what is the kinetic theory of matter?

Answer 1

What is the kinetic theory of matter?
That matter is made up of molecules, and that these molecules are in continual random motion and possessing kinetic energies.

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The three states of matter
They are : SOLID, LIQUID and GAS.


Property Solid Liquid Gas
Intermolecular forces strong less strong much weaker
Motion vibrate about a fixed mean position; therefore do not move through the body of the solid vibrate and move freely at quite high speeds throughout the body of the liquid move randomly and at high speeds
Intermolecular distance molecules are very close together, and therefore gives the solid a rigid shape molecules are slightly further apart than in a solid, and take the shape of the container which holds the liquid molecules are very far apart, and therefore free to take up any space which is available to them




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Evidence for the motion of molecules (Q & A)
A. BROWNIAN MOTION
Do air (or liquid) molecules have motion?
Yes.

Why can't we see the molecules in motion?
They are too small to be seen by the naked eye or even the most powerful microscope.

How do liquid or gas molecules move?
They move in a continuous and random motion, possessing a wide range of velocities.

Evidence?
Provided by the effect called Brownian motion.

Experimental set-up?
See p.121 of text by Gilbert Rowell & Sidney Herbert, "PHYSICS, A Course for GCSE", 3rd ed., Federal Publications, 1994.



What are the bright specks seen dancing about in a jerky, erratic or random way?
They are smoke particles which scatter the light shining on them.

Why are the smoke particles moving in an erratic manner?
The irregular movement of the visible particle (smoke) is due to an uneven bombardment of the particle on all sides by the invisible molecules of air.
The particle is so small that the number of molecules of air hitting one side is not balanced by an equal number of molecules hitting the opposite side at the same instant.
Consequently, the motions of the small particles change direction.
When it moves to another place the same thing happens, thus the Brownian motion was observed.

What is a typical path taken by a smoke particle due to Brownian motion?
See diagram, from the same source book.



Where can Brownian motion be seen?
In liquids and gases.

What, then, is Brownian motion?
It is the continual and random motion of the minute particles (e.g. smoke particles) suspended in a fluid.

Could Brownian motion be due to convection current?
No, because if this were the case, then the particles would move upwards continually and would not move in a zig-zag manner from side to side.

Why don't we observe Brownian motion for a ping-pong ball suspended with a string?
Although the suspended ping-pong ball would be bombarded on all sides by the air particles, the ball would remain fairly still.
This is because the large number of molecules hitting one side of the ball is not relatively much greater than the number hitting the other side at the same instant. Hence the ping-pong ball does not move.

Why is the study of Brownian motion so important?
Brownian motion is important in the kinetic molecular model of matter as it provides experimental evidence that molecules of liquids and gases are continually moving in a random way.


B. DIFFUSION
It is the differential movement of molecules (or ions) from a region of high concentration to a region of lower concentration.


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Relationship between temperature of the gas and the average kinetic energy of the molecules
It is important to bear in mind that the molecules of a gas have a wide range of speeds and therefore kinetic energies.
The temperature of a gas is directly proportional to the average kinetic energy of the gas molecules.
Hence if the average kinetic energy of the gas increases, the temperature of the gas also increases. This will translate into an increase in the speed of the gas particles.

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Using the kinetic theory, some phenomena can be explained
How does a gas exerts pressure on the walls of its container?
During collisions with the walls of the container, the molecules experience a change in velocity with a corresponding change in momentum.
The rate of change of momentum is the force exerted by the molecules on the walls of the container. The force per unit area exerted on the walls is the pressure of the gas.

Why is energy required to evaporate a liquid?
There are intermolecular forces between liquid molecules.
Only the fastest-moving molecules will have large enough energy to overcome the intermolecular forces. Hence energy is required to break the attractive forces.

Why does evaporation occur?
In liquids, there are intermolecular forces holding the liquid molecules together. Due to the constant collisions between molecules, some molecules at the surface may attain greater velocities.
Such molecules, if moving in the upward direction, may have large enough kinetic energy to overcome the downward attractive forces exerted by the molecules in the liquid and break away from the liquid surface and exist independently as vapour molecules. Hence evaporation occurs, and can occur at any temperature.

Why does cooling occur when a liquid evaporate?
Since only the fastest-moving molecules leave the liquid, it follows that the average kinetic energy of the molecules remaining in the liquid is decreased.
Since temperature is directly proportional to the average kinetic energy of the liquid molecules, therefore temperature falls, and cooling occurs.

Why does a liquid evaporate more quickly in a draught?
In a draught, the liquid molecules that leave the surface of the liquid will be blown away. More liquid molecules are able to take up the space left empty by the previous molecules.
At the same time, fewer molecules are able to return to the liquid. Hence liquid evaporate more quickly in a draught.

How does the surface area of a liquid affect evaporation?
Evaporation only takes place at the surface of a liquid.
The larger the surface area, there are more number of liquid molecules that can escape from the surface. Hence evaporation will be rapid. This in turns lead to rapid cooling of the liquid.

How does temperature affect evaporation?
The rate of evaporation is greater at higher temperature.
This is because of the greater average kinetic energy of the molecules, leading to greater velocities. Hence the molecules readily overcome the attractive forces and contribute to the process of evaporation.

What happens when a gas is heated in a container of fixed volume?
When the container is heated, the moving molecules gain internal energy. Since temperature is directly proportional to the average kinetic energy of the molecules, their average speed and hence their kinetic energy are increased.
As the molecules are moving faster than before, they will make more collisions per second with the walls. At the same time, each collision now results in a greater force imparted. Hence pressure is increased.

How does the kinetic theory explain diffusion?
All molecules are in constant motion. Because of the rapid, constant and chaotic motion of molecules, especially in a gas, molecules spread out from a region of higher concentration to a region of lower concentration.
In a gas, the spreading out is faster because of the larger intermolecular distance and negligible force among molecules.

Answer 2

Kinetic Theory Of Matter

Answer 3

gases occupy about 1000 times the volume of the same amount of the liquid phase the molecules are about 10 times as far apart (10x10x10 = 1000) the liquid particles are nearly touching and repel each other strongly not so for gases of course, all bets are off when you have compressed a gas to near the density of its liquid phase

Answer 4

Energy is always conserved
that's it, no matter what you do, react, move, destroy, build, energy is always conserved.