Select the definition of diffusion. The gradual dispersal of one substance through another substance. The movement of a gas particle from one area to another. The escape of a gas through a small opening in a barrier into a region of lower pressure. The random motion of gas particles. Select the definition of effusion. The random motion of gas particles. The gradual dispersal of one substance through another substance. The escape of a gas through a small opening in a barrier into a region of lower pressure. The movement of a gas particle from one area to another.