10th Class Chemistry Notes 2026 — Chapter 2
Class 10 Chemistry — Chapter 02: Matter
Federal Board
1. Multiple Choice Questions (MCQs)
i. What happens to the internal energy of a substance during melting?
a) It decreases
b) It stays the same
c) It increases
Correct Answer: (c) It increases
Concept: Heat energy absorbed during melting increases the potential energy component of internal energy by breaking intermolecular bonds, even though the temperature (kinetic energy) remains constant.
ii. Which process involves a solid turning directly into a gas?
a) Melting
b) Freezing
c) Sublimation
d) Condensation
Correct Answer: (c) Sublimation
Concept: Sublimation occurs when particles of a solid absorb enough thermal energy to bypass the liquid state and change directly into gas.
iii. Boiling occurs at:
a) Any temperature
b) A fixed temperature
c) Only at room temperature
Correct Answer: (b) A fixed temperature
Concept: Boiling takes place at a specific boiling point where the vapour pressure of the liquid equals the external atmospheric pressure.
iv. During condensation, the particles in a gas:
a) Gain energy and move faster
b) Lose energy and move slower
c) Gain energy and move slower
d) Lose energy and move faster
Correct Answer: (b) Lose energy and move slower
Concept: Condensation releases heat energy, causing gas molecules to lose kinetic energy, move more slowly, and come closer together to form a liquid.
v. Which law states that the volume of a gas is directly proportional to its temperature at constant pressure?
a) Boyle’s Law
b) Charles’s Law
c) Avogadro’s Law
d) Dalton’s Law
Correct Answer: (b) Charles’s Law
Concept: Charles’s Law states that volume is directly proportional to absolute temperature when pressure and the amount of gas are held constant.
vi. Evaporation differs from boiling because:
a) Evaporation occurs at the surface of a liquid at any temperature
b) Boiling occurs only at the surface
c) Evaporation requires a fixed temperature
d) Boiling occurs at any temperature
Correct Answer: (a) Evaporation occurs at the surface of a liquid at any temperature
Concept: Evaporation is a surface phenomenon that can occur at temperatures below the boiling point, whereas boiling is a bulk phenomenon that occurs at a fixed temperature for a given pressure.
vii. According to kinetic particle theory, the pressure of a gas increases when:
a) The volume increases
b) The temperature decreases
c) The number of particles decreases
d) The temperature increases
Correct Answer: (d) The temperature increases
Concept: Higher temperature increases the average kinetic energy of gas particles, causing more frequent and energetic collisions against the container walls.
viii. Which factors affect the rate of diffusion of gases?
a) Molecular mass
b) Temperature
c) Both a and b
d) Neither a nor b
Correct Answer: (c) Both a and b
Concept: According to Graham’s Law, the diffusion rate is inversely proportional to the square root of molecular mass. It also increases with temperature because gas particles gain kinetic energy.
ix. Solid air fresheners use the process of:
a) Condensation
b) Deposition
c) Sublimation
d) Freezing
Correct Answer: (c) Sublimation
Concept: Solid air fresheners contain volatile solids, such as naphthalene or p-dichlorobenzene, that change directly into gas and release fragrance.
x. The diffusion of medicine in the body is important because:
a) It controls the temperature of the medicine
b) It ensures the medicine reaches all parts of the body
c) It keeps the medicine in one place
d) It increases the molecular mass of the medicine
Correct Answer: (b) It ensures the medicine reaches all parts of the body
Concept: Diffusion moves dissolved drug molecules from regions of higher concentration to regions of lower concentration, helping them reach target tissues.
2. Short Answer Questions
i. Explain why the temperature remains constant during the phase change from ice to water.
Answer:
During the phase change from ice to water (melting at 0°C), the absorbed heat energy, known as latent heat of fusion, is used to weaken and break the hydrogen bonds holding the ice crystal lattice together.
This energy does not increase the average kinetic energy of the water molecules. Since temperature is a measure of average kinetic energy, it remains constant until all the ice has melted.
ii. What is the name given to the phase change when a solid is converted directly to a gas?
Answer:
The phase change is called sublimation. Examples include dry ice (CO₂), iodine crystals, and camphor.
iii. What is the difference between evaporation and boiling?
Answer:
Feature | Evaporation | Boiling |
|---|---|---|
Location | Surface phenomenon; occurs only at the surface. | Bulk phenomenon; occurs throughout the liquid. |
Temperature | Occurs at any temperature below the boiling point. | Occurs at a specific boiling point. |
Bubbles | No bubble formation. | Bubbles form within the liquid and rise to the surface. |
Energy source | Slow process; uses heat from surroundings and causes cooling. | Requires continuous external heating. |
iv. Interpret a heating curve for water, identifying key phase changes.
Answer:
A heating curve for water plots temperature (°C) against time or heat added. It shows how the temperature changes as water passes through different physical states.
Solid Ice Region (Below 0°C): Temperature rises as ice absorbs kinetic energy.
Flat Region at 0°C (Melting): Solid and liquid coexist. Latent heat breaks the ice lattice, so the temperature remains constant.
Liquid Water Region (0°C to 100°C): Temperature rises as liquid molecules move faster.
Flat Region at 100°C (Boiling): Liquid and gas coexist. Latent heat overcomes intermolecular forces in the liquid.
Steam Region (Above 100°C): Temperature increases as water vapour absorbs more heat.
v. Explain the effect of increasing temperature on the pressure of a gas in a sealed container.
Answer:
According to Gay-Lussac’s Law, increasing the temperature increases the average kinetic energy and velocity of gas particles.
In a rigid, sealed container (constant volume), faster-moving particles collide with the inner walls more frequently and with greater force. This results in an increase in gas pressure.
vi. How does increasing the external pressure affect the boiling point of a liquid?
Answer:
A liquid boils when its vapour pressure equals the external atmospheric pressure. Increasing external pressure raises the vapour pressure required for boiling.
Consequently, liquid molecules require higher kinetic energy and a higher temperature to escape into the vapour state. This increases the boiling point, as observed inside a pressure cooker.
vii. Describe how molecular mass influences the rate of diffusion.
Answer:
According to Graham’s Law of Diffusion, at constant temperature, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.
Lighter molecules have a higher average velocity than heavier molecules at the same temperature and therefore diffuse faster.
For example, ammonia (NH₃), with a molar mass of 17 g/mol, diffuses faster than hydrogen chloride (HCl), with a molar mass of 36.5 g/mol.
viii. Give an example of sublimation and explain its practical application.
Answer:
Example: Solid carbon dioxide, commonly known as dry ice.
Practical Application: Dry ice is used for the commercial cold shipping of biological samples and perishable foods. It sublimates directly into cold CO₂ gas at −78.5°C without leaving liquid residue or water.
ix. Why is the diffusion of gases faster at higher temperatures?
Answer:
At higher temperatures, gas particles gain more kinetic energy.
As a result, gas particles move at higher average velocities and overcome intermolecular resistance more easily. Their increased random motion speeds up mixing and diffusion.
x. How does Avogadro’s Law relate to the volume and number of moles of gas?
Answer:
Avogadro’s Law states that equal volumes of all gases, under identical conditions of temperature and pressure, contain equal numbers of molecules or moles.
The volume of a gas is directly proportional to the number of moles:
At Standard Temperature and Pressure (STP), one mole of an ideal gas occupies a molar volume of 22.4 dm³ (22.4 L).
3. Long Answer Questions
i. Describe in detail the kinetic particle theory and how it explains the phase changes of melting, freezing, boiling, and condensation.
Answer:
Postulates of Kinetic Particle Theory
All matter is made of tiny particles, such as atoms, ions, or molecules.
Particles are in continuous, random motion.
Forces of attraction (intermolecular forces) exist between particles. These forces decrease as the distance between particles increases.
Explanation of Phase Changes
1. Melting (Solid to Liquid)
Heating supplies heat energy to a solid, causing its particles to vibrate more vigorously. At the melting point, particles gain sufficient potential energy to overcome their fixed lattice positions and slide over one another, forming a liquid.
2. Freezing (Liquid to Solid)
Cooling reduces the kinetic energy of liquid particles. They slow down until attractive forces pull them into rigid, fixed crystalline positions, forming a solid.
3. Boiling (Liquid to Gas)
Adding thermal energy enables liquid particles to overcome intermolecular forces. When vapour pressure equals atmospheric pressure, bubbles form throughout the liquid and escape as gas.
4. Condensation (Gas to Liquid)
Gas particles lose thermal energy upon cooling, reducing their kinetic energy. Intermolecular attractions become effective again, bringing the particles closer together and forming a liquid.
ii. Interpret a cooling curve, identifying and explaining the significance of the flat regions on the curve.
Answer:
A cooling curve shows how the temperature of a substance decreases with time as it loses heat and changes from gas to liquid and then from liquid to solid.
Interpretation of Cooling Curve
Sloped Gas Cooling Region: Gas loses kinetic energy, causing its temperature to decrease.
Flat Region 1 (Condensation Temperature): Temperature remains constant. Heat lost to the surroundings corresponds to the latent heat of vaporization released as gas particles form intermolecular bonds and become liquid.
Sloped Liquid Cooling Region: The liquid cools as the average kinetic energy of its particles decreases.
Flat Region 2 (Freezing Temperature): Temperature remains constant. Heat released corresponds to the latent heat of fusion as the liquid changes into a solid crystal lattice.
Sloped Solid Cooling Region: The solid continues to cool until it reaches the ambient room temperature.
Significance of Flat Regions: During condensation and freezing, temperature remains constant because the energy released is associated with the change of state rather than a decrease in the particles’ average kinetic energy.
iii. Explain how diffusion works in gases and discuss the factors affecting the rate of diffusion, including molecular mass and temperature.
Answer:
Mechanism of Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient. It occurs due to continuous random thermal movement and collisions between particles.
Factors Affecting the Rate of Diffusion
1. Molecular Mass (M)
According to kinetic energy theory, at constant temperature, the average kinetic energy of gas particles is equal.
Therefore, lighter molecules move faster and diffuse more rapidly than heavier molecules.
The velocity is inversely proportional to the square root of molecular mass:
Example: Ammonia (NH₃), with a molar mass of 17 g/mol, diffuses almost 1.47 times faster than hydrogen chloride (HCl), with a molar mass of 36.5 g/mol.
2. Temperature (T)
Increasing temperature increases the average kinetic energy and velocity of gas particles, resulting in faster mixing and diffusion.
3. Concentration Gradient
A larger difference in concentration between two regions increases the rate of diffusion.
4. State of Matter / Medium Density
Diffusion is fastest in gases because of the wide spacing between particles and weaker intermolecular attractions compared with liquids and solids.
4. Think Tank
1. Analyze the impact of altitude on the boiling point of water and how it affects cooking times. Provide a detailed explanation based on kinetic particle theory and external pressure.
Answer:
Impact of Altitude
At high altitudes, atmospheric pressure decreases significantly because the column of air above the surface is thinner.
Kinetic and Thermodynamic Explanation
Boiling occurs when the vapour pressure of a liquid equals the external atmospheric pressure.
At high altitudes, external pressure is lower. Therefore, water requires less thermal energy and lower kinetic energy for its vapour pressure to match atmospheric pressure.
Consequently, water boils below 100°C. For example, it may boil at approximately 92°C in Murree and other high-altitude areas.
Effect on Cooking Times
Cooking depends on the temperature at which heat is transferred to food. At high altitudes, water boils at a lower temperature, so food cooks in cooler water.
This slows down chemical denaturation and softening processes, increasing cooking time.
Pressure cookers reverse this effect by trapping steam and increasing internal pressure, which raises the boiling temperature of water above 120°C.
2. Discuss the importance of understanding diffusion rates in the development of pharmaceuticals.
Answer:
Understanding diffusion kinetics is important in modern drug formulation and pharmacokinetics.
1. Transdermal and Oral Drug Delivery
Transdermal patches, such as nicotine or pain-relief patches, rely on controlled diffusion through skin lipid layers into capillaries.
2. Targeted and Sustained Release
Controlling drug particle size and molecular weight helps ensure steady, long-acting diffusion in the bloodstream rather than sudden, potentially toxic increases in drug concentration.
3. Cell Membrane Permeability
Small, lipophilic drug molecules diffuse rapidly across lipophilic cell membranes to reach targets inside cells.
3. Evaluate the environmental and practical implications of using sublimation in various industries, such as air fresheners and 3D printing, considering both benefits and potential drawbacks.
Answer:
Applications and Benefits
1. Solid Air Fresheners
Solid air fresheners provide continuous, long-term fragrance release without liquid leakage or the need for solvent containers.
2. 3D Printing and Dye-Sublimation Printing
Dye-sublimation printing produces high-resolution, durable colour prints on fabrics and plastics. It works by vaporising pigments directly into polymer fibres without liquid ink smudging.
3. Pharmaceutical Freeze-Drying (Lyophilization)
Freeze-drying preserves heat-sensitive drugs and vaccines by sublimating ice under vacuum.
Environmental and Practical Drawbacks
1. Volatile Organic Compounds (VOCs)
Air freshener chemicals, such as p-dichlorobenzene and synthetic fragrances, can contribute to indoor air pollution, respiratory irritation, and bioaccumulation risks.
2. Energy Intensity
Industrial sublimation processes, such as freeze-drying and dye-sublimation heating, require high energy input for vacuum generation and precise temperature control.
Notes by Sir Dilawar
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