On the basis of their chemical composition, substances can be classified either as elements or compounds.
Antoine Laurent Lavoisier defined element as the simplest or basic form of matter that cannot be broken into simpler substances by chemical methods.
OR
A pure substance which is made up of one kind of atom only is called an element.
The majority of the elements are solid. Eleven elements are in gaseous state at room temperature.
Two elements are liquid at room temperature, mercury and bromine.
(1) Metals
Nearly 70 elements belong to a particular class known as metals. In the metals, the atoms are very closely packed together and have special types of bonds known as metallic bonds. Because of very tight packing, the metals are quite hard. Cesium and gallium are two elements which liquify slightly above room temperature.
(2) Non-metals
Nonmetals as the name suggests are opposite to metals which means that their properties are quite different from the metals. Only about 22 elements are nonmetals.
(1) They are generally silver-grey. However, some metals or their alloys have a golden yellow colour.
(2) Metals have a lustre, the freshly cut surface has a shine on it.
(3) They easily conduct heat and electricity.
(4) They are malleable i.e. they can be beaten into sheets.
(5) They are sonorous.
For example, Gold, silver, copper, iron, sodium, potassium, etc. Mercury is the only metal that is liquid at room temperature.
(1) They exist in solid, liquid and gaseous states.
(2) They display a variety of colour.
(3) They are generally neither malleable nor ductile.
(4) They are poor conductors of heat and electricity.
(5) They are not sonorous.
For example, Hydrogen, oxygen, iodine, carbon, etc.
Some elements have intermediate properties of the metals and nonmetals. The elements which exhibit the properties of metals as well as nonmetals are called metalloids. For example, Boron, Silicon, Germanium, etc.
A pure substance, which is composed of two or more elements, combined chemically in a definite ratio by mass, such that it can be broken into elements only by chemical means, is called compound. The two or more elements present in a compound are called constituents or components of the compound. For example, water is a compound of hydrogen and oxygen, combined together in the ratio of 1 : 8 by weight. The water can be broken into its constituents only by electrochemical method i.e. by passing electric current through it. The compounds can be further classified as acids, bases and salts.
Group activity to differentiate between mixtures and compounds.
(a) Divide the class into two groups. Give 5 g iron filings and 3 g of sulphur powder in a china dish to both the groups.
(b) Group-I: Mix and crush iron filling and sulphur powder.
(c) Group-II: Mix and crush iron filings and sulphur powder. Heat this mixture strongly till red hot. Remove the flame and let the mixture cool.
(d) Group-I and II: Check for magnetism in the material obtained. Bring a magnet near the material and check if the material is attracted towards the magnet.
(e) Compare the texture and colour of the material obtained by the groups. Add carbon disulphide to one part of the material obtained. Stir well and filter.
(f) Add dilute sulphuric acid or dilute hydrochloric acid to the other part of the material obtained.
(a) (i) The material obtained by students of group I is a heterogeneous mixture, i.e. when we examine the mixture under a microscope, it is found that though the iron particles lie very close to the sulphur particles, at some places there are more of iron particles whereas at other places, there are more of sulphur particles.
(ii) On the other hand, the material obtained by students of group II is a compound which altogether has different properties as compared to those of its constituents i.e. iron filings and sulphur.
Iron + Sulphur = Iron sulphide
(Element) (Element) (Compound)
(iii) If we examine iron sulphide under a microscope, we find that the composition, texture and colour of the compound(black) is the same throughout.
(b) Material obtained by group I contains iron filings which have magnetic properties. Alternatively, material obtained by group II is a compound which does not contain iron filings and hence does not have magnetic properties.
(c) The material obtained by students of group I is a mixture and hence its components can be separated by simple physical methods such as a magnet or carbon disulphide.
If we put a magnet in the mixture of iron filings and sulphur powder, the iron particles are attracted by the magnet, they stick to the magnet and get separated from sulphur.
Alternatively, if we shake the mixture of iron filings and sulphur powder with carbon disulphide (CS2), sulphur dissolves in it but iron filings do not. On filtration, iron filings are obtained as a residue on the filter paper while sulphur is recovered from the filtrate by evaporating carbon disulphide.
(d) Material obtained by students of both group I and group II on treatment with dilute sulphuric acid or dilute hydrochloric acid evolves a gas but the gas evolved is different in each case.
When dilute sulphuric acid is added to the material (mixture of iron filings and sulphur powder) obtained by students of group I, the iron part reacts with dilute sulphuric acid to produce hydrogen gas (H2) which is a colourless, odourless and combustible gas. When dilute sulphuric acid is added to the material (iron sulphide) obtained by students of group II, hydrogen sulphide gas (H2S) is obtained. It is a colourless gas having the smell of rotten eggs.
A mixture shows the properties of its constituents, but the properties of a compound are altogether different from those of its constituents.
(Session 2025 - 26)