Maharashtra Board Class 12 Chemistry Chapter 2 Solutions PDF Download

2. Solutions

2.1 Introduction

You are familiar with mixtures. The mixture is a combination of two or more substances. Air is a mixture of gases, rock is a mixture of two or more minerals and so forth.

Recall that the mixtures are (a) homogeneous in which mixing of components is uniform or (b) heterogeneous which have nonuniform mixing of components.

We studied in standard XI, that homogeneous mixtures are classified according to the size of their constituent particles as colloids or as true solutions. In this chapter we deal with the properties of homogeneous mixtures especially of true solutions.

Can you recall? The size of particles of colloids and those of true solutions.

The solutions are commonly found in all life processes. The body fluids are solutions. The solutions are also important for industrial processes, and many other areas.

Can you recall? The terms solute and solvent.

The solution is a homogeneous mixture of two or more pure substances. A true solution consists of a solvent and one or more solutes. We explore the properties of binary true solutions containing only one solute.

Teacher's Note

Solutions are everywhere in your life. When you drink milk, it is a solution. When you add salt to water, it becomes a solution. Your body has many solutions like blood and saliva.

Exam Trick

Remember: Homogeneous mixture = uniform mixing. Heterogeneous mixture = non-uniform mixing. You cannot see the particles in a homogeneous mixture.

Points to Remember

A solution has a solute and a solvent mixed together uniformly.
Colloids are different from true solutions because their particles are bigger.
Solutions are found everywhere in nature and in our bodies.
A true solution looks clear like water.

2.2 Types of Solutions

We generally think that a solution is either solid dissolved in liquid or a mixture of two liquids. There are other types of solutions as well. The solute and the solvent may be in any of three states namely, solid, liquid or gas. The solutions thus may involve any combination of these three states of their components. This gives rise to nine types of solutions depending on the states of solute and solvent. These are summarised in table 2.1.

Our focus, in this chapter, will be on the solution of solid in liquid with some attention to a solution of gas in liquid. The solvent in most of the cases will be water.

Can you recall? The different units used to express the concentrations of solutions.

Teacher's Note

There are 9 types of solutions depending on what state the solute and solvent are in. For example, salt water has a solid solute (salt) in a liquid solvent (water). Air is a gas solution.

Exam Trick

Remember: Solid + Liquid = most common type. Gas + Liquid = carbonated drinks. Gas + Gas = Air. There are 9 combinations total.

Points to Remember

A solution can have solute and solvent in any state - solid, liquid, or gas.
The most common solution is solid dissolved in liquid.
Salt in water is a solid-liquid solution.
Air is a gas-gas solution.
There are 9 different types of solutions.

2.3 Capacity of Solution to Dissolve Solute

Chemists need to know the capacity of solutions to dissolve a solute. Suppose that a solute is added to a solvent. It dissolves readily at first. The dissolution then slows down as more solute is added. If we continue the addition of solute, the dissolution stops. The solution at this point is said to be saturated.

A dynamic equilibrium can be reached where the number of solute molecules leaving the crystal to pass into solution is equal to the number returning from the solution to the crystal. Thus,

Solute + solvent \(\rightleftharpoons\) Solution

A saturated solution contains maximum amount of solute dissolved in a given amount of solvent at a given temperature. A solution containing greater than the equilibrium amount of solute is said to be supersaturated solution. Such solutions are unstable. The precipitation occurs by the addition of a tiny crystal of solute and the supersaturated solution changes to saturated solution.

Teacher's Note

When you add salt to water and keep stirring, at some point no more salt will dissolve. That point is called saturation. This happens in your kitchen when making salt water for cooking.

Exam Trick

Remember: Saturated solution = no more solute can dissolve. Supersaturated solution = has extra solute (unstable). One small crystal can make it crash and form a saturated solution.

Points to Remember

A saturated solution has the maximum amount of dissolved solute.
A supersaturated solution has more solute than it should hold.
Saturation happens at a specific temperature.
Dynamic equilibrium means dissolving and forming crystals happen at the same rate.

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