The graph depicts the temperature cures for two substances labeled as substance X and y Why does the temperature rem lain constant as heat is added to each substance in the regions labeled A to B, C to D, and A' to B, and C' to D'? Based on the curve for either substance X or substance Y and using what you know about the particulate nature of matter, and the relationship between temperature and potential or kinetic energy, describe why adding heat leads to the step-wise change in temperature depicted in the graphs.