Characteristics of Particles of Matter – Class 9 Science | Detailed Notes, Q&A, Experiments, MCQs, Case Study

🔬 Chapter: Matter in Our Surroundings

🎯 Topic: Characteristics of Particles of Matter

🟦 Q1: What are the main characteristics of particles of matter?

Ans:
The particles of matter—such as atoms or molecules—exhibit the following four fundamental characteristics:

1. Particles of matter are extremely small (beyond microscopic level).
2. Particles of matter have spaces between them (interparticle spaces).
3. Particles of matter are in constant motion (they never stay still).
4. Particles of matter attract each other (force of attraction/cohesion exists).

Each of these characteristics can be proved through experiments, daily life observations, and scientific reasoning. Let’s now explain each one in detail.

🟦 Q2: How do we know that particles of matter are extremely small?

Ans:
This characteristic can be understood through a simple dilution experiment using potassium permanganate (KMnO₄) and water.

🔬 Experiment: Potassium Permanganate Dilution

Materials Required:

• Crystals of potassium permanganate
• 6 beakers
• Water (distilled if possible)
• Measuring cylinder or dropper

Procedure:

1. Take 2 or 3 crystals of potassium permanganate and dissolve them in 100 mL of water in the first beaker.
2. Take 10 mL of this purple solution and transfer it into another 90 mL of water in the second beaker. Stir well.
3. Again, take 10 mL of the solution from the second beaker and transfer it into a third beaker containing 90 mL water.
4. Repeat this process 5 to 6 times.

Observation:

• The colour gets lighter and lighter, but even after 6 dilutions, a light pink color remains.

Conclusion:

• Even a few crystals can color litres of water!
• This proves that each crystal is made of millions of particles, which keep dividing and spreading.
• Therefore, particles of matter are extremely small in size, beyond our normal visual capacity.

🟦 Q3: What experiment shows that particles of matter have spaces between them?

Ans:
This can be proved through the sugar-water solution experiment.

🔬 Experiment: Dissolving Sugar in Water

Materials Required:

• 100 mL water
• 50 grams sugar
• A transparent beaker
• Stirring rod
• Marker

Procedure:

1. Pour 100 mL water in a beaker. Mark the level using a marker.
2. Add 50 grams sugar slowly and stir it with a rod till it dissolves completely.
3. Observe the water level after the sugar is dissolved.

Observation:

• The water level remains the same, even after dissolving sugar.

Explanation:

• Sugar particles disappear from sight but have not evaporated.
• Instead, they occupy the empty spaces between the water molecules.

Conclusion:

• This proves the existence of interparticle spaces.
• Matter is not completely solid at molecular level—even liquids have gaps where other particles can fit.

🟦 Q4: What proves that particles of matter are continuously moving?

Ans:
This is demonstrated through the natural phenomenon called diffusion, which is the spontaneous intermixing of particles.

There are two classic experiments to show this:

🧪 (A) Diffusion of Gases – Incense Stick (Agarbatti)

Observation:

• When you light an incense stick in one corner of a room, its fragrance spreads all over the room in a few minutes.

Scientific Explanation:

• The burning agarbatti releases aromatic gases.
• These gas particles move in all directions due to kinetic energy.
• They collide with air molecules, which are also moving.
• This process of random mixing is called diffusion.

Conclusion:

• The spreading of smell confirms that particles are always in motion.

🧪 (B) Diffusion in Liquids – Copper Sulphate in Water

Materials:

• Beaker
• Water
• Copper sulphate crystals (blue)

Procedure:

• Place copper sulphate crystals gently at the bottom of water in a beaker without stirring.

Observation:

• The blue color spreads slowly throughout the beaker.

Scientific Explanation:

• The copper sulphate particles dissolve and move on their own.
• The water particles are also moving.
• They intermix due to continuous random motion.

Conclusion:

• This proves that particles of both liquids and solids are in constant motion.
• Heating accelerates diffusion due to increase in kinetic energy.

🟦 Q5: How do we know that particles of matter attract each other?

Ans:
The presence of interparticle force of attraction (cohesion) is evident in the rigidity and strength of different substances.

🔨 Experiment: Breaking Different Materials

Materials:

• Chalk
• Ice cube
• Iron nail
• Hammer

Procedure:

• Try breaking all three materials using the same force.

Observation:

• Chalk breaks easily – particles have weak attraction.
• Ice needs more force – particles have moderate attraction.
• Iron nail doesn’t break – particles have very strong attraction.

Conclusion:

• This shows the different levels of cohesive force:
  • Solids (iron) → Strong attraction
  • Liquids (water) → Weak-moderate attraction
  • Gases (air) → Very weak/negligible attraction

🔁 Another Example: Moving Hand Through Different Substances

• Through Air: Very easy – particles far apart.
• Through Water: Slight resistance – some attraction.
• Through Wood/Brick: Impossible – strong cohesive force.

This shows that the force of attraction varies depending on the state of matter.

🟦 Q6: What is the difference between rigid and fluid substances?

Ans:

Note: Both liquids and gases are fluids because they have the property of flowing.

🟦 Q7: Define solid, liquid, and gas based on their properties.

Ans:

🟦 Q8: Why are liquids and gases called fluids?

Ans:

• Both liquids and gases have the ability to flow.
• They do not retain shape and take the shape of the container.
• This flow property is known as fluidity.
• Hence, they are called fluids.

🟦 Q9: Why do gases need to be stored in closed containers?

Ans:

• Gases have no fixed volume or shape.
• They expand to fill any space and can escape if not contained.
• Gases do not have a free surface, so they cannot be stored in open containers.
• Therefore, gases are stored in airtight cylinders like LPG tanks.

Q10. Why are liquids and gases together called fluids?

Ans:

• Both liquids and gases have the ability to flow.
• This property of flowing is called fluidity.
• Hence, they are collectively known as fluids.

Q11. Why must gases be kept in closed containers?

Ans:

• Gases do not have a free surface and fill the entire container.
• They escape easily if not sealed.
• Therefore, gases are stored in airtight cylinders (like LPG cylinders) to prevent leakage.

📌 Summary Table: Characteristics of Particles of Matter