Solutions - Topics, Books, FAQs

A solution is a homogeneous mixture of two or more substances whose composition can be varied within particular limits. The term homogenous mixture can be defined as a mixture where the composition and properties are steady throughout the mixture. Solution consists of solvent and solute, in which the solvent is the one present in a larger amount, whereas the solute is present in a smaller amount. There are various common examples of solutions that we see in our daily life, such as saltwater, which is formed when we mix salt in water. Vinegar is obtained when we mix acetic acid with water.

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This Story also Contains

1. Important Topics of Solutions
2. Overview of Chapter
3. Important PYQs in this Chapter
4. Some Preparation Tips on 'Solutions'
5. Prescribed Books for Solutions

Important Topics of Solutions

Types of Solution

Solutions are homogeneous mixtures of two or more components. The Type of Solution is decided by the physical states of solute and solvent and solutions are categorized as solid, liquid, or gaseous. For example, a sugar solution is a liquid solution, while air is a gaseous solution.

Expression of Concentration of Solution

The concentration of a solution indicates the amount of solute present in a given quantity of solvent or solution. Common expressions include molarity (moles of solute per litre of solution), molality (moles of solute per kilogram of solvent), mole fraction, and percentage composition.

Vapour Pressure of Solution

The vapour pressure of a solution is the pressure exerted by its vapour when in equilibrium with its liquid phase. It depends on the nature and quantity of the solute and solvent. Non-volatile solutes lower the vapour pressure of the solution compared to the pure solvent.

Formula - $P_{\text{solution }}<P_{\text{solvent }}^0$

Where,
- $P_{\text{solution }}=$ vapour pressure of the solution
- $P_{\text{solvent }}^0=$ vapour pressure of pure solvent

Ideal Solution

An ideal solution follows Raoult’s Law, where the total vapour pressure is directly proportional to the mole fraction of components. Ideal Solutions exhibit no enthalpy change or volume change on mixing. Examples include benzene and toluene mixtures.

Raoult’s Law

Raoult’s Law states that the partial vapour pressure of a component in a solution is equal to the product of its mole fraction and its vapour pressure in the pure state. Raoult’s Law explains the colligative properties and is crucial in studying non-ideal solutions.

Formula:

$P_A=X_A{P}_A^0$
Where:
- $P_A=$ partial vapour pressure of component A
- $X_A=$ mole fraction of component A
- $P_A^0=$ vapour pressure of pure component A

For a two-component solution:

$P_{\text{total }}=P_A+P_B=X_A{P}_A^0+X_B{P}_B^0$

• Ideal and non-ideal solutions- Ideal solutions follow Raoult’s Law at all concentrations and there is no change in volume or enthalpy upon mixing. The interactions between A-A, B-B, and A-B are same whereas non-ideal solutions deviate from Raoult’s Law , either a positive deviation or a negative deviation, depending on A-B interactions, wheather they are weaker or stronger than A-A and B-B.

Positive deviation:$P_{\text{solution }}>X_A{P}_A^0+X_B{P}_B^0$

Negative deviation: $P_{\text{solution }}<X_A{P}_A^0+X_B{P}_B^0$

Azeotropic Mixture

Azeotropes are binary mixtures that boil at a constant temperature and behave as a single substance during distillation. An azeotropic mixture is formed due to strong deviations from Raoult's Law and can be either minimum or maximum boiling azeotropes.

Elevation in Boiling Point

Adding a non-volatile solute to a solvent increases its boiling point. Elevation in boiling point is a colligative property and occurs because the solute lowers the solvent’s vapour pressure, requiring more heat to reach the boiling point.

Depression in Freezing Point

The freezing point of a solution decreases when a solute is added. Depression in the freezing point is a colligative property which is used in applications like antifreeze solutions, where freezing is prevented by lowering the freezing point.

Osmotic Pressure

Osmotic pressure is the pressure required to stop osmosis, the movement of solvent molecules through a semipermeable membrane. It depends on the concentration of solute particles in the solution and is significant in biological processes.

Reverse Osmosis

Reverse osmosis occurs when a pressure greater than the osmotic pressure is applied to a solution, forcing solvent molecules through a semipermeable membrane in the opposite direction. Reverse osmosis is widely used for water purification.

Isotonic, Hypertonic, and Hypotonic Solutions

Isotonic Solutions have equal osmotic pressure to a reference solution, such as body fluids. Hypertonic Solutions have higher osmotic pressure, causing water to move out of cells. Hypotonic Solutions have lower osmotic pressure, causing water to move into cells.

Van’t Hoff Factor and Abnormal Molar Mass

The Van’t Hoff factor accounts for the effect of ionization or association of solutes on colligative properties. It is used to calculate abnormal molar masses in cases where solutes dissociate (e.g., NaCl) or associate (e.g., acetic acid).

Solubility and Henry’s Law

Solubility refers to the maximum amount of solute that can dissolve in a solvent at a given temperature and pressure. Henry’s Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas. This principle is critical in industries like carbonated beverages and scuba diving.

Overview of Chapter

There are various kinds of binary solutions exist in nature as given in the table below:

In this chapter, there are various important topics that you must understand completely:

(i) Solubility: The maximum ability of a substance to get completely dissolved in a solvent. When the solute is solid and the solvent is liquid, then only temperature affects the solubility. With the increase in temperature, the solubility increases. But pressure has no effect in this case as both solids and liquids are incompressible. But is the solute is gas, then pressure also is an important factor that plays a major role in insolubility. For this case, Henry's law is given which states that - "At constant temperature, the solubility of a gas is directly proportional to the partial pressure of a gas present above the solution".

(ii) Vapour pressure of liquid solutions: In this concept, you learn the vapour pressure of volatile liquids in solution when taken in a closed vessel. This phenomenon is explained by Raoult's law, which states that - "The vapour pressure of each volatile liquid present in the solution is directly proportional to the mole fraction of that liquid present in solution".

Mathematically, Raoult's law can be expressed as follows:

$p_{\text{total }}=p_1^0+(p_2^0+p_1^0)x_2$
where, $p_1^0$ = vapour pressure of pure component 1
$p_2^0$ = vapour pressure of pure component 2
x₂ = mole fraction of component 2
(iii) Ideal and non-ideal solutions: Ideal solutions are those solutions which obey Raoult's law at all ranges of concentrations. Whereas the non-ideal solutions are those which do not obey Raoult's law at all ranges of concentrations. The vapour pressure of non-ideal solutions is always higher or lower than as predicted by Raoult's law and thus we say that the solution is exhibiting a positive or negative deviation, respectively.

(iv) Colligative properties: The properties of solutions which depend only on the number of solute particles present in the solution are known as colligative properties.

• Relative lowering of vapour pressure: The actual vapour pressure of a solvent in a solution is less than that of the pure solvent. Raoult explained this behaviour and established that this lowering of vapour pressure depends only on the concentration of the solute particles. Mathematically, it can be expressed as follows:Δp₁p₁₀=p₁₀−p₁p₁₀=x₂
• Elevation of the boiling point: The boiling point is the point of temperature when the vapour pressure of a substance is equal to the atmospheric pressure. This also depends upon the number of solute particles present in the solution.
  

Mathematically, it can be expressed as follows: ΔT_b=K_bm
K_b = Boiling point elevation constant

• Depression of freezing point:

Freezing Point: It is the temperature at which the liquid and the solid form of the same substance are in equilibrium and have the same vapour pressure. A Solution freezes when its vapour pressure is equal to the V.P. of pure solid solvent. Due to the lower vapour pressure of the solution, the solid form of a solution separates out at a lower temperature.

ΔT_f=T_f∘−T_f

• This is also termed cryoscopy and depression of freezing point (ΔT_f)
• It can be measured by Beckmann's thermometer method and Rast's method.
• For a dilute solution, ΔT_f is directly proportional to the molality (m) of the solution.
• Hence ΔT_f ∝ m
  ΔT_f = K_f m
  If the molality of the solution is one, then
  ΔT_f = K_f
  $\mathrm{\Delta }{T}_f$ and $M$ can be found out by using these relations.

  $\begin{array}{rl}& \mathrm{\Delta }{T}_f=K_f\frac{w}{M}\times \frac{1000}{W}\\ & M=\frac{K_f\times w\times 1000}{\mathrm{\Delta }{T}_f\times W}\end{array}$

  Here $w=$ Weight of solute
  $\mathrm{W}=$ Weight of solvent
  ${\mathrm{K}}_{\mathrm{f}}=$ Molal depression constant or cryoscopic constant
  $\mathrm{M}=$ Molar mass of Non-volatile solute.

  ${\mathrm{K}}_{\mathrm{f}}=\frac{{\mathrm{RT}}^2}{1000{\mathrm{L}}_{\mathrm{f}}\text{ or }\mathrm{\Delta }{\mathrm{H}}_{\text{fusion }}}$

  Here ${\mathrm{L}}_f$ or $\mathrm{\Delta }{\mathrm{H}}_{\mathrm{f}}=$ latent heat of fusion.

NOTE: The value of K_v or K_f depends only on the nature of the solvent and not on the nature of the solute.

Abnormal Mass: This concept says, that when a solute is dissolved in a liquid then the solute does not dissociate completely as expected instead it dimerizes and thus the molar mass of the solute becomes double. Such deviation of molar mass from the actual value is known as abnormal molar mass.

To deal with the case of an abnormal mass, a Vant Hoff factor was introduced in 1880, which mathematically is described as follows:
i= Normal Molar Mass Abnormal Molar Mass

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Important PYQs in this Chapter

Q.1 20 mL of sodium iodide solution gave 4.74 g silver iodide when treated with excess of silver nitrate solution. The molarity of the sodium iodide solution is ________ M. (Nearest Integer value)
(Given : $\mathrm{Na}=23,\mathrm{I}=127,\mathrm{Ag}=108,\mathrm{N}=14$, $\mathrm{O}=16\mathrm{g}{\mathrm{mol}}^{-1})$

Ans. Let molarity of Nal solution be $\times \mathrm{M}$
$\mathrm{NaI}+{\mathrm{AgNO}}_3\to \mathrm{AgI}+{\mathrm{NaNO}}_3$
Moles of Agl formed $=\frac{4.74}{235}=0.02$
Moles of Nal $=\frac{20\times \mathrm{x}}{1000}=0.02\mathrm{x}$
$0.02x=0.02$
$x=1$
$\therefore$ Molarity of Nal solution $=1\mathrm{M}$

Hence, the answer is 1.

Q.2 The percentage dissociation of a salt $\left({\mathrm{MX}}_3\right)$ solution at given temperature (van't Hoff factor $\mathrm{i}=$ 2) is__________% (Nearest integer)

Ans. $\begin{array}{rl}& {\mathrm{MX}}_3\to {\mathrm{M}}^{+3}+3{\mathrm{X}}^{\circ }\\ & \mathrm{i}=1+(\mathrm{n}-1)\alpha \\ & \mathrm{i}=1+(4-1)\alpha =2\\ & \alpha =\frac{1}{3}=33.33\mathrm{\%}\approx 33\mathrm{\%}\end{array}$

Hence, the answer is 33%.

Some Preparation Tips on 'Solutions'

• This chapter is a part of the physical chemistry and it is very important from the numerical point of view. The theory part is less but you simply need to work with the formulas and questions of this chapter.
• There are some common terms used in this chapter like vapour pressure and mole fraction, so before coming to this chapter, first, you must finish the two basic chapters - "Mole concept" and "Gaseous state".
• You must have completely memorized all the formulas and must have a very good grasp of solving the numerical problems.
• Usually, most of the students like this chapter as it is very easy and straightforward. This chapter holds a good weight ok marks in board exams as well as in competitive exams like JEE and NEET.

Prescribed Books for Solutions

For this chapter, first, you need to finish the important topics of solutions class 12 thoroughly from the class 12th NCERT book and then simultaneously solve the examples and questions given in the book. Apart from this, if you want to prepare for the advanced level of competitive exams like JEE and NEET, you must prepare from the books - O.P. Tandon and R.C Mukherjee. Meanwhile, in preparation, you must continuously write mock tests to increase your depth of knowledge. Our platform will help you to provide a variety of questions for deeper knowledge with the help of videos, articles and mock tests.

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