This is called a dynamic equilibrium. Therefore, the reaction is constantly going, but the amount of the reactants and products stay the same for they are both reacting at the same rate. So the answer is 4).
Freezing point is the temperature at which a substance undergoes a phase transition from liquid to solid state. Water converts from liquid state to solid ice at a temperature of
.
Mass is the amount of matter contained in a substance. The SI unit of mass is kilogram. 1 kilogram is equal to 1000 g. One kilogram is defined as the mass of one cubic decimeter of water at the freezing point of water.