Nature of Matter

What Matter Is

Chemistry is the study of matter and the changes it undergoes. The starting point, set out in NCERT §1.2, is a definition that is deliberately simple: anything which has mass and occupies space is called matter. A book, a pen, water, air and every living being are all composed of matter because each of them has mass and takes up space. This definition is operational rather than abstract; it tells us what to look for before any chemical question can be asked.

From this single criterion two complementary descriptions of matter follow. The first concerns what matter is made of at the smallest scale — its particulate nature. The second concerns how matter can be organised — first by physical state, and then by chemical composition. The whole of this subtopic is the careful unpacking of those two descriptions, and the two classification schemes they generate.

The Particulate Nature of Matter

Since ancient times people have wondered about the nature of matter. The central question, as the NIOS chemistry text frames it (§1.2), is this: if a piece of rock is broken into smaller and smaller particles, can the process go on for ever, or does it stop at particles that can no longer be broken down? Greek philosophers such as Plato and Aristotle held that matter is continuous and that subdivision can go on indefinitely.

The opposing view prevailed. Many believed that subdivision can be repeated only a limited number of times, until particles are reached that cannot be further divided. On this view, matter is composed of a very large number of extremely tiny particles and therefore has a particulate nature. These smallest indivisible particles were named atoms, from the Greek word meaning "indivisible". The Greek philosopher Leucippus and his student Democritus are generally credited with first proposing this idea about 440 BC, while in India Maharshi Kanad had propounded an atomic concept of matter still earlier, around 500 BC, naming the smallest particle "Parmanu".

This particulate picture is the bridge between the macroscopic world we observe and the microscopic world chemistry actually reasons about. Every physical state and every category in the classification scheme below is ultimately a statement about how these particles are arranged and how they are bonded.

Physical States of Matter

Matter can exist in three physical states: solid, liquid and gas. The distinction between them is, at root, a distinction in how the constituent particles are arranged and how freely they move. In solids the particles are held very close to each other in an orderly fashion, with little freedom of movement. In liquids the particles are close together but can move around. In gases the particles are far apart compared with the solid or liquid states, and their movement is easy and fast.

Because of these differing arrangements, the three states show characteristically different bulk behaviour with respect to volume and shape. The table below summarises the rule that NEET most often tests in a single line.

Read down the "volume" and "shape" columns and the logic of the classification appears. A solid is rigid on both counts. A liquid keeps its volume but surrenders its shape to the container. A gas surrenders both, completely occupying the space available to it. The progression from "definite, definite" to "not definite, not definite" tracks exactly the loosening of particle packing seen in Figure 1.

Interconversion of States

The three states of matter are interconvertible by changing the conditions of temperature and pressure. The chain of conversions runs both ways: a solid usually changes to a liquid on heating, and the liquid changes to a gas (or vapour) on further heating; in the reverse direction, a gas liquefies to the liquid on cooling, and the liquid freezes to the solid on further cooling.

The crucial point for the classification scheme is that interconversion of states is a physical change: the identity of the substance is unaltered. Ice, water and steam are the same compound, $\ce{H2O}$, in three different physical states. Changing state therefore never moves a substance between the chemical categories developed below; it only relabels its physical form.

Chemical Classification of Matter

Alongside the physical classification by state, matter is classified at the macroscopic or bulk level into mixtures and pure substances. The defining test is the chemical nature of the constituent particles. When all the constituent particles of a substance are the same in chemical nature, it is a pure substance. A mixture, by contrast, contains many types of particles — specifically, particles of two or more pure substances, present in any ratio.

Each branch then divides once more. Mixtures split into homogeneous and heterogeneous mixtures according to the uniformity of their composition. Pure substances split into elements and compounds according to whether they contain one type of atom or several types combined in a fixed ratio. The schematic tree below shows the full scheme that the rest of this subtopic explains branch by branch.

A defining feature of the two main branches is their composition. The constituent particles of a pure substance have a fixed composition; copper, silver, gold, water and glucose are examples. Glucose, for instance, contains carbon, hydrogen and oxygen in a fixed ratio, and its constituents cannot be separated by simple physical methods. A mixture has a variable composition because its components — the pure substances forming it — may be present in any ratio. Most substances we meet daily, such as a sugar solution in water, air and tea, are mixtures.

Mixtures: Homogeneous and Heterogeneous

A mixture may be homogeneous or heterogeneous, and the dividing line is the uniformity of composition. In a homogeneous mixture the components completely mix with each other, so the particles of the components are uniformly distributed throughout the bulk and the composition is uniform throughout. Sugar solution and air are the standard examples. In a heterogeneous mixture the composition is not uniform throughout, and sometimes the different components are even visible to the eye; mixtures of salt and sugar, or of grains and pulses along with some dirt such as stone pieces, are examples.

It is worth noting that the components of any mixture, homogeneous or heterogeneous, can be separated by physical methods. This separability by physical means is one of the surest ways to recognise a mixture and to distinguish it from a compound, where the constituents are bonded and yield only to chemical methods.

Pure Substances: Elements and Compounds

Pure substances are further classified into elements and compounds. The particles of an element consist of only one type of atoms. These particles may exist as individual atoms or as molecules: sodium, copper and silver exist as atoms, whereas hydrogen, nitrogen and oxygen exist as molecules in which two atoms combine to give the respective molecule, written $\ce{H2}$, $\ce{N2}$ and $\ce{O2}$. The atoms of different elements are different in nature, but within a single element every atom is of one type.

A compound is obtained when two or more atoms of different elements combine together in a definite ratio. Two facts follow, and NEET tests both. First, the elements are present in a compound in a fixed and definite ratio that is characteristic of that particular compound. Second, the properties of a compound are different from those of its constituent elements. Water, ammonia, carbon dioxide and sugar are common compounds; the molecules of water and carbon dioxide are written $\ce{H2O}$ and $\ce{CO2}$.

Separating the Components of Matter

The classification scheme is reinforced by how the categories respond to attempts at separation, a theme NCERT introduces here and develops fully in its section on separation techniques. The contrast is sharp and examinable.

The decisive question to ask of any material, therefore, is whether its components separate by physical means (mixture) or only by chemical means (compound), or whether it cannot be simplified at all (element). This single line of reasoning resolves most classification statements that appear in examinations.