Elements, Compounds and Mixtures

According to the chemical classification of matter there are two main categories:

• Pure substances

• Mixtures

Lavoisier, a French chemist on the basis of quantitative studies classified pure substances into elements and compounds. He showed that when we heat mercuric oxide it changes into mercury and oxygen.

Mercuric oxide is a compound because it decomposes into simpler substances, whereas mercury and oxygen cannot be further decomposed into anything simpler as they are elements.

Element

It is defined as a substance that cannot be further reduced to simpler substances by ordinary processes. Elements are made up of particles/atoms of only one kind.

For example: Hydrogen and oxygen

There are 114 elements known. Out of these 92 of them occur in nature.

Metals and non-metals

Elements can be further divided into metals and non-metals.

Metals

They are generally solids with characteristics such as hardness, malleability, ductility high tensile strength, lustre and ability to conduct heat and electricity.

For example: Copper, iron, zinc etc.

Non-metals

They are generally non-lustrous, brittle, poor conductors of heat and electricity.

For example: Sulphur, phosphorus, nitrogen etc.

Metalloids

These elements have characteristics common to metals and non-metals.

For example: Arsenic, tin, bismuth etc.

Compound

It is a pure substance that can be decomposed into simpler substances by some suitable chemical technique. A compound is formed by combination of two or more elements in a definite proportion.

For example, water is a compound of hydrogen and oxygen elements present in the ratio of 1: 8.

Properties of compounds

• A compound cannot be separated into its constituents by mechanical or physical means.

For example, if we bring a magnet near a sample of iron sulphide, the iron present in the iron sulphide cannot be separated.

• Properties of a compound differ entirely from those of its constituent elements.

For example, water is made up of hydrogen and oxygen. However, the properties of hydrogen and oxygen (both gases) are different from water (liquid). Hydrogen is combustible, oxygen is a supporter of combustion whereas water (made up of both hydrogen and oxygen) puts out a flame.

• Energy changes are involved in the formation of a compound. For example, iron and sulphur reacts only when heat is supplied.

• The constituent elements in a compound are in a fixed proportion by weight. In water, hydrogen and oxygen are present in a fixed ratio of 1:8 by weight.

• A compound is a homogenous substance. That is it is same throughout in properties and composition.

• A compound has a fixed melting point and boiling point. For example, ice melts at 0^oC.

Mixture

A mixture is a material containing two or more elements or compounds that are in close contact and are mixed in any proportion. The components of a mixture can be separated by physical means.

For example, air, gun powder, etc.

Properties of a mixture

• A mixture may be homogenous or heterogeneous. A homogenous mixture has a uniform composition throughout its mass. For example, sugar or salt dissolved in water, alcohol in water, etc. A heterogeneous mixture does not have a uniform composition throughout its mass. There are visible sharp boundaries.

For example: Oil and water, salt and sand, etc.

• The constituents of a mixture can be separated by physical means like filtration, evaporation, sublimation and magnetic separation.

• In the preparation of a mixture, energy is neither evolved nor absorbed.

• A mixture has no definite melting and boiling point.

• The constituents of a mixture retain their original set of properties. For example, sulphur dissolves in carbon disulphide and a magnet attracts iron filings.

Types of mixtures

Matter	Mixture type	Example
Solid	Solid mixture	Iron filings and sulphur
Solid	Liquid mixture	Common salt and water
Solid	Gas mixture	Air entrapped in soil
Liquid	Gas mixture	Oxygen dissolved in water
Gas	Gas mixture	Air containing hydrogen, oxygen, nitrogen, carbon dioxide etc.
Liquid	Liquid mixture	Water and alcohol

Differences between mixtures and compounds

Mixtures	Compounds
A mixture can be separated into its constituents by physical processes (filtration, evaporation, sublimation, distillation)	A compound cannot be separated into its constituents by physical processes. It can be separated by chemical means
A mixture shows the properties of its constituents	A compound has a new set of properties different from its constituents
Composition of a mixture varies and the constituents are present in any proportion by weight. It does not have a definite formula	The composition of a compound is fixed and the constituents are present in fixed proportions by weight. It has a definite formula
The constituents do not react chemically, thus no energy changes take place	Chemical reactions take place and energy changes in the form of heat and light are involved
A mixture does not have a fixed melting point and boiling point Examples: air, sand and salt	A compound has a fixed melting point and boiling point Examples: H2O (water), FeS (iron sulphide)

 

Water is a compound and not a mixture

• The components hydrogen and oxygen cannot be separated by physical methods such as filtration, evaporation.

• Hydrogen and oxygen are present in a fixed proportion of 1: 8.

• Energy changes accompany the formation of a compound i.e., heat and light are given out.

• Properties of water are entirely different from the constituents, hydrogen and oxygen.

• The boiling point of water is 100^oC at 76 cm Hg i.e., one atmospheric pressure.