Classifying Matter by Composition

Matter is made of atoms, the smallest basic units of matter, and molecules, which are formed by combining two or more atoms together. Based on its composition, matter can be classified into two groups — pure substances and mixtures.

By definition, a matter is said to be pure if it is made of only one component, with a uniform composition throughout. For example, take helium. Helium is made up of only helium atoms. Similarly, water is made of only water molecules. These compositions do not vary from sample to sample. 

Pure substances can be categorized into two types, elements and compounds, depending on whether or not they can be broken down. 

Elements are pure substances that cannot be broken down into simpler substances by chemical means. For example, helium is an element.

Compounds, on the contrary, are substances that can be separated into their constituents only by chemical means. For example, water, also a pure substance, can be chemically broken down into two simpler substances, hydrogen and oxygen, meaning it is a compound. 

On Earth, compounds are more common than pure elements because most elements will readily combine with other elements. For example, chalk used for writing on the blackboard is made of calcium, carbon, and oxygen; sugar is made of carbon, hydrogen, and oxygen. Mixtures, unlike pure substances, are made of one or more components. Their proportion can also vary from sample to sample. For example, lemonade is a mixture of three components — lemon juice, sugar, and water — and their proportions can be varied to make the drink more or less sweet.

Mixtures themselves can be categorized into two types, homogeneous and heterogeneous, depending on how uniformly the substances within them mix.

A homogeneous mixture exhibits a uniform composition and appears visually the same throughout. It is often referred to as a solution. For example, a spoonful of salt dissolved in water gives a homogeneous mixture as every drop of that mixture has the same composition. Similarly, lemonade is also a homogeneous mixture.

In a heterogeneous mixture, the components are not distributed uniformly, and thus the composition varies from point to point. For example, sand mixed with iron filings. Here, different parts of this mixture will likely have different amounts of iron filings and sand. 

Regardless, any mixture, whether homogeneous or heterogeneous, can be separated into its pure components by physical means without changing the identities of the components. For instance, salt can be recovered from the water solution by heating the mixture to dryness. Similarly, iron filings can be separated from sand using a magnet.