When a substance undergoes a phase change, which of the following is true?

When a substance undergoes a phase change, such as melting, boiling, or condensing, the temperature remains constant while heat is added or removed. This phenomenon occurs because the energy that is being added or removed from the substance during a phase change is used to overcome intermolecular forces rather than increasing or decreasing the temperature of the substance itself.

For example, when ice melts to water, the temperature of the ice-water mixture remains at 0 degrees Celsius until all the ice has melted. Similarly, during the boiling of water, the temperature remains constant at 100 degrees Celsius (at 1 atm pressure) until all the water has turned to steam. This characteristic is fundamental in thermodynamics and is described by the concepts of latent heat, where heat is absorbed or released during phase changes.

In contrast, during temperature increases or decreases, such as when a substance is heated without undergoing a phase change, the temperature rises. However, during phase changes, the added energy goes into changing the state of the substance rather than raising its temperature.