any ideas for a science working model for science exhibition???? i'm in 9th

You can make:

1) Solar Powered device (battery etc.)

2) Build a small Wind mill and show it produces electricity

3) Hydro-electricity

4) You can make a model in which you will be showing industries discharging sewage in rivers by showing by some black colour water flowing into a river using a motor. You can also show a chimney emitting smoke by keeping some dhoopbatti inside it and moreover you can show some speakers producing unpleasant sound using a cell or a battery.

Hope this helps.