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ICSE Class 8 Answered

how is the formation of water by combining hydogen and oxygen a chemical change.I think it is a physical change because water can be changed to hydrogen and oxygen by the analysis.  
Asked by ajishvpm | 23 Jun, 2018, 05:25: PM
answered-by-expert Expert Answer
Lets first clear the idea of physical change and chemical change with respect to water
 
Physical changes of water: 
1. Boiling of water
2. Freezing of water
3. Melting of ice 
 
In a physical change, the state of water changes not the constituents present in it like hydrogen and oxygen and this a reversible change.
 
When water boils it evaporates as a steam ( water in a gaseous or vapour state) which has same constituents as water i.e., H2O. On boiling water, it does not form Hydrogen H2 and oxygen O2 hence boiling of water is a physical change not chemical.
 
Chemical change of water:
This change is related to the electrolysis of water.
When a current is passed through the acidified water it decomposes water H2O to form the Hydrogen H2 and oxygen O2.
Hydrogen is collected at the cathode (negative electrode). Oxygen is collected at the anode (positive electrode).
Now, in this case, water changes to hydrogen and oxygen that means the constituents of water H2O are break down to form molecular Hydrogen H2 and oxygen Oand this reaction is not a reversible reaction 
 
The reaction takes place as:    
 

           2H2O            2H2  ↑   +     O2

  Acidulated water                            Hydrogen     Oxygen

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