Answer:
When thermal energy is added to a substance at its melting or boiling point, the temperature of the substance remains constant as the added energy is used to break intermolecular bonds and cause a phase change from solid to liquid or from liquid to gas. In contrast, adding thermal energy at any temperature other than the melting or boiling point will cause the temperature of the substance to increase, as the added energy increases the kinetic energy of the particles, causing them to move faster and collide more frequently. At other temperatures, the intermolecular forces between the particles are not strong enough to cause a phase change until the substance reaches its melting or boiling point.