When steel is heated, what is the first physical change it undergoes?

When steel is heated, the first physical change it undergoes is that it expands and loses strength. As the temperature increases, the atomic structure of the steel begins to vibrate more vigorously, which causes the atoms to move further apart from one another. This expansion is a characteristic behavior of most materials when subjected to heat.

In terms of strength, heating steel reduces its hardness and makes it more ductile. This loss of strength occurs because the heat energy disrupts the bonds between atoms and alters the microstructure of the steel, leading to a reduction in its mechanical properties. It is particularly important for firefighters to understand this change, as expanding and weakening steel can influence the safety and integrity of structures during a fire.

Unlike the other choices, which describe different phenomena, the expansion and loss of strength occur first as a direct consequence of heat application. It is critical for those in firefighting and rescue operations to recognize these changes to make informed decisions about structural safety and potential hazards.