Extra 20 long questions and answers from the chapter:1 "Matter in Our Surroundings" for Class 9 CBSE Science:

 

 

1. Explain the characteristics of the particles of matter.
Answer: The characteristics of particles of matter are:

Made of tiny particles

– Matter is composed of small particles.

Have space between them

– Particles have intermolecular spaces, which are more in gases, less in liquids, and least in solids.

Always in motion

– Particles possess kinetic energy and move continuously.

Attract each other

– Intermolecular forces hold the particles together, strongest in solids and weakest in gases.

Diffusion

– Particles can mix with other particles by diffusion.

Can change state

– By heating or cooling, matter can change from one state to another.

 

2. Differentiate between solids, liquids, and gases based on their properties.
Answer:

Property	Solids	Liquids	Gases
Shape	Fixed	No fixed shape, takes container's shape	No fixed shape, fills the container
Volume	Fixed	Fixed	No fixed volume
Compressibility	Negligible	Low	Highly compressible
Intermolecular Force	Strongest	Medium	Weakest
Intermolecular Space	Least	More than solids	Largest
Diffusion	Very slow	Faster than solids	Fastest

 

3. Explain the process of evaporation and the factors affecting it.
Answer:
Evaporation is the process by which a liquid changes into vapor at a temperature below its boiling point.
Factors affecting evaporation:

Temperature

– Higher temperature increases the rate of evaporation.

Surface area

– More surface area leads to faster evaporation.

Humidity

– More humidity reduces evaporation.

Wind speed

– Higher wind speed increases evaporation.

Nature of liquid

– Volatile liquids like alcohol evaporate faster.

Pressure

– Lower atmospheric pressure increases evaporation.

 

4. What is diffusion? Explain diffusion in solids, liquids, and gases with examples.
Answer:
Diffusion is the movement of particles from a high concentration to a low concentration area.

Diffusion in gases

– Fastest due to large intermolecular spaces. Example: Perfume spreading in a room.

Diffusion in liquids

– Slower than gases. Example: Ink mixing in water.

Diffusion in solids

– Very slow due to tightly packed particles. Example: Gold and silver atoms intermixing in

jewellery

.

 

5. Why does evaporation cause cooling? Give examples.
Answer:
Evaporation causes cooling because:

Liquid absorbs heat energy from surroundings.

The absorbed heat breaks intermolecular bonds.

Particles with higher energy escape, leaving the remaining liquid cooler.

The loss of heat energy lowers the temperature.

 

Examples:

Sweating

cools our body.

Earthen pots

keep water cool.

Rubbing alcohol

on the skin feels cold.

 

6. Define boiling and explain how it differs from evaporation.
Answer:
Boiling is the rapid change of liquid into vapor at a fixed temperature (boiling point).

Feature	Boiling	Evaporation
Speed	Fast	Slow
Temperature	At boiling point	Any temperature
Energy	Requires heat supply	Takes heat from surroundings
Surface	Occurs throughout the liquid	Only at the surface
Example	Boiling water at 100°C	Clothes drying in the sun

 

7. What is sublimation? Give examples and explain why some substances sublime.
Answer:
Sublimation is the process where a solid directly converts into gas without becoming liquid.
Examples: Camphor, Naphthalene, Dry ice.
Reasons:

Weak intermolecular forces.

High vapor pressure of solid.

Direct conversion of solid particles to vapor.

Heat energy overcomes molecular bonds.

Low boiling point.

 

8. Explain the role of kinetic energy in the change of state of matter.
Answer:

Solids to Liquids (Melting)

– Heating increases kinetic energy, particles vibrate faster, and break bonds.

Liquids to Gases (Boiling)

– High kinetic energy overcomes intermolecular forces, turning liquid into gas.

Gases to Liquids (Condensation)

– Cooling reduces kinetic energy, particles slow down, and form a liquid.

Liquids to Solids (Freezing)

– Loss of kinetic energy makes particles move slower and form a solid.

Sublimation

– High kinetic energy allows some solids to directly change into gases.

 

9. Why does the boiling point of a liquid increase with pressure?
Answer:

Higher pressure makes it harder for particles to escape as vapor.

More energy is required to break bonds and change state.

Water boils at

100°C at 1

atm

but at

120°C in a pressure cooker

.

Boiling point increases in deep sea and high-pressure environments.

This principle is used in industrial distillation.

 

10. Explain why gases can be compressed easily but solids cannot.
Answer:

Gases have large intermolecular spaces.

Weak intermolecular forces allow compression.

Solids have strong molecular bonds and little space.

Applying pressure on solids does not reduce volume.

Examples: Air can be compressed into cylinders, but wood cannot.

 

11. Explain the factors affecting the boiling point of a liquid.

Answer: The boiling point of a liquid depends on the following factors:

Atmospheric Pressure:

Higher pressure increases the boiling point, while lower pressure decreases it (e.g., water boils at a lower temperature on mountains).

Intermolecular Forces:

Stronger forces (e.g., hydrogen bonds in water) require more energy to break, increasing the boiling point.

Purity of Liquid:

Impurities raise the boiling point (e.g., salt water boils at a higher temperature than pure water).

Nature of Liquid:

Different liquids have different boiling points due to molecular structure (e.g., ethanol boils at 78°C, water at 100°C).

Altitude:

At higher altitudes, air pressure is lower, reducing the boiling point.

External Pressure Applied:

Increasing pressure (e.g., in a pressure cooker) raises the boiling point.

Surface Area:

Larger surface area allows faster heat absorption, affecting the boiling rate but not the boiling point.

 

12. Explain why solids are rigid and incompressible.

Answer: Solids have the following properties that make them rigid and incompressible:

Strong Intermolecular Forces:

The attractive forces between solid particles are very strong, keeping them tightly bound.

Definite Shape and Volume:

Particles in solids are arranged in a fixed pattern, giving them a definite shape and volume.

Very Small Intermolecular Spaces:

The particles are closely packed, leaving little room for movement or compression.

Fixed Position of Particles:

The particles in a solid only vibrate about their fixed positions but do not move freely.

High Density:

Due to the close packing of particles, solids have a higher density compared to liquids and gases.

Low Compressibility:

Since there is very little space between the particles, solids cannot be compressed easily.

High Resistance to Deformation:

Solids require an external force to change their shape, unlike liquids and gases.

 

13. Differentiate between evaporation and condensation.

Answer:

Feature	Evaporation	Condensation
Definition	The process of liquid changing into gas at a temperature below boiling point.	The process of gas changing into liquid upon cooling.
Temperature	Can occur at any temperature.	Occurs when the gas cools below its condensation temperature.
Energy Requirement	Absorbs heat from surroundings.	Releases heat into surroundings.
Effect	Causes cooling.	Releases heat and warms the surroundings.
Example	Sweat evaporating from skin.	Water droplets forming on a cold glass.
Rate depends on	Temperature, humidity, wind speed, surface area.	Cooling rate, air pressure, humidity.
Applications	Drying clothes, cooling effects of earthen pots.	Formation of clouds, dew formation.

 

14. Why do gases have neither a fixed shape nor a fixed volume?

Answer:

Weak Intermolecular Forces:

The forces between gas particles are very weak, allowing them to move freely.

Large Intermolecular Spaces:

The particles are far apart, making gases highly compressible.

No Fixed Shape:

Gas particles move randomly in all directions, filling the shape of their container.

No Fixed Volume:

Gases expand to occupy any available space because the particles are in continuous motion.

Diffusion:

Gas particles spread out quickly and mix with other gases due to random motion.

Compressibility:

Since gas particles are far apart, they can be easily compressed into a smaller volume.

Expansion:

Gases expand when heated as particles gain kinetic energy and move faster.

 

15. Why do clothes dry faster on a windy day than on a calm day?

Answer:

Increased Wind Speed:

Wind carries away water vapor, allowing fresh dry air to replace it, speeding up evaporation.

Reduction in Humidity:

Dry air absorbs more water vapor, enhancing the drying process.

Higher Rate of Evaporation:

Faster airflow increases evaporation by removing vapor from the surface.

Increased Kinetic Energy:

Moving air molecules transfer energy to water molecules, making them evaporate quickly.

Greater Surface Exposure:

The movement of clothes due to wind spreads water molecules, increasing the evaporation rate.

Lower Atmospheric Moisture Near Clothes:

Since wind moves moisture away, saturation does not occur, ensuring continuous drying.

Practical Use:

This principle is used in drying machines where hot air is blown over wet clothes.

 

16. Explain why different substances have different melting and boiling points.

Answer:

Nature of Particles:

Different substances have different molecular structures affecting intermolecular forces.

Intermolecular Forces:

Stronger forces require more energy to break, increasing melting and boiling points.

Molecular Mass:

Heavier molecules require more energy to change states, increasing their melting and boiling points.

Purity of Substance:

Impurities raise the boiling point and lower the melting point of a substance.

External Pressure:

Increasing pressure increases boiling points, as seen in pressure cookers.

Type of Bonding:

Ionic compounds have higher melting and boiling points than covalent compounds.

Example:

Water has a boiling point of 100°C, but ethanol boils at 78°C due to weaker intermolecular forces.

 

17. Explain why we feel comfortable wearing cotton clothes in summer.

Answer:

Cotton Absorbs Sweat:

Cotton is highly absorbent and helps in soaking up sweat from the skin.

Facilitates Evaporation:

The sweat absorbed by cotton evaporates, cooling the body.

Allows Air Circulation:

Cotton is a breathable fabric, allowing air to pass and keeping the body cool.

Lightweight and Loose:

Loose cotton clothes help air circulate freely, preventing heat

build up

.

Non-Sticky Fabric:

Unlike synthetic fabrics, cotton does not stick to the skin when sweating.

Reflects Heat:

Light-

coloured

cotton clothes reflect sunlight, keeping the body cooler.

Comfortable to Wear:

Cotton prevents skin irritation and allergies caused by synthetic clothes in hot weather.

 

18. Explain why pressure cookers are used in high-altitude areas for cooking.

Answer:

Low Atmospheric Pressure:

At high altitudes, air pressure is lower, reducing the boiling point of water.

Food Takes Longer to Cook:

Lower boiling points mean water boils at a lower temperature, slowing the cooking process.

Pressure Cookers Increase Pressure:

The sealed environment increases pressure inside, raising the boiling point.

Faster Cooking:

Higher boiling temperature ensures food cooks faster.

Prevents Water Loss:

Cooking in open vessels at high altitudes leads to excessive water evaporation.

Energy Efficient:

Pressure cooking reduces cooking time and saves fuel.

Common Use:

This method is widely used in mountain regions where cooking in open vessels is ineffective.

 

19. Why does a metal plate feel colder than a wooden table in winter, even if both are at the same temperature?

Answer:

Metal is a Good Conductor:

Metals transfer heat quickly from our body to the surroundings.

Wood is a Poor Conductor:

Wood does not absorb heat from our hand as quickly as metal.

Heat Loss from the Body:

When we touch metal, heat from our hand moves into it rapidly, making us feel cold.

Same Temperature, Different Conductivity:

Both objects are at room temperature, but metal feels colder due to better conduction.

Thermal Conductivity of Materials:

Conductors (metals) absorb and transfer heat faster than insulators (wood).

Real-Life Application:

Cooking utensils are made of metal for efficient heat transfer.

Similar Example:

Tiles feel colder than carpets in winter due to the same reason.

 

20. Why do we see steam rising from a hot cup of tea?

Answer:

Evaporation of Hot Water:

Some liquid particles gain energy and change into vapor.

Condensation of Water Vapor:

The vapor near the surface cools and forms tiny water droplets.

Difference in Temperature:

When warm vapor meets cooler air, it condenses.

Formation of Foggy Appearance:

The condensed droplets scatter light, making the steam visible.

Fast Evaporation at High Temperature:

Hot tea loses heat quickly, forming visible steam.

Wind Speed Effect:

On windy days, steam disperses faster due to increased evaporation.

Daily Observation:

Similar to steam from boiling water or geysers in winter.