science working models for class 9 chemistry

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i need to make a project on any principle of chemistry.
the requirements are that it should be a working model with a suitable activity written below it.
I need the ideas to make a running model based on any principle of chemistry.
PS:if you can manage to give me and idea where materials found easily at home can be used i would be very grateful.
this is for a pproject in ninth grade...
thanx