How are the particles or molecules of a fluid behaving when the fluid is heated?

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a ... A liquid is made up of tiny vibrating particles of matter, such as atoms, held ... Unlike a solid, the molecules in a liquid have a much greater freedom to move. Liquids generally expand when heated, and contract when cooled.

The mass has escaped into a gas which you can not see so the law stays true.