Anything that occupies space and has mass is matter. Matter may exist as solid, liquid or gas. Matter can be classified in several ways. Three important ways in which matter can be classified is
Based on the particle nature.
Based on physical states.
Based on chemical composition.
Particle nature of
matter
The great Indian Philosopher ‘Kanada’ of Vaiseshika Siddhanta
(Metaphysics) proposed that matter is composed of very small, tiny particles
called Anus and Paramanus (molecules and atoms in the modern concept). Greek
philosophers Democritus and Luccipus are credited with the atomic theory of matter.
However, it is John Dalton, a British teacher who is regarded
as the father of modern atomic theory. Atoms are tiny particles which cannot be
further broken down. Atoms join together to form matter. A molecule is a group of
atoms which is capable of independent existence.
Physical states of
matter
Solids
They are rigid which means intermolecular distances or
spaces are fixed and they are the least.
Molecules are closely packed and have maximum attraction.
They cannot move or flow.
They have a definite shape and size.
They cannot be compressed easily and appreciably without
breaking.
A solid expands or contracts very little on heating or
cooling. E.g. Book, chair, wood, stone etc.,
Liquids
They are not rigid
They have a definite volume.
They do not have higher and lower levels.
The force of attraction between the molecules is weak.
They can be compressed to a small extent.
A liquid expands or contracts more than a solid, on being
heated or cooled.
The intermolecular distances or spaces are not fixed and they
are more than in solids. For example, water, milk, and honey.
Gases
They are not rigid.
They have neither a definite shape nor a definite volume. They
assume the shape of the container.
They can flow easily in all directions.
They can be compressed to a large extent easily.
Intermolecular distances or spaces are varying and they are large
distances.
Molecules in gases have the least attraction.
A gas expands or contracts to a large extent on being heated or
cooled.
Classification of
matter based on chemical composition
Based on chemical composition matter is classified into
elements, compounds and mixtures.
Elements
The simplest form of a pure substance which cannot be further
divided into another simple is known as an element. It is made up of only one kind
of atom.
For example: Iron, copper, gold, oxygen, lead, nitrogen,
silver. Elements are basic units or building blocks of a variety of substances.
About 114 elements are known today, out of which 92 are found in nature.
The rest of the elements are artificially made by man.
Some elements exist in a solid state, e.g. gold, silver,
copper, and aluminium. Some elements do exist as liquids, for example, bromine and mercury.
Some few exist as gases, e.g. nitrogen, oxygen, and helium.
Elements can be conveniently classified as,
Metals
Non-metals
Metalloids
Inert gases or noble
gases
Symbols for elements
To denote or identify the elements some simple rules are
followed.
Usually, an element is denoted by the first letter (in English
or Latin language) by a capital letter. For e.g., nitrogen and carbon have
symbols N and C.
When the first letter is the same, as it happens for several elements
two letters are taken, the first letter being a capital letter followed by
another alphabet as a small letter.
E.g. Cadmium – Cd
Magnesium – Mg
Chromium – Cr
Calcium – Ca
Manganese – Mn
Compounds
Substances made up of atoms of two or more different elements
combined chemically in a fixed ratio. For example, water molecules are made up of two
different elements hydrogen and oxygen, with two hydrogen atoms and one oxygen
atom thus water contains the elements hydrogen and oxygen in a fixed ratio of 2:1. It
is denoted by the formula H2O.
Similarly, many substances like carbon dioxide, sulphur
dioxide, acids, bases and salts are compounds.
How to distinguish a
compound from elements
Following are the
simple rules
If a pure substance cannot be broken down into two or three
different atoms (instead of the same kind of identical atom), by heat, light or
electricity; then it is an element.
On the other hand, if a pure substance can be broken down into
two or more simpler substances by applying heat, light or electricity; then it
will be a compound. This can be demonstrated by a simple experiment. A pinch of
sugar is taken in a test tube and heated.
It can be found that the sugar is charred which is nothing
but a form of carbon an element and water vapor condenses on the walls of the
test tube. Therefore, it can be concluded that the sugar has been decomposed
into carbon, a charred mass and water vapour. Thus sugar cannot be an element
but a compound.
Mixtures
Elements and compounds are pure substances. Many substances
which we see around us are not pure substances. They are in many cases
mixtures.
A mixture is a substance that consists of two or more
elements or compounds which are not chemically combined together. For example, the air
is a mixture of several gases like nitrogen, oxygen, helium (elements), Carbon
dioxide, water vapour, and sulphur dioxide (compounds). E.g. Seawater, lime water,
muddy water, kerosene, brass, soil, sugar solution, wood, and alloys.
Types of mixtures
The mixtures can be of several types given as follows:
Gas in the Gas mixture – e.g. air
Gas in the Liquid mixture – e.g. soda water (carbon dioxide in
water)
Liquid in the liquid mixture – vinegar (acetic acid in water),
kerosene in water, and alcohols in water are examples.
Solid in Liquid mixture – e.g. sugar solution (sugar in
water), salt solution (salt in water)
Solid in Solid mixture – e.g. soil, mixture of spices, alloys
of metals (steel, brass, bronze, lead-tin solder)
Solid in Gas mixture – e.g. smoke (dust or solid carbon
particles suspended in gaseous air).
There is, yet, another way of classifying mixtures as
homogeneous and heterogeneous mixtures.
Homogeneous mixture
If one constituent spreads uniformly into another constituent
without being identified or noticed, it is a homogeneous mixture. E.g. sugar
solution, salt solution.
Heterogeneous mixture
If the constituents are unevenly distributed and can be
physically identified or noticed, it is a heterogeneous mixture. E.g. sand in
sugar, salt in sugar.
It is to be remembered that generally in a mixture the
constituents will not be in a fixed ratio, except in certain mixtures, like solid
solutions (alloys).
The mixture can be separated into its constituents easily by
physical methods like filtration, evaporation, sublimation and distillation.
Differences between
mixtures and compounds
|
Compounds |
Mixtures |
|
A compound is a
pure substance. Its composition is always the same. It contains only one type
of molecule. |
A mixture is not a
pure substance, its composition can vary and it contains different types of molecules. |
|
A compound
contains the constituent elements in a fixed ratio. |
The constituents
of a mixture may present in any ratio. |
|
It is homogeneous. |
It is usually
heterogeneous. |
|
It exhibits
definite properties. |
Its properties are
variable. |
|
Energy is absorbed
or released during the formation of compounds. |
Energy is neither
absorbed nor released. |
|
Its components can
be separated by chemical methods. |
Its components can
be separated by physical methods. |
The modern concept of atom
and molecule
Now atoms can be defined as the smallest part of an element
which may or may not have independent existence, but always takes part in
chemical reactions. The molecule is now considered the smallest unit of a
compound or of an independently existing pure substance.
E.g. CO2, H2O, H2SO4
are molecules of compounds.
O2, N2, and H2 are molecules of
elements.
Atomicity
Number of atoms present in a molecule is called atomicity.
For e.g., Hydrogen cannot exist as an independent atom. It has to exist as a
molecule consisting of two hydrogen atoms. (H2). Hence its atomicity
is 2, diatomic. Similarly in some compounds, 3 atoms are present. They are
triatomic for instance, Carbon dioxide (CO2), Ozone (O3).
Some elements like Helium (He) and argon (Ar) can exist as a single atom. Their
atomicity is 1, monoatomic. Many compounds do exist as polyatomic (more than 3
in a molecule). E.g. Methane (CH4) and ethylene (C2H4).
Valency
According to the old concept, valency is the combining capacity
of an element with the other atoms is called the valency of the element. It is
defined as the number of hydrogen atoms that combine with or can be displaced
by one atom of the element. Therefore, the valency of oxygen is 2 and the
valency of hydrogen is taken to be 1.
Modern concept
It is defined as several electrons either lost or gained by
an atom of an element.
Ion
Atom is basically neutral. If it loses an electron or gains
an electron it becomes a charged particle called ‘Ion’. When an atom of an
element loses electrons, the positive charges in the nucleus outnumber the
negative charges outside the nucleus. Hence it becomes a positively charged ion
called Cation.
When an atom gains electrons it becomes as negatively charged
as the number of negative charges outnumber the positive charges in the
nucleus. It is called anion.
Radical a compound
Usually, a compound has two parts head and a tail for instance H2SO4. Hydrogen and sulphate are called radicals. Simple radicals have only single atoms like Na and CI.