Solution and its Concentration

A solution (a homogeneous mixture) is formed when one or more substances (the solute) are completely dissolved in another substance (the solvent). The most common examples are the solutions that are obtained by dissolving solids in water.

Sugar or common salt dissolved in water gives this type of solution. Two-third of the Earth’s surface is covered by a solution. The sea-water is a solution of water and soluble minerals. It also contains gases like oxygen, nitrogen and carbon dioxide. Such dissolved gases are very important for aquatic life to survive in oceans.

There are some solutions of two or more than two liquids. For example, ethyl alcohol mixes with water in all proportions to form a solution. Iodine (solid) dissolved in ethyl alcohol gives tincture of iodine which has antiseptic properties.

A solution made of solid dissolved in a liquid has two parts:

  1. The solid that dissolves is called the solute.
  2. The liquid, in which the solid is dissolved, is called the solvent.

Solutions are not confined to only solids dissolved in liquids. There are other types of solutions as well. In each case the substance which is present in bigger quantity is normally taken as solvent and substance which is present in smaller quantity is normally taken as solute.

When a substance dissolves in a solvent it is said that particular solute is soluble in that particular solvent. If it does not dissolve then it is insoluble. Water is a commonly used solvent as it dissolves a large number of substances. Because of this property water is called a universal solvent. Different types of substances dissolve in water. Because of this unique property of water, plants can take minerals from the soil.

Being a good solvent, water is used in many ways. However, there are some disadvantages also which result from this unique property of water. Water becomes easily contaminated. Therefore, purifying water for drinking and other uses is a major challenge.

There are other important solvents, for examples organic liquids. The organic solvents are important because, unlike water, they dissolve organic substances. Ethyl alcohol and benzene are examples of such organic solvents.

Concentration of a Solution

Concentration of a solution is expressed in terms of the amount of solute present in a given mass or in a given volume of a solvent. Usually concentration of a solution is defined as the mass of solute present in a definite volume of a solution (which is usually taken as 1 litre). Concentration of a solution may also be expressed in terms of per cent by mass of solute (in gram). This gives the mass of solute per 100 mass units (grams) of solution as:

% of solute = (mass of solute / mass of solution) × 100.

A solution of 10% glucose by mass means that 100 grams of the solution contains 10 gram of glucose. This means 10 grams of glucose is dissolved in 90 grams of water.

When you dissolve a particular substance (for example, sugar) in water, the solution becomes more concentrated as you add more and more sugar. A concentrated solution contains a high proportion of the solute. A dilute solution contains a small proportion of solute.

If you keep on adding solute to a solvent, keeping the temperature constant you reach a point where no more solute will be dissolved. At this point, the solution has become saturated with respect to solute. However, if you increase the temperature, more solute will get dissolved. The concentration of a solute in a saturated solution at a definite temperature is called solubility of that solute in that particular solvent.