Classification of Matter

Classification of Matter

At the macroscopic level — the level we can see and handle — matter is first divided into two broad categories: pure substances and mixtures. This classification, which you first encountered in Class IX, is shown in Figure 1.2 of the textbook.

A pure substance is one in which all constituent particles are identical in chemical nature. Every particle of a pure substance has the same composition and the same set of properties. Copper, silver, gold, water, and glucose are examples. Glucose, for instance, contains carbon, hydrogen, and oxygen in a fixed ratio, and every particle of glucose has that same composition.

A mixture, by contrast, contains two or more different kinds of particles. These particles come from two or more pure substances that are physically mixed together. The pure substances that form a mixture are called its components. The key point is that the components can be present in any ratio — the composition of a mixture is variable. Sugar solution in water, air, and tea are all mixtures.

Homogeneous and Heterogeneous Mixtures

Mixtures are further classified based on the uniformity of their composition.

A homogeneous mixture is one in which the components are completely mixed with each other. The particles of the different components are uniformly distributed throughout the bulk of the mixture, so the composition is the same at every point. You cannot see the separate components, even with a microscope. Sugar solution and air are homogeneous mixtures.

A heterogeneous mixture is one in which the composition is not uniform throughout. Different regions of the mixture have different compositions, and sometimes the different components are visible to the naked eye. A mixture of salt and sugar, or grains mixed with stones, are heterogeneous mixtures.

Separating Mixtures

The components of a mixture can be separated by physical methods. Because the components are only physically mixed — not chemically combined — you can use techniques that exploit differences in their physical properties. Common methods include:

• Simple hand-picking
• Filtration
• Crystallisation
• Distillation

Pure Substances: Elements and Compounds

Pure substances have a fixed composition, and their constituents cannot be separated by simple physical methods. Pure substances are further classified into elements and compounds.

An element is a pure substance whose particles consist of only one type of atom. These particles may exist as individual atoms or as molecules. Sodium, copper, silver, hydrogen, and oxygen are examples of elements. All atoms of a given element are of one type, but atoms of different elements are different in nature.

Some elements, such as sodium or copper, have atoms as their constituent particles. In other elements, the constituent particles are molecules formed by two or more atoms of the same element. For example, hydrogen gas ($H_2$), nitrogen gas ($N_2$), and oxygen gas ($O_2$) consist of diatomic molecules — two atoms of the same element chemically bonded together.

A compound is a pure substance formed when two or more elements combine chemically in a fixed and definite ratio by mass. This ratio is characteristic of that particular compound. The properties of a compound are completely different from the properties of its constituent elements.

Consider water ($H_2O$). Hydrogen and oxygen are both gases at room temperature. Hydrogen burns with a pop sound; oxygen supports combustion. Yet when they combine in the fixed ratio of 2:1 by number of atoms (or 1:8 by mass), the resulting compound is a liquid that is used as a fire extinguisher. The compound has properties that neither element possesses.

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