1. Kinetic molecular model explains the three states of matter assuming that

• matter is made up of particles called molecules.
• the molecules remain in continuous motion.
• molecules attract each other.

At very high temperatures, the collision between atoms and molecules tears off their electrons. Atoms become positive ions. This ionic state of matter is called the plasma-the fourth state of matter.

Density is the ratio of mass to volume of a substance. The density of water is 
$1000 kgm^{-3}$. 

Pressure is the normal force acting per unit area. Its SI unit is $Nm ^{-2}$ or pascal (Pa). 

The instruments that measure atmospheric pressure are called barometers.

Liquids transmit pressure equally in all directions. This is called Pascal's law.

When a body is immersed wholly or partially in a liquid, it loses its weight equal to the weight of the liquid displaced. This is known as the Archimedes principle.

The property of matter by which matter resists any force which tries to change its length, shape, or volume is called elasticity.

Stress is the deforming force acting per unit area.

The ratio of change of length to the original length is called tensile strain.

The ratio between stress and tensile strain is called Young's modulus.