What is the true solution to

A solution in which particles of the solute are broken down to such a fine state, that they cannot be seen under a powerful microscope is called a true solution.

Characteristics of a True Solution :

• A true solution is always clear and transparent, i.e., light can easily pass through it without scattering.
• The particles of a solute break down to almost molecular size and their diameter is of the order of 1 nm (10-9 m) or less.
• A true solution can completely pass through a filter paper as particle size of solute is far smaller than the size of pores of filter paper.
•  A true solution is homogeneous in nature.
•  In a true solution, the particles of solute do not settle down, provided temperature is constant.
• From a true solution, the solute can easily be recovered by evaporation or crystallisation.

Concentration of a Solution : 

It is defined as the amount of solute present in a given quantity of the solution. The most common method for expressing the concentration of a solution is called percentage method. The concentration of solution refers to the percentage of solute present in solution. Furthermore, the percentage of solute can be expressed in terms of :

• mass of the solute
• volume of the solute.
• Concentration of a solution in terms of mass percentage of solute : If a solution is formed by dissolving a solid solute in a liquid solvent then the concentration of solution is expressed in terms of mass percentage of solute and is defined as under :

The concentration of solution is the mass of the solute is grams, which is present in 100 g of a solution.

The percentage concentration of a solution refers to mass of solute in 100 g of solution and not 100g of solvent, i.e., water.

The concentration of a solution in terms of mass percentage of solute is calculated by the formula given below :

Concentration of solution

(ii) Concentration of a solution in terms of volume percentage of solute : If a solution is formed by dissolving a liquid solute in a liquid solvent, then the concentration of the solution is expressed in terms of volume percentage of solute. The concentration of a solution is the volume of the solute in milliners, which is present in 100 milliliters of a solution.

It is very important to keep in mind that the percentage concentration of solution refers to volume of solute in 100 ml of solution and not 100 ml of solvent, i.e., water.

The concentration of a solution in terms of volume percentage of the solute is calculated by the formula given below :

Concentration of solution

Saturated, Unsaturated and Supersaturated Solutions:

A solution, in which no more solute can be dissolved at that temperature, is called a saturated solution.

A solution, in which more quantity of solute can be dissolved without raising its temperature, is called an unsaturated solution. 

A solution which temporarily contains more solute than the saturation level (i.e. maximum solute) at a particular temperature, is called a supersaturated solution. 

Test for saturated, unsaturated and supersaturated solutions:

In order to test, whether a given solution is saturated or unsaturated, add some more solute to this solution and try to dissolve by stirring with glass rod keeping temperature constant. If more solute does not dissolve in the given solution, then it must be a saturated solution and if more solute dissolves, it must be an unsaturated solution. 

On the other hand, supersaturated solution can be easily distinguished from the saturated solution simply by adding a few crystals of solute dissolved. If the precipitation of some additional solute occurs, it is a supersaturated solution; otherwise it is a saturated solution. 

Ex.1 What is the meaning of 15% solution of NaCl ?

Solution: 15% solution of NaCl is a solution 100 g of which contains 15 g of NaCl and 85 g of water.

Ex.2 Calculate the amount of glucose required to prepare 250 g of 5% solution of glucose by mass.

Solution:  % of solute

Mass of solute =

Ex.3 A solution contains 50 mL of alcohol mixed with 150 mL of water. Calculate concentration of this solution.

Solution: This solution contains a liquid solute (alcohol) mixed with a liquid solvent (water), so we have to calculate the concentration of this solution in terms of volume percentage of solute (alcohol). Now, we know that:

Concentration of solution

Here, Volume of solute (alcohol) = 50  mL

And. Volume of solvent (water) = 150 mL

So, Volume of solution = Volume of solute + Volume of solvent

= 50 + 150 = 200 mL

Now, putting these values of ‘volume of solute’ and ‘volume of solution’ in the above formula we get :

Concentration of solution

Thus, the concentration of this alcohol solution is 25 per cent or that it is a 25%.

Ex.4 How much water should be added to 16 ml acetone to make its concentration 48%?

Solution:  Concentration of solution

x =

Volume of solvent = 33.33 - 16 = 17.33 ml.

SOLUBILITY:

The maximum amount of solute which can be dissolved in 100 gms of a solvent at a specified temperature is known as the solubility of that solute in that solvent (at that temperature).

e.g. A maximum of 36 gms of common salt (NaCl) can be dissolved in 100 g. of water at 20°C (or 293 K). Therefore, the solubility of common salt in water at 20°C is 36 g.

Different substances have different solubilities in the same solvent. Let us understand with the help of an experiment :  

• Take approximately 50 mL of water each in two separate beakers.
• Add common salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring.
•  When no more solute can be dissolved, heat the contents of the beaker.
• Start adding the solute again.
• Is the amount of common salt and sugar or barium chloride, that can be dissolved in water at a given temperature, the same?
• What would happen if you were to take a saturated solution at a certain temperature and cool it slowly?

Discussion:

• The amounts of common salt, sugar and barium chloride that can be dissolved in water (50 mL) at room temperature are different.
• When a saturated solution at a certain temperature is cooled, the solubility decreases and the amount of the solute which exceeds the solubility at lower temperature crystallizes out of the solution.

Conclusion: Different substances have different solubilities in a given solvent at the same temperature and, in general, the solubility decreases as the solution is cooled and the extra amount of solute crystallizes out.

Ex. 2.5 g of a solute is dissolved in 25 g of water to form a saturated solution at 298 K. Find out the solubility of the solute at this temperature.

Solution:  Mass of the solute = 2.5 g

Mass of the solvent = 25 g

 ∴ Solubility of the solute =

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