MATTER IN OUR SURROUNDINGS

 

CHARACTERISTICS OF PARTICLES OF MATTER.

• Extremely Small Size: Particles of matter are inconceivably small, far beyond the visibility of the naked eye.
• Spaces Between Particles: There are voids or intermolecular spaces between particles, which allows substances to mix (e.g., dissolving sugar in water). This space is minimal in solids, higher in liquids, and maximum in gases.
• Constant Motion: Particles possess kinetic energy and are always in motion. In solids, they vibrate in place, while in liquids and gases, they move randomly. Increased temperature leads to faster particle movement.
• Attractive Forces: Particles of matter attract each other with varying force (intermolecular force), which dictates the state of matter—strongest in solids, weaker in liquids, and weakest in gases.
• Possess Mass: Each particle has mass, contributing to the overall mass and volume of the substance.

2. Give reasons for the following observation: 
The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
Ans-The hot food particles have more kinetic energy  so it travel faster and more distance .Que-A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Ans- water molecules have weak intrrmolecular force of attraction between them so they can be easily cut thats why  diver can cut the water easily.
Give reasons 
(a) A gas fills completely the vessel in which it is kept..

Ans-Gas molecules have higher intermolecular force and lower intermolecular force of attraction between them,thats why gas fill the vessel completely.

b)A gas exerts pressure on the walls of the container.

Ans-due to the continuous, rapid, and random motion of gas molecules, which frequently collide with the walls.so gas exert pressure on the wall.

🔹 Change of State of Matter

1️⃣ Melting

• Solid → Liquid

• Example: Ice → Water

• Melting point of ice = 0°C

2️⃣ Freezing

• Liquid → Solid

• Example: Water → Ice

3️⃣ Evaporation

• Liquid → Gas (below boiling point)

• Happens at all temperatures

4️⃣ Condensation

• Gas → Liquid

• Example: Water droplets on cold glass

5️⃣ Boiling

• Liquid → Gas at fixed temperature

Boiling point of water = 100°C

Factors effecting rate of evaporation-

 Factors Affecting Rate of Evaporation

1️⃣ Surface Area

• Larger surface area → faster evaporation

• Example: Clothes spread out dry faster than folded clothes.

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2️⃣ Temperature

• Higher temperature → faster evaporation

• Example: Clothes dry faster in summer.

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3️⃣ Wind Speed

• Higher wind speed → faster evaporation

• Wind carries away water vapour.

• Example: Clothes dry faster on a windy day.

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4️⃣ Humidity

• Lower humidity → faster evaporation

• If air already has more water vapour, evaporation slows.

Example: Clothes dry slowly on a humid (rainy) day.

 Melting Point (Class 9)

✅ Definition

Melting point is the temperature at which a solid changes into a liquid.

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📌 Important Example

• Melting point of ice = 0°C (273 K)

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🧠 Key Points to Remember

• At melting point, solid and liquid exist together.

• During melting, temperature remains constant.

• The heat supplied is used as latent heat of fusion.

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✍️ Why temperature remains constant at melting point?

Answer:
Because the heat supplied is used to change the state of matter, not to increase temperature.

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⭐ Importance of Melting Point

• Helps to identify a substance

• Indicates purity of a solid

• Solids with high melting point have strong force of attraction.

9️⃣ Why does a desert cooler cool better on a hot dry day?

Answer:
On a hot dry day, the air has low humidity, so evaporation of water is faster. Faster evaporation absorbs more heat from the surroundings, causing greater cooling. Hence, a desert cooler works better on a hot dry day.

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🔟 How does evaporation cause cooling?

Answer:
During evaporation, liquid particles take heat from surroundings, causing cooling.

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1️⃣1️⃣ Why should we wear cotton clothes in summer?

Answer:
Cotton absorbs sweat and allows it to evaporate easily, giving cooling effect.

1️⃣ Which of the following are matter?

Answer:
Matter: Chair, air, almonds, cold drink, smell of perfume
Not matter: Love, hate, thought, cold

Reason: Matter has mass and occupies space.

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2️⃣ Give reasons for the following:

a) Smell of hot food reaches far away
Because particles move continuously and show diffusion. At high temperature diffusion is faster.

b) A diver cuts through water
Because force of attraction between liquid particles is weak.

c) Water flows but wood does not
Water is a liquid and can flow. Wood is a solid and has fixed shape.

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3️⃣ Characteristics of particles of matter

Answer:

1. Particles are very small

2. There is space between particles

3. Particles are continuously moving

4. Particles attract each other

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4️⃣ Physical state of water

a) 25°C → Liquid
b) 0°C → Solid and liquid
c) 100°C → Liquid and gas

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5️⃣ Increasing order of force of attraction

a) Oxygen < Water < Sugar
b) Helium < Air < Oxygen

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6️⃣ Give reasons:

a) Water at room temperature is liquid

• Has fixed volume

• Has no fixed shape

• Can flow

b) Iron almirah is solid at room temperature

• Has fixed shape and volume

• Strong force of attraction

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7️⃣ Why is ice at 0°C more effective in cooling than water at 0°C?

Answer:
Ice absorbs latent heat of fusion while melting, so it removes more heat from surroundings.

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8️⃣ What is evaporation? Explain factors.

Answer:
Evaporation is the change of liquid into gas at any temperature below boiling point.

Factors affecting evaporation:

1. Surface area

2. Temperature

3. Wind speed

4. Humidity

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9️⃣ How does evaporation cause cooling?

Answer:
During evaporation, particles take heat from surroundings, causing cooling.

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🔟 Why should we wear cotton clothes in summer?

Answer:
Cotton absorbs sweat and allows fast evaporation, which cools the body.

🔥 Latent Heat of Vapourisation

✅ Definition

Latent heat of vapourisation is the amount of heat required to change a liquid into gas at its boiling point without any change in temperature.

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📌 Example

• When water at 100°C changes into steam at 100°C, it absorbs latent heat of vapourisation.

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🧠 Key Points (Very Important)

• Temperature remains constant during the change of state.

• Heat is used to overcome the force of attraction between particles.

• Gases have more energy than liquids.

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✍️ Why is latent heat of vapourisation needed?

Answer:
Because extra heat is required to completely separate liquid particles and change them into gas.

❄️ Latent Heat of Fusion

✅ Definition

Latent heat of fusion is the amount of heat required to change a solid into a liquid at its melting point without any change in temperature.

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📌 Example

• When ice at 0°C changes into water at 0°C, it absorbs latent heat of fusion.

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🧠 Key Points (Important)

• Temperature remains constant during melting.

• Heat is used to break the force of attraction between solid particles.

• Both solid and liquid exist together at melting point.

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✍️ Why is latent heat of fusion needed?

Answer:
Because extra heat is required to overcome the strong attraction between particles of a solid.