When matter changes state, which property remains unchanged?

• A. Energy
• B. Temperature
• C. Chemical composition
• D. Density

Answer: (C)

During a phase change, the substance’s chemical composition stays the same. The molecules are unchanged chemically (for example, ice, liquid water, and steam are all H2O); only the arrangement and energy of the molecules are altered. Latent heat is involved—the energy absorbed or released changes the state but not the identity of the material. Temperature may remain constant at the phase-change temperature for a pure substance, but that doesn’t affect the fact that the composition remains unchanged. In contrast, properties like energy, temperature behavior, and density can change with different phases, so they aren’t the invariant feature here.