What occurs to the molecules of iron in a sword when it is heated until it glows red hot?

When iron is heated until it glows red hot, the correct interpretation is that the iron atoms are vibrating quickly in place. Heating a solid like iron provides energy to the atoms, increasing their kinetic energy. As a result, the atoms vibrate more rapidly while remaining in their fixed positions within the solid structure. This increased vibration is what causes the iron to glow red, as the energy is emitted in the form of light due to thermal excitation.

Although a significant amount of heat could potentially lead to the iron melting, reaching the liquid state requires temperatures above the melting point of iron, which typically occurs at around 1,538 degrees Celsius (2,800 degrees Fahrenheit). At lower temperatures, such as those where iron glows red hot, it remains in a solid state, and the primary change is the increased vibrational motion of the atoms. Hence, the assertion that the iron atoms are vibrating quickly in place accurately describes the behavior of the iron at that temperature.