As physical changes do not change the chemical composition of any substance it is known as physical change in the same manner stretching of a rubber is a physical change because it doesn't change its chemical properties or composition.
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Stretching of a rubber a physical change because the streching doesn't change the chemical composition of rubber. rubber are made up of cross-linked polymer,long-chain molecules connected to each other that initially exist in  a collide state . stretching 
these chains in order to lengthen  and align them in the direction of the applied force , we see that no chemical change has happened because if we will leave it we will see tht it comes back to his original shape. so it is concluded that streching of rubber is a physical change .
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As the physical state of substance remained the same & bcoz No New Substance is formed.
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