Composition of matter

In terms of composition, matter can be broadly classified as elements, compounds and mixtures.

Elements

Element is a basic form of matter that can not be broken down into simpler substances by chemical reactions.

Thus elements are pure substances made of only one kind of particles. The smallest particle of an element is called an atom. The atoms of a given element are all alike but they are different from that of the other element.

For some elements, pairs of atoms combine to form independent units called molecules. The properties of an element depend upon the structure and number of atoms in a molecule.

Elements can be classified into four categories (i) metals (ii) nonmetals (iii) metalloids and (iv) noble gases. Each category has its own characteristics. Most of the elements are solids. Two elements are liquids and eleven are gases. Elements may be monoatomic, diatomic, triatomic or polyatomic. As on today , 118 elements are known.

Examples of elements Copper, silver, gold, iron, silicon, nitrogen, oxygen, helium etc.

Compounds

A compound is a substance composed of two or more elements chemically combined with one another in a fixed proportion.

Many substances are made up of two or more elements. They are called compounds. Different elements combine as a result of chemical reaction. After the chemical reaction, these elements lose their identity and can not be separated from each other by simple means.

Characteristics of compounds

  1. Compounds are pure substances chemically formed by the union of two or more elements.

  2. A compound is homogeneous i.e. all particles of a compound are identical in composition and they have the same properties throughout.

  3. Compounds have a fixed composition by weight.

  4. The properties of a compound are different from those of its constituent elements.

  5. The components of a compound can not be separated by simple physical means.

  6. During the formation of a compound, energy is evolved or absorbed. Sometimes, this energy is in the form of heat, light or electricity.

Examples of compounds Common salt, sugar, water, rust ( iron oxide),washing soda, soap, copper sulphate etc.

Mixtures

A mixture is a substance made of two or more elements or compounds mixed in any proportion.

In every day life, we come across many substances which are mixtures. For example, air, sea water, ores, minerals etc. are mixtures. In a mixture, the components do not lose their identity even after mixing. These components also do not tend to chemically interact mutually in normal course (except where heat or moisture is present)

Characteristics of mixtures

  1. Components of a mixture can be mixed in any proportion by weight. So the mixture is usually heterogeneous.

  2. The individual components of a mixture retain their properties even after mixing.

  3. The components of a mixture can be separated by simple physical means.

  4. No energy change is involved in the preparation of a mixture.

Examples of mixtures Mixture of two liquids like water and alcohol, salt solution, milk, gun powder, earth crust etc.

Difference between a mixture and a compound

Let us illustrate the difference between a mixture and a compound by a simple illustration. Carry out the following experiment under the supervision of a teacher.

Divide the class into two groups. Give 7g of iron filings and 4g sulphur powder in a china dish to each group. Ask group I to mix and crush iron filings and sulphur powder. Ask group II to mix and crush iron filings and sulphur powder and heat them strongly for 5 minutes. Let the mixture cool. Check the magnetism in the material obtained for both the groups. Bring the magnet near the material and check if the material is attracted towards the magnet. Compare the texture and colour of the material obtained by the two groups. Add carbon disulphide to one part of the material taken in a test tube. Stir well and filter. Add dilute HCl or H2SO4 to the other part of the material taken in another test tube. At the end, one concludes that the material left with group I is a mixture while the material left with group II is a compound. This conclusion is drawn on the basis of following observations.

Sr.No & Mixture & Compound

1 & When iron filings and sulphur are mixed in a china dish,you can still see the black specks of iron and the yellow sulphur powder. & When iron filings and sulphur are heated together, a black shining compound, iron (II) sulphide ( FeS )is formed.
2 & When a magnet is brought near this mixture of two elements, the iron filings are pulled away and stick to the magnet. Sulphur remains in the china dish. & When a magnet is brought near this heated mixture of two elements, the iron of the iron (II)sulphide is not attracted by the magnet.
3 & When an acid like HCl is added to the mixture of iron and sulphur,a colourless, odourless gas,hydrogen is evolved which burns with a blue flame and is extinguished with a pop sound. Unreacted sulphur remains at the bottom of the test tube.
Fe + S + 2HCl →  FeCl2 + H2 + S & When an acid like HCl is added to the compound iron sulphide,a colourless gas with a smell of rotten eggs is evolved. The gas is hydrogen sulphide.
FeS + 2HCl  →  FeCl2 + H2S
4 & If liquid carbon disulphide is added to the mixture, sulphur dissolves but not iron. & If liquid carbon disulphide is added to the compound, iron (II)sulphide does not dissolves but sinks to the bottom of the test tube.

 

Difference between a mixture and a compound

Sr.No & Mixture & Compound
1 & A mixture is made of two or more substances in any proportion & A compound is made of two or more substances in a fixed proportion.
2 & A mixture does not involve a chemical change & A compound is obtained as a result of chemical change.
3 & A mixture may be homogeneous or heterogeneous & A compound is always homogeneous.
4 & Each component of the mixture retains its properties & The components of a compound lose their properties
5 & Components of a mixture can be separated by a simple physical method & Components of a compound can not be separated by physical methods.
6 & Mixture does not have a sharp melting point and boiling point. & Compound has a sharp melting point and boiling point.
7 & Formation of a mixture is not accompanied by energy change. & Usually, compound formation is accompanied by energy change in the form of light or heat.

 

Types of mixture

There are two types of mixtures (i) Homogeneous mixtures and (ii) Heterogeneous Mixtures

  1. Homogeneous mixture It is a mixture, the composition of which is uniform throughout. It is also called a solution. Air is a homogeneous mixture of nitrogen, oxygen, carbon dioxide and water vapour. These gases are physically mixed and not chemically combined. Alloys like brass ( copper and zinc ),steel (iron and carbon )are homogeneous mixtures. Other examples include cement, petrol, kerosene oil.

  2. Heterogeneous mixtures It is a mixture that consists of physically different parts, each with different properties. e.g. sodium chloride and iron filings, sulphur powder and iron filings, mixture of kerosene oil and water etc. In this mixture, particles of different components can be seen e.g. yellow coloured sulphur particles and black iron filings can be seen in a mixture of iron filings and sulphur powder. Iron filings can be separated from the mixture with the help of a magnet i.e. it can be separated by a simple physical method.

Matter ( Solid, Liquid, Gas )
{2}{c}{\textbf{Pure substances} }
{1-2}Elements & Compounds
Can not be broken down to simpler substances & Have fixed composition.Can be broken down into elements by chemical or electrochemical reactions
e.g. Cu,O2, N2, C & e.g. H2O, CH4, sugar, salt

 
Matter ( Solid, Liquid, Gas )
{2}{c}{\textbf{Mixtures} }
{1-2}Homogeneous & Heterogeneous
Uniform composition & No uniform composition
e.g. sugar in water,sulphur in CS2 & e.g. salt and sand,water in oil
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