水100度先會變蒸氣?

There is no doubt that the boiling point for water is 100 degree, but however, water do become steam when water temperature reaches close to the boiling point, before the water reaches 100 degree, some water molecules would vibrate enough to escape the water and become gas, namely the steam.

You can easily set up an experiment to see this happen (it is advice to be under a teacher supervision).

如果係正常大氣壓力下就係100度先會變蒸氣
如果壓力大過就會高溫d先變
壓力細就快d變

Yes, under 1 atmosphere pressure, water would boil and change its state to
steam at 100 degree C.

The higher the pressure it is, the higher the boiling point it is. Vice versa.