中4 phy question

A student performs an experiment to find the specific latent heat of vaporization of water. A beaker containing water is placed on an electronic balance. the water is heated by a 100W immersion heater, which is immersed in the water such that it does not touch the beaker.

Q: It is found that there is a slight decrease in the mass of water in the beaker before the water boils. Explain briefly in terms of the molecular motion.