The Definition of Matter

Matter is anything that has weight and occupies space. Every object or substance is made up of matter. Many of the properties and behavior of substances can best be explained by assuming that all substances are composed of small particles called molecules. The assumption that matter is made up of tiny particles (molecules) which are in constant motion is known as the “molecular theory of matter.”

Structure of Matter

Evidence of the Particle Nature of Matter

(i) Many substances in solid form can easily be crushed to powder form e.g. piece of chalk, lump of clay, charcoal and piece of stone.

(ii) A dry stick or dry wood is easily broken into smaller bits

(iii) Solubility – if you drop a cube of sugar into a cup of water and turn the water, the sugar “disappears”. That is, it dissolves in water.

(iv) If you scrape the surface of a piece of chalk, you will see thousands of very tiny particles flake off and float through the air

(v) If a beam of light (e.g. sunlight) is entering a dusty room through a window, you will observe a chaotic motion of the dust particles in the air.

Experimental Evidence of the Particle Nature of Matter

Experimental evidence of the atomic or molecular nature of matter is the Brownian motion named after the botanist, Robert Brown, who discovered the phenomenon in 1827. While he was observing tiny pollen grains suspended in water under a microscope, Brown noticed that the tiny pollen grains moved about in zigzag paths even though the water appeared to be perfectly still. The here and there by the molecules of water pollen grains were being knocked about which were vigorously moving about.

Brownian motion can be demonstrated by the smoke-cell experiment. The apparatus is shown in the figure below.

Molecular motion in gas: Brownian (random) motion

Collect some smoke from a smoldering piece of cloth or wood by means of a syringe and introduce it into the cell. Replace the cover quickly and adjust the focus of the microscope until the fine particles come into view clearly. You will observe the smoke particles as black dots which move about irregularly like a (drunkard) drunken man staggering about. The particles dart from one place to another very suddenly, some going out of focus, others coming into focus, but always in motion.

What you see through Microscope in Smoke cell experiment:

Explanation of Brownian Motion

The irregular movement of the smoke particles is due to the motion of the invisible air molecules which bombard each particle from all sides. The particle is very small and the number of molecules of air hitting one side is not balanced by the number of molecules hitting the opposite side at the same instant. Therefore, the particle moves in the direction of the resultant force and when it moves to another place, the same thing happens. Why can’t the motion of the particles be due to convection? In that case the particles would move upwards continually and not zigzag from side to side.

Brownian motion in liquids can be demonstrated similarly as follows:

Place on a clean microscope slide a few drops of diluted aquadag (finegraphite particles suspended in water) or photopake (a similar suspension used for blacking negatives) and cover the liquid with a cover-slip. Project an image of the slide on a screen using a micro projector so that the particles can be seen. The graphite particles are then seen to be moving about in an irregular manner, thus showing Brownian motion in a liquid. In this case, the irregular motion of the graphite particles is due to their bombardment by the surrounding water molecules which are constantly moving about in different directions. Brownian motion is important for two reasons.

(i) It provides evidence for the existence of the tiny particles of matter called molecules

(ii) It gives evidence that molecules are in a constant state of random motion.