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๐ฌ Think about it
Drop a colour into water and it slowly spreads on its own. Light an incense stick and soon the whole room smells of it. Particles are always moving, even when you can't see them. Let's catch them in action.
How do we know liquid particles are always moving?
- A grain of potassium permanganate sends pink streaks into water.
- In time the whole water turns a uniform pink.
- Water particles are in constant motion.
- They pull out and spread the colour particles around.
Example:
A drop of ink slowly colours a whole glass of water without any stirring.
In this Activity, we will drop potassium permanganate into water to see that particles move on their own.
Safety first
Do not touch potassium permanganate with your hands. Use a spoon or a spatula to handle it.
Materials needed
A glass tumbler, water, a few grains of potassium permanganate, a spoon or spatula.
Procedure
1. Take a glass tumbler containing water.
2. Put a few grains of potassium permanganate into it.
3. Watch the streaks of pink colour spreading from the grain.
4. Wait and observe over time without stirring.
2. Put a few grains of potassium permanganate into it.
3. Watch the streaks of pink colour spreading from the grain.
4. Wait and observe over time without stirring.
Observation
First, pink streaks spread out from the grain. With time, the whole water becomes a uniform pink.
Explanation
The water particles are in constant motion. They pull the colour particles out of the grain and hit them, spreading them throughout the liquid. Strongly held substances like sand cannot be pulled apart, so they are insoluble.
โ Summary
- Colour grain added
- Pink streaks spread
- Whole water pink
- Particles keep moving
๐ฌ Think like a scientist
- Take three tumblers: hot water, room-temperature water, ice-cold water.
- Drop a grain of potassium permanganate into each.
- The colour spreads fastest in hot water, slowest in cold water.
- So the movement of particles increases when heat is provided.
How can we show that gas particles move?
- Light an incense stick in one corner of a room.
- Soon you smell the fragrance everywhere.
- The fragrance particles spread to fill the room.
- This shows air particles are constantly moving.
Example:
Perfume sprayed in one corner is soon smelled across the whole room.
In this Activity, we will light an incense stick to show that gas particles spread and move across a room.
Materials needed
An incense stick, a room.
Procedure
1. Light an incense stick in one corner of the room.
2. Wait for a few minutes.
3. Notice whether you can smell the fragrance from a distance.
2. Wait for a few minutes.
3. Notice whether you can smell the fragrance from a distance.
Observation
At first the fragrance is felt only near the stick; shortly, it is smelled throughout the room.
Explanation
The fragrance particles spread and fill the whole room. This shows that air particles are constantly moving. The moving air particles hit the fragrance particles and help them spread everywhere.
โ Summary
- Incense lit
- Smell spreads out
- Reaches far away
- Gas particles move
๐ก Ever heard of ...
- The particulate nature of matter helps in everyday processes.
- When we wash oily clothes, soap particles surround the oil.
- One end of a soap particle attaches to the oil.
- The other end mixes with water and lifts the oil away.
What finally decides the state of matter?
- Thermal (heat) energy decides the interparticle distance.
- Low energy: particles stay close — a solid.
- More energy: particles move around — a liquid.
- High energy: particles move freely — a gas.
Thermal Energy Decides the State
Low thermal energy
Particles close; only vibrate — solid.
↓
More thermal energy
Particles move within limited space — liquid.
↓
High thermal energy: free particles — gas
Important Points
- Particles in liquids and gases are in constant motion.
- Heating makes particles move faster.
- Thermal energy decides whether matter is solid, liquid, or gas.
Important Definitions
- Thermal energy — the heat energy of particles that decides their motion and the state of matter.
โก A step further
- The tiny particles that make up all matter are atoms and molecules.
- For example, iron is made of iron atoms; gold is made of gold atoms.
- Atoms of some elements (hydrogen, oxygen, sulfur) cannot exist independently.
- In such cases, atoms of the same element combine to form a molecule.
- Two hydrogen atoms combine to form one molecule of hydrogen.
- A water molecule is made of two hydrogen atoms and one oxygen atom.
- You will learn more about atoms and molecules in higher grades.
โ Test Yourself
-
Why does a colour spread in still water?
View Answer
The water particles are in constant motion. They pull out and carry the colour particles until the whole water is coloured. -
In which water does potassium permanganate spread fastest?
View Answer
In hot water, because the particles move faster when heat is added. It spreads slowest in ice-cold water. -
How does the smell of incense reach across a room?
View Answer
Moving air particles hit the fragrance particles and help them spread until they fill the whole room. -
What decides the state of matter?
View Answer
The thermal (heat) energy of the particles, which sets how far apart they are and how freely they move. -
How does heat change particle movement?
View Answer
Adding heat makes particles move faster, so substances change from solid to liquid to gas.
๐ NCERT Question 6 — Represent diagrammatically the changes
Represent diagrammatically how particle arrangement changes as ice melts and turns into water vapour.
View Answer →
