Classification of Matter

Matter is anything which has mass and occupies space. Examples of matter are books, phones, laptops, water, juices, coffee, air, Earth, Moon, Sun, and anything which is composed of atoms. Based on physical characteristics, we can classify matter into two main categories: pure substances and mixtures. Mixtures can further divide into homogeneous and heterogeneous mixtures and pure substances into elements and compounds. The figure below explains the same.

Pure Substances

A substance is a pure substance when all constituent particles of the substance have the same chemical properties. In pure substance, the composition of constituent particles is fixed i.e. it does not vary from sample to sample. Most of the substances that we use in our day to day life are not pure substances. Some of the examples of pure substances are water; metals like iron, copper, calcium, silver, gold etc; salts like sodium chloride, calcium carbonate, potassium nitrate etc; gases like oxygen, carbon dioxide, nitrogen, hydrogen, helium etc.

Pure substances are normally not found in nature. But they are produced by industries, for example, refining of dore bar (semi-pure gold) by the Wohlwill process, extraction of calcium carbonate from a quarry, electrolysis of sodium chloride to produce hydrogen gas.

Consider a gold bar. Constituent particles in the gold bar are its atoms. Each atom in the gold bar is identical to other atoms and they all have the same chemical properties. Thus, we say it is a pure substance. Similarly, the constituent particles of pure water are its molecules (H2O). The water is represented by these molecules, which are identical and show the same chemical properties.

Based on the nature of constituent particles, we can subcategorise pure substances into elements and compounds.

Elements

Pure substances in which constituent particles are composed of the one type of atoms are called elements. Some of the examples are elements are sodium, calcium, silver, gold, sulphur, phosphorus, helium, neon, hydrogen, oxygen, nitrogen &c. The constituent particles of elements can be atoms or molecules. Atoms include sodium (Na), calcium (Ca), silver (Ag), gold (Au), helium (He), neon (Ne). Molecules include hydrogen (H₂), oxygen (O₂), nitrogen (N₂), chlorine (Cl₂) etc. The figure below depicts the same.

For an element, a constituent particle can consist of one or more atoms, but an important point is all the atoms should be the same type. As of 2019, there are 118 elements known to us, and they are listed in the modern periodic table. The most abundant elements in the universe are hydrogen and helium.

Compounds

When constituent particles of a pure substance are composed of different atoms, it is called a compound. Examples of compounds are water (H₂O), carbon dioxide (CO2), hydrogen chloride (HCl), ammonia (NH₃), methane (CH₄), glucose (C₆H₁₂O₆), sodium chloride (NaCl), potassium hydroxide (KOH) etc.

In compounds, two or more different atoms combine to form a constituent particle. The atoms of a constituent particle are chemically bonded to each other. We cannot separate the atoms of a constituent particle by any physical methods.

Elements can interact among themselves to form compounds, for example, hydrogen and oxygen react to form water. The properties of elements may or may not resemble its respective compound. Hydrogen and oxygen are gases while their compound water is liquid. Hydrogen is flammable and oxygen supports fire while water extinguishes the fire. It is interesting to note, just by combinations of a few elements, we can get countless compounds. Water is the most abundant compound not only in Earth but also in the entire universe.

Mixtures

In contrast to pure substance, a mixture contains different constituent particles. A mixture is a combination of two or more pure substances. In a mixture, the composition of individual components can vary at any scale. Most of substances in the world are mixtures. Some of the examples of mixtures are air, seawater, steel, sand, rocks, concrete, petrol, diesel, medicines, milk, juices, tea, coffee, food and many others.

In mixture, each ingredient retains its properties. Consider an example of coffee. Typical ingredients in coffee are roasted coffee, sugar, cream, spice like cinnamon, and water. When a person drinks coffee, each ingredient in coffee contributes to its taste. Also, in mixture individual components are not bonded chemically. There is no chemical bond among the ingredients of coffee. Seawater is a mixture of water, sand, minerals and salts dissolved in it. Steel is also a mixture mainly of carbon and iron. Air support life on the Earth is also a mixture of various gases. The main components in the air are nitrogen which roughly accounts for 79 % and oxygen which is approximately 21 %. It is possible to separate the components of a mixture by physical methods such as hand picking, distillation, filtration, crystallisation, sedimentation etc.

Homogeneous Mixture

In homogeneous mixtures, the components of the mixture are uniformly distributed through the mixture. The composition of components does not vary from sample to sample provided the sample size is considerably large to the size of components. The components in a homogeneous mixture are well mixed. The examples of homogeneous mixtures are air, sugar solution, tap water etc. Homogeneous mixtures aka solutions. Air is a solution of various gasses like nitrogen, oxygen, carbon dioxide, water vapour. Tap water is a solution of water and other components dissolved in it such as calcium carbonate, sodium chloride etc.

Heterogeneous Mixture

Heterogeneous mixtures are do not have a uniform distribution of components. The composition of components does vary from sample to sample. Sometimes, the components are visible by naked eyes and can be handpicked, for example, the mixture of sand and gravel or the mixture of marble balls and stones. We can easily identify the individual components and separate them by our hands. Consider the below figure the mixture of sunflower seed and sand. It is a heterogeneous mixture. We can comfortably identify the black seeds from the sand and we can separate them manually.

Suspension

Suspensions are a special type of heterogeneous mixtures in which components can be separated through sedimentation. In sedimentation, the particles get separated with the help of gravity. When a heterogeneous mixture is immersed in a fluid, the heavier particles settle at the bottom leaving the lighter particles to float on the surface of the fluid. Examples are dust on grains, chalk powder mix with gravel, mud etc. In suspension, the diameter of particles should be sufficiently large (more than 1 micron) to settle down.

Colloid

A colloid is another case of heterogeneous mixtures in which microscopic particles are suspended in a fluid. The difference between suspension and colloid is the size of suspended particles. In colloids, the size of suspended particles is smaller than that of suspensions. Because of the small sizes, the particles in colloid stay suspended in the fluid for a very long period of time. Colloids apparently appear as homogeneous mixtures but are not. The classical example of colloids is milk. Milk appears as a homogeneous mixture which is false. When we examine milk under a microscope, we can observe fats and protein suspended in water. With the help of filtration, we can separate these fats and proteins from the water. Other examples are fog, blood, gelatin, ink.

Summary

The table below summarises the entire article.

Associated Articles

• Four states of matter
• Properties of matter