Royal Grammar School
Class 10: Chemistry
Chapter 14: States of Matter and Phase Changes
EXERCISE - COMPLETE SOLUTION
A) Multiple Choice Questions
Q1. According to kinetic theory, basic difference between solid, liquid and gas is due
to:
Options:
(a) movements of particles
(b) chemical properties
(c) size
(d) shape
Answer: (a) the difference in the movements of the particles
Q2. Upon heating the rate of evaporation:
Options:
(a) decreases
(b) increases
(c) remains same
(d) initially decreases then increases
Answer: (b) increases
Q3. Inter-particle attractions are strongest in:
Options:
(a) Solids
(b) Liquids
(c) Plasma
(d) Gas
Answer: (a) Solids
Q4. Cooling vapours of gases change them directly into solid state. This phenomenon
is called:
Options:
(a) evaporation
(b) condensation
(c) sublimation
(d) deposition
Answer: (d) deposition
Q5. Physical state in which particles possess maximum energy:
Options:
(a) Solid
,(b) Liquid
(c) Gaseous
(d) Vapour
Answer: (d) Vapour
Q6. How does evaporation depend on force of attraction?
Options:
(a) decreases with strength
(b) increases with strength
(c) independent
(d) first increases then decreases
Answer: (a) It decreases with the increasing strength of attraction
Q7. Which gas will diffuse at the fastest rate?
Options:
(a) HCl
(b) SO₂
(c) H₂S
(d) CO₂
Answer: (a) HCl - Lighter gas diffuses faster by Graham's Law
Q8. Phase changes A → liquid → B → solid are:
Options:
(a) Melting, evaporating
(b) Condensation, melting
(c) Condensation, freezing
(d) Boiling, freezing
Answer: (d) Boiling, freezing
Q9. Frost disappearing into water vapours:
Options:
(a) Melting
(b) Evaporation
(c) Sublimation
(d) Deposition
Answer: (c) Sublimation
B) Short Answer Questions
Q14.1 From where energy for evaporation comes?
Answer:
Energy comes from surrounding molecules and container walls. Fast-moving molecules take
heat from slower ones and escape into air. This makes the remaining liquid cooler.
Q14.2 Is condensation endothermic?
Answer:
No, condensation is an exothermic process. When gas changes to liquid, heat energy is
released to surroundings. Example: Mirror fogs up in bathroom due to released heat.
,Q14.3 Why naphthalene balls disappear?
Answer:
Naphthalene undergoes sublimation. It changes directly from solid to vapour without
becoming liquid. Vapours spread in air so solid vanishes slowly with time.
Q14.4 Why is the temperature constant during phase change?
Answer:
During phase change heat is used to break intermolecular bonds, not to increase
temperature. This heat is called latent heat. Temperature stays constant until the whole
substance changes state.
Q14.5 Can liquid be compressed like gas?
Answer:
No, liquids cannot be compressed like gases. Liquid particles are already very close with no
empty space between them. Gases have large gaps so they compress easily under
pressure.
Q14.6 Can you change the boiling temperature of water?
Answer:
Yes, boiling point depends on external pressure. At low pressure water boils below 100°C. At
high pressure it boils above 100°C. Pressure cooker uses this principle.
Q14.7 In which season wet clothes dry late?
Answer:
Wet clothes dry slowly in winter and rainy season. Cold temperature and high humidity slow
down evaporation. Hot and dry air in summer increases evaporation rate.
Q14.8 What happens to vibrating particles when solid heated?
Answer:
On heating, particles gain kinetic energy. Their vibration about fixed positions increases. If
heating continues, vibration becomes strong enough to break bonds and solid melts.
Q14.9 Do solids and liquids diffuse like gases?
Answer:
Yes, solids and liquids also diffuse but very slowly. Diffusion rate is fastest in gases, slower in
liquids, and slowest in solids. Example: Sugar dissolves slowly in cold water.
Q14.10 Why do dew forms in the early morning?
Answer:
At night the temperature drops below dew point. Water vapour in air condenses on cool
surfaces like grass and leaves. This condensed water appears as dew in the early morning.
C) Constructed Response Questions:
Q14.1 Differentiate between evaporation and boiling
Answer:
, Evaporation occurs only at the surface of liquid at all temperatures. It is a slow process and
causes a cooling effect. Boiling occurs at whole liquid at specific boiling point with bubble
formation. Boiling is a fast process. Evaporation increases with surface area while boiling
point is fixed at given pressure. Evaporation needs no external heat but boiling needs
continuous heat supply. Example: Sweat dries by evaporation, water boils at 100°C.
Q14.2 Describe how boiling relates to external pressure
Answer:
The boiling point of liquid depends directly on external pressure. At low atmospheric
pressure, boiling point decreases. That is why water boils below 100°C on mountains. At
high pressure, boiling point increases. In a pressure cooker, high pressure makes water boil
above 100°C and food cooks faster. At sea level and 1 atm pressure, pure water boils
exactly at 100°C. Pressure and boiling point are directly related.
Q14.3 Why evaporation needs no input of energy?
Answer:
Evaporation occurs due to high kinetic energy molecules at liquid surfaces. Fast molecules
escape into air without external heating. They take energy from nearby slow molecules.
Remaining molecules have lower average kinetic energy so liquid cools down. This
phenomenon is called evaporative cooling. That is why we feel cool after sweating and when
we come out of the swimming pool.
Q14.4 Why heating and cooling curves adopt the same path?
Answer: Heating and cooling curves follow the same path because phase changes are
reversible processes. Melting and freezing occur at same temperature. Boiling and
condensation also occur at same temperature. Heat absorbed during melting is released
during freezing. Flat portions of curve show phase change where temperature remains
constant. Only direction of heat flow changes.
Q14.5 Why does ice melt when pressure is exerted on it?
Answer:
Applying pressure lowers the melting point of ice. This property is called pressure melting or
regulation. Skates exert high pressure on ice which melts thin layers into water. Water acts
as lubricant for smooth skating. When pressure is removed, water refreezes back to ice. This
property is unique to water due to its molecular structure and hydrogen bonding.
Q14.6 Why do phase changes occur?
Answer:
Phase changes occur due to change in kinetic energy and intermolecular forces between
particles. On heating, kinetic energy increases and particles overcome attractive forces.
Solid changes to liquid then gas. On cooling, kinetic energy decreases and particles come
closer. Gas changes to liquid then solid. Pressure also affects phase changes by changing
boiling and melting points of substances.
D) Descriptive Questions -
Q14.1 Explain diffusion rate difference based on kinetic theory
Answer:
Class 10: Chemistry
Chapter 14: States of Matter and Phase Changes
EXERCISE - COMPLETE SOLUTION
A) Multiple Choice Questions
Q1. According to kinetic theory, basic difference between solid, liquid and gas is due
to:
Options:
(a) movements of particles
(b) chemical properties
(c) size
(d) shape
Answer: (a) the difference in the movements of the particles
Q2. Upon heating the rate of evaporation:
Options:
(a) decreases
(b) increases
(c) remains same
(d) initially decreases then increases
Answer: (b) increases
Q3. Inter-particle attractions are strongest in:
Options:
(a) Solids
(b) Liquids
(c) Plasma
(d) Gas
Answer: (a) Solids
Q4. Cooling vapours of gases change them directly into solid state. This phenomenon
is called:
Options:
(a) evaporation
(b) condensation
(c) sublimation
(d) deposition
Answer: (d) deposition
Q5. Physical state in which particles possess maximum energy:
Options:
(a) Solid
,(b) Liquid
(c) Gaseous
(d) Vapour
Answer: (d) Vapour
Q6. How does evaporation depend on force of attraction?
Options:
(a) decreases with strength
(b) increases with strength
(c) independent
(d) first increases then decreases
Answer: (a) It decreases with the increasing strength of attraction
Q7. Which gas will diffuse at the fastest rate?
Options:
(a) HCl
(b) SO₂
(c) H₂S
(d) CO₂
Answer: (a) HCl - Lighter gas diffuses faster by Graham's Law
Q8. Phase changes A → liquid → B → solid are:
Options:
(a) Melting, evaporating
(b) Condensation, melting
(c) Condensation, freezing
(d) Boiling, freezing
Answer: (d) Boiling, freezing
Q9. Frost disappearing into water vapours:
Options:
(a) Melting
(b) Evaporation
(c) Sublimation
(d) Deposition
Answer: (c) Sublimation
B) Short Answer Questions
Q14.1 From where energy for evaporation comes?
Answer:
Energy comes from surrounding molecules and container walls. Fast-moving molecules take
heat from slower ones and escape into air. This makes the remaining liquid cooler.
Q14.2 Is condensation endothermic?
Answer:
No, condensation is an exothermic process. When gas changes to liquid, heat energy is
released to surroundings. Example: Mirror fogs up in bathroom due to released heat.
,Q14.3 Why naphthalene balls disappear?
Answer:
Naphthalene undergoes sublimation. It changes directly from solid to vapour without
becoming liquid. Vapours spread in air so solid vanishes slowly with time.
Q14.4 Why is the temperature constant during phase change?
Answer:
During phase change heat is used to break intermolecular bonds, not to increase
temperature. This heat is called latent heat. Temperature stays constant until the whole
substance changes state.
Q14.5 Can liquid be compressed like gas?
Answer:
No, liquids cannot be compressed like gases. Liquid particles are already very close with no
empty space between them. Gases have large gaps so they compress easily under
pressure.
Q14.6 Can you change the boiling temperature of water?
Answer:
Yes, boiling point depends on external pressure. At low pressure water boils below 100°C. At
high pressure it boils above 100°C. Pressure cooker uses this principle.
Q14.7 In which season wet clothes dry late?
Answer:
Wet clothes dry slowly in winter and rainy season. Cold temperature and high humidity slow
down evaporation. Hot and dry air in summer increases evaporation rate.
Q14.8 What happens to vibrating particles when solid heated?
Answer:
On heating, particles gain kinetic energy. Their vibration about fixed positions increases. If
heating continues, vibration becomes strong enough to break bonds and solid melts.
Q14.9 Do solids and liquids diffuse like gases?
Answer:
Yes, solids and liquids also diffuse but very slowly. Diffusion rate is fastest in gases, slower in
liquids, and slowest in solids. Example: Sugar dissolves slowly in cold water.
Q14.10 Why do dew forms in the early morning?
Answer:
At night the temperature drops below dew point. Water vapour in air condenses on cool
surfaces like grass and leaves. This condensed water appears as dew in the early morning.
C) Constructed Response Questions:
Q14.1 Differentiate between evaporation and boiling
Answer:
, Evaporation occurs only at the surface of liquid at all temperatures. It is a slow process and
causes a cooling effect. Boiling occurs at whole liquid at specific boiling point with bubble
formation. Boiling is a fast process. Evaporation increases with surface area while boiling
point is fixed at given pressure. Evaporation needs no external heat but boiling needs
continuous heat supply. Example: Sweat dries by evaporation, water boils at 100°C.
Q14.2 Describe how boiling relates to external pressure
Answer:
The boiling point of liquid depends directly on external pressure. At low atmospheric
pressure, boiling point decreases. That is why water boils below 100°C on mountains. At
high pressure, boiling point increases. In a pressure cooker, high pressure makes water boil
above 100°C and food cooks faster. At sea level and 1 atm pressure, pure water boils
exactly at 100°C. Pressure and boiling point are directly related.
Q14.3 Why evaporation needs no input of energy?
Answer:
Evaporation occurs due to high kinetic energy molecules at liquid surfaces. Fast molecules
escape into air without external heating. They take energy from nearby slow molecules.
Remaining molecules have lower average kinetic energy so liquid cools down. This
phenomenon is called evaporative cooling. That is why we feel cool after sweating and when
we come out of the swimming pool.
Q14.4 Why heating and cooling curves adopt the same path?
Answer: Heating and cooling curves follow the same path because phase changes are
reversible processes. Melting and freezing occur at same temperature. Boiling and
condensation also occur at same temperature. Heat absorbed during melting is released
during freezing. Flat portions of curve show phase change where temperature remains
constant. Only direction of heat flow changes.
Q14.5 Why does ice melt when pressure is exerted on it?
Answer:
Applying pressure lowers the melting point of ice. This property is called pressure melting or
regulation. Skates exert high pressure on ice which melts thin layers into water. Water acts
as lubricant for smooth skating. When pressure is removed, water refreezes back to ice. This
property is unique to water due to its molecular structure and hydrogen bonding.
Q14.6 Why do phase changes occur?
Answer:
Phase changes occur due to change in kinetic energy and intermolecular forces between
particles. On heating, kinetic energy increases and particles overcome attractive forces.
Solid changes to liquid then gas. On cooling, kinetic energy decreases and particles come
closer. Gas changes to liquid then solid. Pressure also affects phase changes by changing
boiling and melting points of substances.
D) Descriptive Questions -
Q14.1 Explain diffusion rate difference based on kinetic theory
Answer:
