How does temperature affect pressure inside a bottle for a water rocket?
The water will expand somewhat when the temperature rises and therefore contribute slightly to the increase in pressure. The majority of pressure increase, however, comes from the air in the bottle if it is only partially filled. You can roughly calculate the pressure increase using an equation based on the ideal gas law: delta_P*V = n*R*delta_T delta_P is the pressure difference (Unit used: atmospheres, atm) V is the volume (Unit used: Liter, L) (only the part of the bottle filled with air counts) n is the amount of substance (loosely number of moles of gas) R is the gas constant (Unit and number used: 0.0821 L·atm·mol-1·K-1 delta_T is the thermodynamic temperature difference (Unit used: Kelvin).
