Which of the following is not a phase change?
melting of snow
evaporation of water from the ocean
condensation of water vapor to form clouds
hydrogen and oxygen gas combining to form water vapor

hydrogen and oxygen gas combining to form water vapor is the correct answer.

Answer : The correct option is hydrogen and oxygen gas combining to form water vapor.

Explanation :

Phase change : It is a change from one state to another state without changes in chemical composition.

Phase change such as solid to liquid, liquid to vapor, vapor to liquid, liquid to solid, vapor to solid, solid to vapor.

Melting of snow is a fusion process in which phase changes from solid to liquid.Evaporation of water from the ocean is a evaporation process in which phase changes from liquid to vapor.Condensation of water vapor to form clouds  is condensation process in which phase changes from vapor to liquid.Hydrogen and oxygen gas combining to form water vapor, there is no phase change in this reaction because a gas combine with another gas to form a water vapor as a gas.