1. Matter Is Made of Tiny Particles
The basic idea of the molecular nature of matter is that everything around us—solids, liquids, and gases—is made of extremely small particles called atoms or molecules. These particles are far too small to be seen directly, but all physical properties arise from their behaviour.
The large objects we see are just huge collections of these particles.
2. Continuous Motion of Particles
One of the key ideas is that molecules are always in motion. Their movement depends on the state of matter:
- In solids: molecules vibrate about fixed positions.
- In liquids: molecules move around but stay close together.
- In gases: molecules move freely at high speeds.
2.1. Example
When a drop of ink is placed in water, it slowly spreads throughout. This happens because the ink molecules and water molecules are constantly moving and mixing.
3. Large Gaps Between Particles
Particles of matter have empty space between them. The amount of space varies in solids, liquids, and gases:
- Very small gaps in solids
- Moderate gaps in liquids
- Large gaps in gases
These gaps explain why gases can be compressed easily while solids cannot.
3.1. Everyday Observation
When a syringe filled with air is pushed, it compresses easily because gas particles have empty space between them.
4. Forces Between Particles
There are attractive forces between particles. These forces keep solids rigid, allow liquids to flow, and let gases spread out:
- Strong forces in solids
- Moderate forces in liquids
- Very weak forces in gases
4.1. Why This Matters
The strength of these forces explains why solids hold shape, liquids take the shape of their container, and gases expand indefinitely.
5. Evidence of Molecular Motion
The presence of molecules and their motion was historically verified using simple experiments.
5.1. Brownian Motion
When tiny pollen grains are seen under a microscope floating in water, they show a random zig-zag motion. This happens because water molecules, which are invisible, continuously collide with them. This motion is called Brownian motion, and it proves the existence of molecules.
6. Molecular Explanation of States of Matter
The behaviour of molecules explains the three common states of matter:
6.1. Solids
Molecules are tightly packed and can only vibrate. This gives solids a fixed shape and volume.
6.2. Liquids
Molecules are close but can move past one another. This allows liquids to flow and take the shape of their container.
6.3. Gases
Molecules are far apart and move freely in all directions. This makes gases expand to fill all available space.