Theory:

Pure substances are those that are completely made up of one form of particle. Therefore, physical processes cannot distinguish pure substances from other forms of matter.
Example:
Salt, sugar, oxygen, copper, iron etc.,
Based on the kind of atom, we classify pure substances as
  • Elements
  • Compounds
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Compounds: It is a form of matter created by combining two or more elements in a specific mass ratio. To decompose it into its constituent components, we use chemical methods.
Water\(H_2O\), oxygen \(O_2\), nitrogen dioxide \(NO_2\), salt \(NaCl\), and so on.
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Elements are the fundamental substances that cannot be separated into simpler substances by any chemical methods. All the elements in the periodic table is an example for elements — such as Oxygen, iron, etc.
In \(1661\), Robert Boyle became the first scientist to use the term 'element'.
 
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Robert Boyle
 
A French chemist named Antoine Laurent Lavoisier \(1743-1794\) was the first to define an experimentally useful description of an element.
 
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Antoine Laurent Lavoisier
According to Antoine Laurent Lavoisier, an element is a fundamental type of matter that cannot be broken down into simpler substances by chemical reactions.
Based on the physical and chemical properties, the elements are classified as
  • Metals
  • Non-metals
  • Metalloids
Properties of metals:
1. Malleable (can be drawn into thin sheets)
 
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2Ductile (can be drawn into wires)
 
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3. Lustre (shine) with silvery grey or golden yellow
 
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4. Sonorous (capable of producing a deep or ringing sound when hit)
 
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Example:
Gold, copper, silver, iron, aluminium, etc.
Properties of non-metals:
1. Non-metals come in a wide range of colours.
 
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2. They are bad conductors of heat and electricity and are known as insulators.
 
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3. They lack lustre, sonority, and malleability since they are brittle and soft in nature.
Example:
Oxygen, nitrogen, sulphur, and diamond, etc.
Properties of metalloids:
  • Substances that show the characteristics of both metals and non-metals are known as metalloids.
  • There are around \(8\) elements in the periodic table that are called metalloids.
Example:
Boron (B), Silicon (Si), Germanium (Ge), Arsenic (As), Antimony (Sb), Tellurium (Te), Polonium (Po).
Note:
1. Mercury (metal) and bromine(non-metal) are two elements that exist as liquids at room temperature.
2. Diamond is a non-metal which is hard and lustre in nature.
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Pure Substance
Element or Compound
Reason
Water \(H_2O\)Compound
Can be separated as
Water = Hydrogen + Oxygen
Lead \(Pb\)Element
Cannot be separated
Phosphorous \(P_4\)Element
Cannot be separated
Sodium Chloride \(NaCl\)Compound
Can be separated as
Sodium Chloride = Sodium + Chloride
Sulphur \(S_8\)Element
Cannot be separated