MATTER AND ITS PROPERTIES II

KINETIC THEORY OF MATTER

This article deals with the kinetic theory of matter at a basic level meant for Grade 9 and 10 students of CBSE, ICSE, IGCSE, and equivalent levels in other state education boards.

The kinetic theory of matter helps explain many properties of matter at bulk-scale (or at macro-level). In its most simple form, it consists of two main postulates or assertions apart from the fact that all matter is composed of small particles. Its two main postulates and their implications are discussed below:

The first and the most important point of this theory is

There is a force of attraction between the particles of different forms of matter. This force of attraction is maximum in solids, lesser in liquids, and minimum in gases.

We must understand the implications apart from memorizing the above postulate.

So, what are the implications:

i) If the force of attraction between the particles is maximum in solids, the inter-particle space must be minimum and the distance to which an individual particle can move about is least. In fact, the particles in solids are thought of as vibrating along their mean positions (see the figure below)

The particles in solids can never move a distance greater than their extremities. However, this distance is dependent on conditions of temperature and pressure. Greater the temperature, the greater is the distance moved from their mean position and the faster they cover this distance.

So, basically, the forces of attraction between the particles of solids do not let each other wander too far from their mean positions. How far they can go? depends on their temperature. With increase in their temperature, they start vibrating with greater speeds and start to go farther from their mean positions. The increase in their speeds means that their kinetic energy, which is dependent upon speed, also increases.

The force of attraction between particles decreases until they become relatively free to move about. That is what happens with most materials or solids. They then become liquids and even further increase in their temperature can turn them into gases. Refer to our ‘change of state’ for more details.

The second point of the Kinetic Theory of Matter

The particles of any form of matter have inter-particle distance between them. This distance is least in solids and greatest in gases. In liquids, this distance is between that of solids and gases but does not necessarily lie in the middle.

If we analyze closely, the second postulate is the implication of the first postulate (given above) which is regarding the forces of attraction between the particles. Greater the forces of attraction, lesser the inter-particle distance.

Now, let us go into the detailed implication of lesser or greater inter-particle distance and applied external pressure:

What do you think, the application of pressure would do to solids, liquids, or gases?

Think of it in terms of inter-particle distance….

Well, the inter-particle spaces would tend to reduce in size. But, the particles in solids already have the least inter-particle spaces. It would be difficult to reduce them further. So, solids are referred to as incompressible. Liquids as slightly compressible and gases as highly compressible. Obviously, there is so much scope to reduce inter-particle space in gases by applying external pressure.

Also, think about the implications of the above two postulates of the kinetic theory of matter on the bulk-scale properties like shape, volume, and ability to flow.

The maximum amount of inter-particle forces of attraction in solids and the least inter-particle distance mean that they have a definite shape as well as volume. The inter-particle forces of attraction decrease in liquids and inter-particle distances are comparatively increased. In gases, the forces of attraction are minimum and inter-particle distances maximum. This means that both liquids and gases do not have a definite shape and both of them can flow. That’s why both liquids and gases are collectively called fluids. There is a difference, however, in inter-particle forces of attraction and distances in liquids and gases. This fact on micro-scale results in one important difference at the bulk scale too. This difference at the bulk-scale is with respect to volume. Liquids have definite volume while gases do not have a definite volume, that is, they can occupy the entire volume available. Exactly, this is what we observe at the bulk-scale with gases. The perfumes, room-fresheners, and other gases leaked from containers do spread quickly while a spilled-liquid just changes its shape and spreads on the floor.

The following table summarizes the properties of three major forms of matter (solids, liquids, and gases) at bulk scale as predicted from the kinetic theory of matter, which deals with matter at particle- or micro-level

Property	Solids	Liquids	Gases
Mass	Definite	Definite	Definite
Volume	Definite	Definite	Not definite
Shape	Definite	Not definite	Not definite
Ability to flow	No	Yes	Yes
Compressibility	Not compressible	Slightly compressible	Highly compressible

Misconception:

Some sources say that the kinetic energy of particles in solids is minimum and maximum in gases. In the case of liquids, it lies between the solid and gaseous states. This is not always so. The kinetic energy of individual gaseous particles may be much more than that of solid or liquid states as the inert-particle forces in the gaseous state are minimum but when we talk of average kinetic energy, it may be the same in a solid, liquid, or gaseous state. In fact, the measure of the average kinetic energy of particles in a substance is given the name of ‘temperature’. So, for example, an Iron bar, water, and air can be at the same temperature and this means that the average kinetic energy of particles in them is the same.

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