COMPONENTS OF A SOLUTION

In our day to day life, we deal with many substances and most of them are mixtures i.e. pure substances are very rare.

When two or more chemically non-reacting substances are mixed, they form mixtures. A mixture may be heterogeneous or homogeneous.

Heterogeneous mixture

A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. For example, vegetable soup is a heterogeneous mixture because any given spoonful of soup will contain varying amounts of the different vegetables and other components of the soup. A heterogeneous mixture consists of distinct phases and the properties which are observed in it are just the sum of the properties of individual phases.

Phase

A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance or a homogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases.  When oil and water are combined, they do not mix evenly, but instead form two separate layers. Each of the layers is called a phase. The oil phase is less dense than the water phase and so the oil floats on top of the water.

Homogeneous mixture

However, a homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. For example, salt water is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample. A homogeneous mixture consists of a single phase and its properties may differ drastically from those of the individual components.

A homogeneous mixture which consists of two or more than two components is called a solution. The constituents of a solution CANNOT be separated by filtration, settling or centrifugal action. All solutions are characterized by:

(i) homogeneity;

(ii) absence of settling; and

(iii) the molecular or ionic state of sub-division of the components.

When the solution is composed of only two chemical substances, it is termed a binary solution. If it is composed of three components, then it is called ternary solution and if it is composed of four components, then it is called  quaternary solution and so on. Thus, a solution may be regarded as a substance consisting of a single phase containing more than one component.

Solute and Solvent

A solution consists of a solvent and one or more solutes. Solvent is that component of a solution that has the same physical state as the solution itself. Solute is the substance that is dissolved in a solvent to form a solution.

Generally, the component present in greater amount than any or all the other components is called the solvent and, in most of the cases, we find that solvent is a liquid but it is not always true. A solvent can be a solid, a gas or a supercritical fluid also.

How to determine which is solvent and which is solute ?

If a solution is formed from a solid and a liquid in which the liquid component is present in large excess over the solid, then the physical state of the solution will surely be the same as that of the liquid component and hence the liquid will be the solvent and the solid will be the solute. There is no ambiguity here.

If a solution is formed from a solid and a liquid in which the liquid component is present in smaller amount as compared to the solid component but the physical state of the solution is the same as that of the liquid component, then the liquid will be the solvent (even if it is present in smaller amount) and the solid will be the solute because the phase of the solution determines the solvent.

If a solution is formed from a liquid and a liquid, then the solution will surely be in liquid phase. In this case, the liquid which is present in larger proportion by mass is called the solvent. However, if both the liquids have the same masses in the solution, then it becomes difficult to know which is solvent and which is solute.