Classification of Matter, Kinetic Particle Theory, Brownian Motion and Diffusion

Matter. Everyone knows what is Matter. However, does everyone know the difference between the three different stages? Why does the solid have a definite volume and shape? Why does the gas have no definite shape and volume? These questions are some of the points being discussed in this blog post about matter.
Not only that, let us think about these: Why does pollen grains move in water? Why does the food colouring spread out in a bucket of water? All these will be explained in this post.

Firstly, let us state the simple knowledge everyone should know.
Matter is anything that has mass and occupies space.
All matter can exist in 3 physical states: solid, liquid & gas, depending on the temperature and pressure of their surroundings.

What are the differences among solids, liquids and gases?

• A solid has a fixed shape and a fixed volume. It is not compressible.
• A liquid has a fixed volume but it does not have a fixed shape. It can flow and takes the shape of its container. It is not compressible.
• A gas has no shape, no surface and no fixed volume. It is highly compressible.

Let us look at the Model of the Three States of Matter:

	Solid
Diagram
Arrangement of the particles	Closely packed Regular pattern
Movement of the Particles	Cannot move freely or move from place to place Vibrate Strong attractive forces

	Liquid
Diagram
Arrangement	Particles slightly further apart Random
Movement	Particles slide past one another Free to move about . Have attractive forces between particles

	Gas
Diagram
Arrangement	Very far apart.   Randomly Arranged Takes up the space of the container
Movement	Little attraction between them  Move about randomly at a very high speed.

That is how, in detail, we classify matter. Now let us go to the complicated topics. What is the Kinetic Particle Theory?

The Kinetic Particle Theory states that matter is made of a large number of tiny particles (atoms or molecules), which are in continuous and random motion.

The Kinetic Particle Theory could be proven by Brownian Motion and Diffusion. 

Brownian Motion, is the continuous and random motion of small solid particles in fluids (liquids and gases). Brownian motion had also proved the existence of particles that cannot be observed with a normal microscope.
Brownian Motion can be observed by placing smoke particles in air. The smoke particles, when seen under the microscope, can be seen to be moving.  If heat is supplied, the motion of the smoke particles become more vigorous. Brownian Motion can also be observed by placing pollen grains in water. The pollen grains's movement is caused by the bombardment by the water molecules.
Below is an Java applet. Using small particles as the particles of a gas or liquid, and a big particles as a small solid, the Brownian Motion can be shown. Also, watch the effect when the heat has been changed.

Click to Run

Diffusion shows that particles move randomly from a region of high concentration to lower concentration.

If a bottle of perfume is opened in one corner of a room, we can smell it in another corner after a very short time as the perfume molecules have traveled from the bottle to your nose through the air. This spreading of molecules is called diffusion.

Bromine vapour can be used to show diffusion of gases. Bromine is a red brown liquid at room temperature. When it evaporates, it becomes a brown vapour. When a little bromine vapour is released into a vacuum, the brown vapour spreads through the vacuum almost at once, showing that the bromine molecules are moving at very high speed.

Diffusion of Bromine

If bromine vapour is released into a similar space full of air, the brown vapour still spreads quickly through the space but very much slower than in a vacuum. This is because the bromine molecules keep hitting the air molecules which get in the way. The air acts as a resistance.

The rate of diffusion of gases depends on the temperature and the density of the gases. The higher the temperature, the faster the diffusion, but the greater the density of the molecules, the slower the diffusion. Gases can also diffuse through walls which have pores slightly bigger than the size of the gas molecule.

Diffusion also takes place in liquids, though at a very much slower rate. An unstirred cup of coffee with milk will become uniformly coloured after many hours. In the school laboratory, diffusion of liquids can be shown using copper (II) sulfate solution and water as shown in the figure below. The two layers become uniformly mixed after a while.

Diffusion of Liquids