A solution is a homogeneous mixture of two or more substances in which one substance is dissolved in the other. Homogeneous means that no particles or parts of different substances can be seen. When one substance dissolves, the solution looks exactly the same. A substance that is dissolved is called a solute and a substance in which it is dissolved is called a solvent. In a solution, the particles are microscopic, less than 1 nm in diameter. A solution is a very stable mixture, and the solute does not separate from the solvent itself.
Solvent + Solute = Solution
Fig 2.3: Formation of Solution
In a salt solution, salt is the solute and water is the solvent. More than one solute may be present in a solution. For example, in soft drinks, water is a solvent while other substances like sugar, salts, and CO₂ are solutes. Consider the example of air where Nitrogen gas is the solvent and oxygen, carbon dioxide, and trace gases are solutes. On the basis of physical states, solvent and solute can be categorized as solid, liquid, and gaseous solutions. Generally, solutions are found in three physical states depending upon the physical state of the solvent, e.g., air is a gaseous solution, seawater is a liquid solution, and alloy is a solid solution in real life.
In gaseous solutions, the solvent is a gas and the solute can be a gas, liquid, or solid. For example, a mixture of nitrogen and hydrogen used in Haber's process (ammonia formation) and another is a mixture of ammonia and carbon dioxide used for urea preparation. Fog, clouds, and mist are examples of solutions where liquid water (solute) is dissolved in air (solvent). Smoke is a solution of carbon particles in gaseous air in our daily life.
Carbonated drinks are solutions where the solvent is liquid water and the solute is gaseous carbon dioxide. Rectified spirit produced by the fermentation of sugar cane and vinegar (acetic acid in water) are examples of solutions where liquid dissolves in liquid. Brine and sugar syrup are solutions of solid salt and sugar in water, respectively.
Hydrogen gas on the nickel metal surface is used in the ghee industry where hydrogen gas is the solute and nickel catalyst is the solvent. A solution of any metal (solid) in liquid mercury is called amalgam. The alloy industry is very common these days. Alloys are formed by mixing different metals (Brass, Bronze, steel).
An aqueous solution is formed by dissolving a substance in water. The dissolved substances in an aqueous solution may be solids, gases, or other liquids. In order to be a true solution, a mixture must be stable. For example, sugar in water and table salt in water. Water is called a universal solvent because it dissolves the majority of compounds present in the earth's crust. Aqueous solutions are mostly used in laboratories.
A solution containing the maximum amount of solute at a given temperature is called a saturated solution. When a small amount of solute at a given temperature is added to a solvent, the solute dissolves very easily in the solvent. If the addition of solute is kept on, a stage is reached when the solvent cannot dissolve any more solute. At this stage, further added solute remains undissolved and settles down at the bottom of the container. On the particle level, a saturated solution is one in which undissolved solute is in equilibrium with dissolved solute. At this stage, dynamic equilibrium is established. Although dissolution and crystallization continue at a given temperature, the net amount of dissolved solute remains constant.
A solution that contains a lesser amount of solute than that which is required to saturate it at a given temperature is called an unsaturated solution. Such solutions have the capacity to dissolve more solute to become a saturated solution.
When saturated solutions are heated, they develop further capacity to dissolve more solute. Such solutions contain a greater amount of solute than is required to form a saturated solution, and they become more concentrated. The solution that is more concentrated than a saturated solution is known as a supersaturated solution. Supersaturated solutions are not stable. Therefore, an easy way to get a supersaturated solution is to prepare a saturated solution at a high temperature. It is then cooled to a temperature where excess solute crystallizes out and leaves behind a saturated solution.
Take 100g of water in a beaker. Add a teaspoon of sugar to it. Stir it. The sugar will dissolve. Repeat the process, and the added sugar will again dissolve in it. A solution that can dissolve more of the solute at a given temperature is called an unsaturated solution. Go on adding sugar to the above solution until it starts settling down at the bottom of the beaker at a particular temperature. The solution that cannot dissolve more solute at a particular temperature is called a saturated solution.
Now heat the solution, stir it, add more sugar, and it will dissolve. Go on adding more sugar and stir it. A stage will reach when no more sugar will dissolve and will start settling down at the bottom of the beaker. This solution is called a supersaturated solution. A solution that contains more of the solute than is contained in the saturated solution is called a supersaturated solution.
How to know whether a solution is saturated or supersaturated? A supersaturated solution is not stable in the presence of crystals of solute. If you add a crystal of sodium thiosulfate to its saturated solution, it will simply drop to the bottom without dissolving. But if you add a crystal of sodium thiosulfate to a supersaturated solution of sodium thiosulfate (see Figure 2.4), crystallization will start. When crystallization has finished, you will have a saturated solution in the presence of sodium thiosulfate crystals.
Unsaturated Solution
Saturated Solution
Supersaturated Solution
Fig 2.4: Different types of solutions
Solutions are classified as dilute or concentrated on the basis of the relative amount of solute present in them. Dilute solutions are those which contain relatively small amounts of dissolved solute in the solution. Concentrated solutions are those which contain relatively large amounts of dissolved solute in the solution. For example, brine is a concentrated solution of common salt in water. These terms describe the concentration of the solution. The addition of more solvent will dilute the solution and its concentration decreases.
Solubility is the maximum amount of solute which dissolves in a specified amount of solvent at a specific temperature. The solubility of a substance depends on the solvent used, as well as temperature and pressure. See Table 2.2.
The solubility of solutes depends on temperature. Depending on the nature of the solute, there is either: