A pure substance is a single substance and made up of one kind of particles. The particles may be either atoms or molecules.
A pure substance is uniform or homogeneous throughout because it consists of only one kind of particles.
These particles are similar to one another and cannot be separated by physical method.
There are two types of pure substances
The pure substance which is made up of one kind of atoms is called an element. Examples Hydrogen, Carbon, Oxygen etc.
TYPES OF ELEMENTS
Elements can be classified into Metals, Non-metals and metalloids on the basis of their properties.
1. Metals are solid at room temperature except mercury, which is a liquid at room temperature.
2. Metals are malleable which means that they can be hammered into thin sheet. Example Gold, Silver
3. Metals are ductile i.e. they can be drawn into thin wires. Gold is most ductile followed by silver, copper and aluminium.
4. Metals have high tensile strength. Sodium and potassium are quite soft hence have low tensile strength.
5. Metals are good conductors of heat and electricity. Silver is the best conductor and lead is the poorest conductor of heat.
silver is the best conductor of electricity followed by copper, gold, aluminium and tungsten.
6. Metals have generally high density.
7. Metals are sonorous i.e. they produce a ringing sound when hit.
8. Metals are generally hard. Sodium and potassium are quite soft and cut with a knife.
9. Metals have lustre i.e. they have shining surfaces. Silver has a white shining surface and gold has yellow shining surface.
10. Metals have generally high melting and boiling points, but sodium and potassium have low melting and boiling points 371 K and 336 K respectively.
1. Non-metals are solids, liquids and gases at room temperature. Bromine is liquid at room temperature. Boron, Carbon, sulphur and phosphorous are solids. Hydrogen, oxygen, nitrogen, etc. are gases.
2. Non-metals are brittle. They break into pieces when hammered.
3. Non-metals are bad conductors of heat and electricity. Graphite an allotropic form of carbon is good conductor of electricity.
4. Non-metals are generally soft. Diamond an allotropic form of carbon is the hardest natural substance known.
5. Non-metals have comparatively low melting and boiling points except boron, graphite and diamond which have high melting and boiling points.
6. Non-metals have low tensile strength i.e. they are not strong and their bonds break easily.
7. Non metals have no lustre. Iodine has a shining lustre and crystalline structure.
Elements which have properties of both metals and non-metals are called metalloids or semi metals. Their intermediate conductivity makes them semiconductors, Ex. silicon, germanium, arsenic, antimony, tellurium.