Pure Substances – (A) Elements
WWhen it is said that something is pure, it means that all the constituent particles of that substance are the same in their chemical nature. For ex. N2, O2, H2, Hg, sugar, common salt, water, iron, copper etc.
Thus, a pure substance consists of a single type of particles.
How can we identify a pure substance?
The purity of a substance can be tested by checking its melting / boiling point. A pure substance has a fixed melting point or boiling point at constant pressure. The melting and boiling point of a substance will change if it contains even a tiny amount of another substance.
- A pure substance may be solid, liquid or gas.
- A pure substance represents a single substance.
– A material containing only one kind of particles is considered as a pure substance (element).
Example – hydrogen (H2), oxygen (O2) etc.
– A substance made up of two or more different kinds of particles combined together in a fixed ratio by weight is also considered as a pure substance (compound).
For e.g. sodium chloride is made of sodium and chlorine even then it is one substance because its chemical constituents cannot be separated by any physical processes. Similarly, sugar or water is a substance because it contains only one kind of pure matter and its composition is same throughout. But a solution of salt or sugar in water is not a pure substance as it contains two kinds of particles that can be separated by evaporation.
Pure substances can be further classified as elements and compounds, on the basis of their chemical composition.
(A) Elements –
Robert Boyle was the first scientist to use the term element. Antoine Laurent Lavoisier, a French chemist was the first to establish an experimentally useful definition of an element.
“It is a basic form of matter which can neither be broken down into simpler substances nor be formed from two or more simpler substances by any physical or chemical process. ”
(a) Oxygen is an element. Each oxygen molecule consists of two atoms of oxygen.
(b) An iron nail is made of atoms of only one kind.
- Elements are represented by symbols. For e.g. hydrogen – H, oxygen – O, calcium – Ca, Chlorine -Cl.
Elements are of three types. They are metals, non metals and metalloids.
(1) Metals –
example – gold, silver, iron, copper, sodium, potassium etc.
(i) Metals are generally solid at room temperature and silvery-grey or golden-yellow in colour.
- Exception – Mercury is only metal that exists in liquid state at room temperature.
(ii) When metals are in pure state or freshly prepared, they show shining surface called metallic luster.
(iii) Metals are generally hard.
(iv) Malleability is a property by which some metals can be beaten into thin sheets. Gold and silver are the most malleable metals.
(v) Metals can be drawn into thin wires without breaking. This ability of metals is known as ductility. e.g. copper, aluminium wires.
- Silver foil used for decorating sweets, aluminium foil used for wrapping food and medicines, show malleable nature of silver and aluminium metal.
Mnemonic – Drawn into wires -Ductility
(vi) Metals are good conductor of heat and electricity.
(vii) Metals produce a sound (sonorous) when hit with a hard object. This property (sonorous nature of metals) is called sonorousness or sonority.
(2) Non-metals –
example – hydrogen, oxygen, iodine, carbon, bromine, chlorine etc.
(i) Non-metals exists in all three physical state.
- Carbon, sulphur, phosphorous etc. are solids.
Oxygen, nitrogen, chlorine etc. are gases whereas bromine is a liquid.
(ii) Non- metals are not lustrous, sonorous, ductile or malleable.
(iii) They are generally soft materials.
(iv) Non-metals cannot be beaten into sheets. They are brittle, break into pieces when hammered.
(v) Non-metals are generally bad conductor of heat and electricity.
(vi) They generally possess low melting and boiling points.
♦ Carbon exists in different physical forms such as diamond, graphite etc. are called allotropes.
♦ The most abundant metal in the earth’s crust is aluminium (7%).
♦ The most abundant non-metal in the earth’s crust is oxygen (50%).
♦ Most elements are solids, two (mercury and bromine) are liquids and eleven are gases.
♦ Gallium and caesium becomes liquid at a temperature (30oC) which is slightly above room temperature.
(3) Metalloids –
e.g – boron, silicon, germanium etc.
Metalloids are elements which show intermediate properties or some properties of metals and some properties of non metals.
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(B) Pure Substances -Compounds