Why does a plastic bottle deformed when we add hot water to it?

Akhilesh actually plastic bottle is made of special type of  polymer i.e Polyethylene terephthalate. This polymer comes into the category of thermoplastic. Such, thermoplastic nature of polymer  tends to have the property to deform on heating and  harden on again cooling due to presence of long chain with the absence of cross linking. This absence of cross-linking between the molecules make the bottle material to get easily effected by the heat and material become weak due to breaking of bond in the materials and deforms.

Note- You will study more about polymer, cross linking  and thermoplastic in higher classes.