Question

Why are soft drink bottles not completely filled?

Answer 1

Soft drinks are actually flavoured carbonated water. So the pressure within the bottle is higher than the pressure outside. If the bottle is completely filled then pressurized CO2 may break the bottle when shaked vigorously. So some space is left to allow the CO2 to get some space by compressing air.

Answer 2

Soft Drink bottles are filled with Carbon Dioxide which is responsible for the 'fizz' in it .Carbon dioxide is added to increase the pressure in the bottle which is also responsible for its preservation.