Theory:

A compound is formed due to the chemical combination of two or more elements in a fixed ratio by mass.
The properties of the compounds vary from their constituent.
  
Example for compounds:
  • Water (\(H_2O\))
  • Backing soda (\(NaHCO_3\))
  • Common salt or Table salt (\(NaCl\))  
Where, water is composed of one oxygen atom and two hydrogen atoms in the ratio \(1 : 2\) by volume or \(8 : 1\) by mass.
 
Classification of compounds:
  
Compounds are classified into inorganic compounds and organic compounds based on the origin of chemical constituents.
  
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Solid compounds:
  
Compound
Constituent elements
Silica (sand)
Silicon, Oxygen
Potassium hydroxide (Caustic potash)
Potassium, Hydrogen, Oxygen
 
Sodium hydroxide (Caustic soda)
Sodium, Oxygen, Hydrogen
Copper sulphate
Copper, Sulphur, Oxygen
Zinc carbonate (Calamine)
Zinc, Carbon, Oxygen
  
Liquid compounds:
  
Compound
Constituent elements
Water
Hydrogen, Oxygen
Hydrochloric acid
Hydrogen, Chlorine
Sulphuric acid
Hydrogen, Sulphur, Oxygen
Acetic acid (vinegar)
Carbon, Hydrogen, Oxygen
  
Gaseous compounds:
 
Compound
Constituent elements
Carbon dioxide, carbon monoxide
Carbon, Oxygen
Sulphur dioxide
Sulphur, Oxygen
Methane
Carbon, Hydrogen
Nitrogen dioxide
Nitrogen, Oxygen
Ammonia
Nitrogen, Hydrogen
 
Difference between the state of solids, liquids and gases:
  
Properties
Solid
Liquid
Gas
1. Arrangement of particles Regular arrangementNo regular arrangementNo regular arrangement
2. VolumeDefinite volumeDefinite volumeNo definite volume
3ShapeDefinite shapeNon definite shapeNon definite shape
4. DensityAffected by densityAffected by densityNot affected by density
5. DiffusionMinimumMinimumMaximum
6. Force of attractionMaximumMaximumMinimum
7. Moment of particlesCannot moveSlowlyHighly moveable
8. CompressibilityNot compressibleHardly compressibleHighly compressible
Reference:
file:///C:/Users/user/Desktop/Books/8th_Science/TN/8th_Std_Term_I_Science_EM.pdf