O-Level Physics Notes

1. KINETIC MODEL OF MATTER!
Introduction:
Hey again everyone! Hope you are all fine. Today the topic we are going to study is the Kinetic Model of Matter. We
have all studied it in the previous classes, easy wasn’t it? Let’s get started!
States of Matter:
Okay so matter can exist in three states as we all know namely the solids, liquids and gases. These are called the
states of matter. As shown below, water (liquid) can exist in ice (solid) and water vapour (gas).
So what is the kinetic model of matter? It states that:
1. All matter is made of tiny particles which exist as atoms, molecules and ions.
2. The particles are always in continuous, random motion and hence, possess kinetic energy. The kinetic energy of a
particle increases with temperature and pressure and at fixed temperature, lighter particles move faster.
The three states differ in arrangement and movement.
Here’s a simple diagrammatic representation of the states:
Now lets get into detail about these.
Solids
● Solid particles of are tightly packed,
● Solid particles vibrate about a fixed position,
● Solids have a definite shape and a definite volume,
● Solids cannot be easily compressed because of very less free space between particles and
● Solids do not flow because the particles cannot move or slide over one another.
Liquids
● Liquids have a definite volume, but do not have a definite shape since because of less tightly packed liquid
particles as compared with solids,
● Liquid particles are far enough apart to slide over one another and therefore, liquids flow easily and
● Liquids cannot compress easily since there is less free space between particles as compared with gases.
Gases
● Particles of gases are much far apart and move freely and therefore, gases do not have a definite shape or
a definite volume,

2. ● Because of more free space between particles, gases can be easily compressed and
● Because of the random and faster movement of the particles as compared with solids or liquids, gases can
flow very easily.
Now if you relate the properties with one another, they’ll be easy to memorize and will seem pretty much logical. Like
for instance, why can’t solids be compressed while gases can be? That difficult? Oh no, it’s not! Gases move at very
high speed, we know, which causes them to have a lot of space between their particles. So, they can be
compressed and brought closer but just think, can you bring the already so close particles of solid more closer? Of
course not. So just try to make sense out of it, it’s not that difficult.
Pressure in Gases:
First of all, what is pressure? We have already studies in the tutorial on Pressure that is the force acting per unit area.
p-T relationship:
How are pressure and temperature related. Do you know that if you heat a container containing gas, the gas particles
will gain kinetic energy and move faster? When they’ll move faster, they’ll hit the walls of container more often and
more force will be exerted on the walls per unit area. This increases the pressure. Hence:
p α T (pressure is directly proportional to temperature)
V-T relationship:
When a gas is heated, it’s temperature rises, causing the molecules to move at higher speeds, we just discussed.
The pressure increases, that is, the particles collide with the walls of container more frequently. When this happens,
the gas will expand in order to keep the pressure constant. So it will occupy more volume if it can. Logical, right?
Therefore, we can conclude:
V α T (volume is directly proportional to temperature)
p-V relationship:
Okay so now, what will happen to pressure if volume of gas is increased or decreased? Take a look at the two
syringes below:
The volume in 2 is decreased by pushing the same syringe further which has caused the volume to halve the original
one in syringe 1. When we halve the volume, the number of molecules of air per unit volume will be doubles, this will
double the frequency of collisions with the wall of the syringe and consequently, will double the pressure. Similarly, if
you double the volume, the frequency of collisions per unit volume will decrease and the pressure will decrease to
half.
So to conclude:
p α V (pressure is directly proportional to volume)