Chapter 7 — Particulate Nature of Matter

The chapter explains that all matter is made up of extremely small constituent particles held together by interparticle forces of attraction. It shows how the strength of these forces and the spacing between particles determine whether a substance is a solid, liquid, or gas, and how thermal energy drives changes between states.

A constituent particle is the basic unit that makes up a larger piece of a substance or material. For example, when chalk or sugar is broken down as far as possible, the tiny units obtained at that stage are its constituent particles. These particles are so small they cannot be seen even through an ordinary microscope.

Interparticle attractions are the attractive forces that hold the constituent particles of matter together. Their strength depends on the nature of the substance and the distance between particles — even a slight increase in distance decreases these forces drastically. The strength of these forces ultimately decides the physical state (solid, liquid, or gas) of the substance.

In solids, interparticle attractions are very strong and spacing is minimum, so particles are held in fixed positions and can only vibrate — giving solids both a definite shape and volume. In liquids, the attractions are slightly weaker, allowing particles to move within a limited space; liquids therefore take the shape of their container but retain a definite volume.

The melting point is the minimum temperature at which a solid melts to become a liquid at atmospheric pressure. When heated to this temperature, the thermal energy is sufficient to overcome the interparticle attractive forces, allowing particles to leave their fixed positions. For example, ice melts at 0 °C, urea at 133 °C, and iron at 1538 °C.

Boiling is the rapid conversion of a liquid into vapour that occurs at a specific temperature (the boiling point) at atmospheric pressure; it happens not only at the surface but also within the liquid, seen as bubble formation. Evaporation is a slower process of vapour formation that occurs only at the surface and at any temperature, even below the boiling point.

In gases, the interparticle spacing is maximum and the particles move freely, meaning there is a lot of empty space between them that can be reduced by applying external pressure (as shown by pushing the plunger of a syringe filled with air). In liquids, the particles are already much closer together, so there is very little space to compress — water is described in the chapter as practically incompressible.

When sugar dissolves in water, it breaks up into its constituent particles, which are too small to see. These sugar particles move into and occupy the interparticle spaces already present between the water particles. This is why the volume of the sugar solution is less than the combined volume of water and sugar measured separately.

When a grain of potassium permanganate is placed in water, streaks of pink colour initially spread from the grain, and over time the entire water turns a uniform pink. This happens because water particles are in constant motion — they pull permanganate particles out of the grain and then knock them throughout the liquid. In hot water this spreading happens fastest, and in ice-cold water it happens slowest, showing that particle movement increases with temperature.

When an incense stick is burnt in one corner of a room, the fragrance is at first only noticed near the stick. Within a short time, it spreads throughout the entire room. This happens because air particles are in constant motion and collide with the fragrance particles, spreading them in all directions — demonstrating that gas particles move freely and continuously.

Thermal energy determines the physical state of matter. In the solid state, thermal energy is low, so particles stay close and experience strong attractions, restricting them to small vibrations. At the melting point, thermal energy overcomes these attractions and the solid becomes a liquid. In the gaseous state, particles have enough thermal energy to completely overcome interparticle attractions and move freely in all directions.

Acharya Kanad, an ancient Indian philosopher, first spoke about the idea of Parmanu (atom). He believed matter is made up of tiny, indivisible, eternal particles called Parmanu, and recorded this idea in his work called Vaisheshika Sutras.

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