CBSE Class 12 Chemistry (Physical Chemistry)
Chapter 1: Solutions
20 Important Questions and Answers
As per CBSE 2026–27 Syllabus
1. What is a solution? Classify solutions on the basis of physical states.
Answer:
A solution is a homogeneous mixture of two or more substances. The component present in a larger amount is called the solvent, while the component present in a smaller amount is called the solute. Solutions can be classified according to the physical states of solute and solvent. They may be gaseous solutions (air), liquid solutions (salt in water), or solid solutions (alloys like brass). Depending on the combination of physical states, there are nine possible types of solutions. Solutions have uniform composition throughout, and the particles of the solute are molecularly dispersed, making them stable and transparent. They cannot be separated by ordinary filtration.
2. What is meant by concentration of a solution? Name different methods of expressing concentration.
Answer:
The concentration of a solution refers to the amount of solute dissolved in a given quantity of solvent or solution. It indicates the strength of the solution. Various methods are used to express concentration, including mass percentage, volume percentage, mass by volume percentage, parts per million (ppm), mole fraction, molarity, and molality. Mass percentage is the mass of solute per 100 g of solution. Molarity is the number of moles of solute per litre of solution, whereas molality is the number of moles of solute per kilogram of solvent. Different concentration units are chosen according to the requirements of the experiment.
3. Define mass percentage and volume percentage.
Answer:
Mass percentage is the mass of a component present in 100 parts by mass of the solution. It is calculated using the formula:
Mass Percentage = (Mass of Component / Total Mass of Solution) × 100
Volume percentage is the volume of a component present in 100 parts by volume of the solution. It is calculated as:
Volume Percentage = (Volume of Component / Total Volume of Solution) × 100
Mass percentage is commonly used for solid-liquid solutions, while volume percentage is useful for liquid-liquid solutions. These concentration terms help compare different solutions and determine the exact amount of solute present in a mixture.
4. What is mole fraction? State its advantages.
Answer:
Mole fraction is the ratio of the number of moles of one component to the total number of moles of all components present in a solution. It is represented by X.
For a binary solution:
X₁ = n₁/(n₁ + n₂)
where n₁ and n₂ are the moles of the two components.
The sum of mole fractions of all components in a solution is always equal to one. Mole fraction is independent of temperature because it depends only on the number of moles. It is widely used in calculations involving vapour pressure and colligative properties. It provides an accurate representation of composition for scientific studies.
5. Differentiate between molarity and molality.
Answer:
Molarity (M) is defined as the number of moles of solute dissolved in one litre of solution. Molality (m) is defined as the number of moles of solute dissolved in one kilogram of solvent.
Molarity depends on the volume of the solution, which changes with temperature. Therefore, molarity is temperature-dependent. In contrast, molality depends on the mass of the solvent, which remains constant with temperature; hence, it is temperature-independent. Molality is preferred in studies involving colligative properties. Molarity is commonly used in laboratory preparations because measuring volume is easier than measuring mass.
6. What is solubility? Discuss factors affecting solubility of solids in liquids.
Answer:
Solubility is the maximum amount of a solute that can dissolve in a specified amount of solvent at a particular temperature to form a saturated solution. The solubility of solids in liquids depends mainly on temperature and the nature of solute and solvent. Most solid solutes become more soluble with an increase in temperature because dissolution is generally endothermic. The principle “like dissolves like” also affects solubility; polar solutes dissolve well in polar solvents, while non-polar solutes dissolve better in non-polar solvents. Pressure has little effect on the solubility of solids in liquids because solids are nearly incompressible.
7. State Henry’s Law and its applications.
Answer:
Henry’s Law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid.
Mathematically:
p = KH × x
where p is the partial pressure, KH is Henry’s constant, and x is the mole fraction of the gas.
Applications of Henry’s Law include the bottling of soft drinks, where carbon dioxide is dissolved under high pressure. It explains why divers suffer from bends when ascending rapidly. It is also used in calculating the solubility of gases in blood and in industrial processes involving gas absorption in liquids.
8. Why are soft drinks bottled under high pressure?
Answer:
Soft drinks contain carbon dioxide gas dissolved in water. According to Henry’s Law, the solubility of a gas in a liquid increases with an increase in pressure. Therefore, soft drinks are bottled under high pressure to dissolve a larger amount of carbon dioxide in the beverage. When the bottle is opened, the pressure decreases suddenly, reducing the solubility of carbon dioxide. As a result, excess gas escapes in the form of bubbles, producing the characteristic fizz. Bottling under high pressure helps maintain the taste, freshness, and carbonation of the drink until it is opened by the consumer.
9. What are ideal solutions? Give examples.
Answer:
Ideal solutions are those solutions that obey Raoult’s Law over the entire range of concentration and show no change in volume or enthalpy during mixing. In ideal solutions, the intermolecular forces between unlike molecules are nearly equal to those between like molecules. Therefore, mixing occurs without heat absorption or evolution.
Examples include:
- Benzene and toluene
- n-Hexane and n-heptane
- Chlorobenzene and bromobenzene
Ideal solutions have ΔHmix = 0 and ΔVmix = 0. They serve as reference systems for studying the behaviour of real solutions and understanding deviations from ideality.
10. State Raoult’s Law.
Answer:
Raoult’s Law states that the partial vapour pressure of each volatile component of an ideal solution is directly proportional to its mole fraction in the solution.
Mathematically:
P₁ = X₁P₁⁰
where P₁ is the partial vapour pressure, X₁ is the mole fraction, and P₁⁰ is the vapour pressure of the pure component.
For binary solutions, the total vapour pressure is the sum of the partial vapour pressures of the components. Raoult’s Law helps explain vapour-liquid equilibrium and forms the basis for understanding colligative properties and deviations shown by real solutions.
11. What are non-ideal solutions?
Answer:
Non-ideal solutions are solutions that do not obey Raoult’s Law throughout the entire concentration range. In such solutions, intermolecular forces between unlike molecules differ from those between like molecules. As a result, changes in volume and enthalpy occur during mixing.
Non-ideal solutions may show positive or negative deviations from Raoult’s Law. Examples of positive deviation include ethanol-acetone mixtures, while chloroform-acetone mixtures show negative deviation. These deviations arise due to differences in molecular interactions. Non-ideal solutions are commonly encountered in practical chemistry and help explain azeotrope formation and various industrial separation processes.
12. What is positive deviation from Raoult’s Law?
Answer:
Positive deviation from Raoult’s Law occurs when the observed vapour pressure of a solution is greater than the value predicted by Raoult’s Law. This happens when intermolecular attractions between unlike molecules are weaker than those between similar molecules. As a result, molecules escape more easily into the vapour phase.
Such solutions have positive enthalpy of mixing (ΔHmix > 0) and positive volume change (ΔVmix > 0). Examples include ethanol-acetone and acetone-carbon disulphide mixtures. These solutions often form minimum boiling azeotropes. Positive deviation indicates weaker interactions and greater volatility of the solution compared to ideal behaviour.
13. What is negative deviation from Raoult’s Law?
Answer:
Negative deviation from Raoult’s Law occurs when the actual vapour pressure of a solution is lower than that predicted by Raoult’s Law. This happens because intermolecular attractions between unlike molecules are stronger than those between similar molecules. Consequently, fewer molecules escape into the vapour phase.
Such solutions have negative enthalpy of mixing (ΔHmix < 0) and negative volume change (ΔVmix < 0). Examples include chloroform-acetone and nitric acid-water mixtures. These solutions generally form maximum boiling azeotropes. Negative deviation reflects stronger molecular interactions and reduced volatility compared to ideal solutions.
14. What are azeotropes?
Answer:
Azeotropes are liquid mixtures that boil at a constant temperature and possess the same composition in both liquid and vapour phases. Because of this property, their components cannot be completely separated by simple fractional distillation.
There are two types of azeotropes:
- Minimum boiling azeotropes – formed by positive deviation from Raoult’s Law.
- Maximum boiling azeotropes – formed by negative deviation from Raoult’s Law.
Examples include ethanol-water (minimum boiling) and nitric acid-water (maximum boiling). Azeotropes are important in industrial separation processes because they impose limitations on purification through ordinary distillation techniques.
15. What are colligative properties?
Answer:
Colligative properties are those properties of dilute solutions that depend only on the number of solute particles present and not on their chemical nature. These properties arise due to the lowering of solvent vapour pressure when a non-volatile solute is added.
The four main colligative properties are:
- Relative lowering of vapour pressure
- Elevation of boiling point
- Depression of freezing point
- Osmotic pressure
Colligative properties are useful in determining molar masses of solutes and studying solution behaviour. They play important roles in biological systems, industrial processes, and various analytical methods.
16. What is relative lowering of vapour pressure?
Answer:
Relative lowering of vapour pressure is the ratio of the decrease in vapour pressure to the vapour pressure of the pure solvent. It occurs when a non-volatile solute is dissolved in a solvent, reducing the number of solvent molecules available for evaporation.
Mathematically:
(P⁰ − P)/P⁰
where P⁰ is the vapour pressure of the pure solvent and P is the vapour pressure of the solution.
According to Raoult’s Law, this quantity is equal to the mole fraction of the solute in dilute solutions. It is one of the important colligative properties used for determining molar masses.
17. What is elevation of boiling point?
Answer:
Elevation of boiling point is the increase in the boiling point of a solvent when a non-volatile solute is dissolved in it. The dissolved solute lowers the vapour pressure of the solvent, requiring a higher temperature for the vapour pressure to equal atmospheric pressure.
The increase is represented as:
ΔTb = Kb × m
where ΔTb is elevation in boiling point, Kb is the molal elevation constant, and m is the molality of the solution.
This is a colligative property because it depends only on the number of solute particles present. It is commonly observed when salt is added to water.
18. What is depression of freezing point?
Answer:
Depression of freezing point is the lowering of the freezing point of a solvent when a non-volatile solute is dissolved in it. The presence of solute particles interferes with the formation of the solid phase, causing freezing to occur at a lower temperature.
The relationship is:
ΔTf = Kf × m
where ΔTf is depression in freezing point, Kf is the molal depression constant, and m is the molality of the solution.
This property is used in antifreeze solutions in automobiles and for determining molar masses of solutes. It depends only on the number of dissolved particles and not on their nature.
19. What is osmotic pressure?
Answer:
Osmotic pressure is the minimum pressure required to stop the flow of solvent through a semipermeable membrane from a dilute solution to a concentrated solution. This phenomenon is called osmosis.
Osmotic pressure is expressed by:
π = CRT
where π is osmotic pressure, C is molar concentration, R is the gas constant, and T is absolute temperature.
It is one of the most important colligative properties because it can be measured accurately even for dilute solutions. Osmotic pressure plays a significant role in biological systems, including water transport in plant and animal cells.
20. What is the van’t Hoff factor? Explain its significance.
Answer:
The van’t Hoff factor (i) is the ratio of the actual number of particles present in solution to the number expected from the chemical formula. It accounts for association or dissociation of solute particles in solution.
Mathematically:
i = Observed Colligative Property / Calculated Colligative Property
For electrolytes such as sodium chloride, dissociation increases the number of particles, making i greater than 1. For associated substances like acetic acid in benzene, i becomes less than 1. The van’t Hoff factor helps explain abnormal molar masses and deviations in colligative properties, making it essential for accurate solution chemistry calculations.