Final answer:
Phase changes in substances are caused by the addition or removal of heat, which changes the temperature of the substance to its melting or boiling point. These phase changes occur at fixed temperatures for given substances and involve the absorption or release of energy described as latent heat.
Step-by-step explanation:
A substance undergoes a phase change when there is a transfer of heat, leading to an increase or decrease in the substance's temperature, reaching a point where it changes its state of matter. At low temperatures, substances are generally solid; as the temperature is raised, they transition to a liquid state, and further heating leads to a gaseous state. Conversely, cooling a substance can cause it to go from gas to liquid, and from liquid to solid.
The temperatures at which these changes occur are known as the melting point and boiling point. The melting point is where the solid becomes liquid, and the boiling point where the liquid becomes gas. These phase changes happen at fixed temperatures for a specific substance, provided that the pressure is constant. It's also important to note that the process of phase change involves the absorption or release of energy, which is described by the latent heat of the substance.
The precise relationship between heat and phase change is given by the equation Q = mL, where 'Q' is the heat absorbed or released, 'm' is the mass of the substance, and 'L' is the latent heat coefficient, which is characteristic of the substance and the specific phase change it undergoes.