid, liquid in gas, gas in solid systems are heterogeneous in nature and thus, it is better to call them colloidal solution rather than calling true solution.
Among these solutions the most important solutions are those which include liquid phase as solvent (will be discussed in detail later), i.e.,
a. Solid (solute) in liquid (solvent) solutions
b. Liquid in miscible liquid solutions
c. Gas in liquid solutions
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