Tuesday 1 December 2015

The Particle Theory - Checkpoint 1 (grade 7)



– A Linkage between the macroscopic (large) and microscopic (small) world

Melting of ice at room temperature and dissolving of sugar in water is a very common phenomenon but how can this be explained. Scientists propose (postulate) that all substances are made up of tiny particles.

The theory states that all matters are made of particles is called particle theory of matter.

1. Evidences of presence of particles

a) Diffusion

When the glass plate is removed, the brown colour diffuses up and fills up the upper and lower gas jars evenly. Despite the fact that nitrogen dioxide is denser than air, the brown colour still moves up. Therefore, this must not be caused by gravity. This can be explained by the constant motion of air and nitrogen dioxide particles. Eventually, all particles mix up thoroughly.

b) Brownian Movement

When pollen grains are placed on water surface, it is observed that they move randomly even without being blown by the wind. This was firstly observed by Robert Brown in 1827 and is known as Brownian movement.

Macroscopic Observation


Microscopic Interpretation

Pollen grains are hit by rapid moving water particles from all directions. This makes the pollen grains move randomly on the water surface.

c) Physical states of matter

The existence of 3 physical states of matter (solid, liquid and gaseous state) can also be explained by particle theory of matter.

In solid states, the particles are closely packed together and they can only vibrate about their equilibrium positions. Therefore, solid is incompressible and has a definite shape.

In liquid states, the particles are fairly close packed but are free to move. Therefore, liquid is also incompressible but do not has a definite shape.

In gaseous state, the particles are very far from each other and free to move at a great speed. Therefore, gas is freely compressible and do not has a definite shape.

When a cube of ice melts to water, there is almost no change in volume. But when a drop of water evaporates, the steam formed has a volume 1000 times larger than the drop of water. This proved that the particles in a gas are more widely separated than in a liquid.

N.B. The sizes of particles are all the same in different physical states, the difference is only the spacing among the particles.

Summary of differences among solid, liquid and gas






  • Fixed

Takes the shape of the container

Takes the shape of the container

Diagram showing the arrangement of particles




Forces of attraction between particles

(Intermolecular forces)

  • Very strong 

  • Strong

  • Very weak

Motion of particles

  • Vibrate about their fixed positions

  • Roll and slide past each other

  • Random motion

  • Move about freely at high speeds

  • Random motion

Energy content

  • Low

  • Moderate

  • High


  • Fixed

  • Takes the shape of container

  • Takes the shape of the container


  • Fixed

  • fixed

  • Same as the volume of the container


  • High

  • Moderate

  • Low


  • Cannot be compressed

  • Cannot be compressed

  • Highly compressible

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