Solutions and Phase Equilibrium, Chemistry tutorial

Solutions and Phase Equilibrium


We have been depicted to various substances in life. Particularly, we should have familiarized ourselves by pure substances such as sugar, sodium chloride, water and ethyl alcohol. We will concur that these substances are different. By just looking at them, you can state some probable differences. For illustration sodium chloride is powder whereas sugar is crystalline, however you can't recognize some properties by just looking at the two substances for illustration: sodium chloride is an ionic compound whereas sugar is the organic compound however we can state that both are solids. They encompass different definite density and melting points that we can prove.

Pure Substances and Solutions:

Quickly perform this simple activity on your own.

Aim: To examine the differences between the pure substances and solutions.

Apparatus: Sodium Chloride salt, ethyl alcohol, granulated Sugar, distilled water as reagents, 4 big test-tubes, a chemical balance and filter paper.


1) Measure 0.5g of NaCl, 0.5 g of sugar and 5cm3 of ethyl alcohol.

2) Measure 100 ml of water into three test tubes.

3) Pour each of the substances in Step (1) into the three (3) test tubes having water.

4) Repeat the activity by employing ethyl alcohol rather than water.

5) Record your observation.

Did you notice:

a) NaCl and Sugar dissolved in water.

b) Ethyl alcohol and water formed a liquid alike in appearance to the two original liquids.

These salt-water mixtures, the sugar-water mixture and the alcohol-water mixtures are known as Solutions.  A solution can be stated as a homogeneous mixture of two or more substances.  In another words, the mixture made is stated to be a single phase.

Let us take salt-water mixture. There are two constituents in the mixture. One is a solute and the other a solvent. Supposing you have more than three constituents in the solution, one will be the solvent whereas all other constituent's in that solution are termed to as the solutes.

Supposing we add 0.5g of NaCl to 500 cm3 of water as compared to 0.5g of sugar we added to water. The mixture of 0.5 g in 500 cm3 of water can be stated to have relatively small quantity of solute.  Such a solution is stated to be diluted.  The mixture of 0.5 g in water that can be stated to have relatively large quantity of solute is stated to be concentrated.

Differentiating Between Pure Substances and Solutions:

In nature, we know that we have both pure and impure substances. We have taken NaCl and C6H1206 which are solutes. Each of such two substances includes particles that are uniform. After we obtained the salt-water and sugar-water mixtures, the substances are as well homogeneous. We can state then that both the pure substances (Illustrations: NaCl and Sugar) and solutions (illustrations - salt-water and sugar-water mixtures) are homogeneous. However can you tell the difference? A pure substance is the homogeneous material which includes only one substance while a solution is a homogenous material which includes more than one substance. A pure substance is further characterized via definite properties like density, vapor pressure, boiling and melting points, the properties of a solution based on the relative amounts of constituents. For illustration - the salt-water mixture comprises of a single phase. Its density is more than that of the pure water. It is uniformly found out that its vapor pressure is lower than the vapor pressure of pure water. Do you concur with that last sentence? If you do, provide a reason for your agreement. The reason provided should be close to the fact that to increase the vapor pressure to 760 mm, pure water should be heated to 100oC. However to reach this vapor pressure, it is discovered that salt-water should be heated above 100oC. One can deduce that the boiling point of salt-water is above the boiling point of pure water.

Phase equilibrium:

It is a well-known fact that pure substances tend to exist in one of three different states: solid, liquid and gas. For illustration - take water. As ice is heated at atmospheric pressure, it all of a sudden melts to liquid at a particular temperature. As the liquid carries on to be heated, it ultimately reaches a temperature at which it spontaneously vaporizes to a gas. Such transitions are discontinuous, that is, they take place at particular state conditions. At precisely those conditions, the system can exist in more than one form in such a way that the two (or more) phases are in equilibrium with one other.

However we are in general familiar with phase behavior at atmospheric pressure, most of the substances experience a diverse set of stages over a broad range of pressures. Pure substances frequently have more than one crystal phase, based on the pressure. The figure below depicts a schematic of a PT diagram of water which describes the type of complex phase behavior that can exist. In the case of mixtures, there are even more possibilities for the phase equilibrium: for illustration, one can have equilibrium between the two liquids of different compositions, or among multiple solid and liquid phases.

1811_Phase diagram of water.jpg

Fig: Phase diagram of water

Definition of Phase: A phase is the homogeneous area of matter in which there is no spatial variation in the average density, composition, energy or other macroscopic properties.

Phases can as well be dissimilar in their molecular structure. For illustration: water consists of multiple ice phases which differ in their crystallographic structure.

A phase can be taken as a different system having boundaries which are interfaces by container walls or other phases.

A phase diagram is a graph exhibiting the relationships between the solid, liquid and gaseous phases of a substance as the function of pressure and temperature.

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