What happens to a 25W 220V bulb when connected across 440V line?

The real-world answer is that the bulb glows with dazzling brilliance for a few

seconds and then burns out.

Electrically, the technical answer is:

Assuming we're working with a traditional incandescent bulb, the filament is designed

to dissipate 25W when connected to 220V. We could easily calculate the resistance of

the filament from that information, but for this discussion, it doesn't matter. It's just 'R'.

The Power dissipated by anything is V2/R

So if the voltage across the device is doubled, the power dissipation increases by

a factor of 4 . Your incandescent bulb begins to dissipate 100w instead of 25w.

Refinement:

The resistance of any metallic conductor increases when the temperature of the

conductor rises. The temperature of the filament certainly increases when its

power dissipation increases.

So 'R' increases, and the dissipation settles at something a little less than 100

watts. But have no fear ... at 100% over-voltage, your bulb will definitely blow.