1. Color Change: Nickel undergoes a color change as the temperature increases. Initially, it appears silvery-white, but as it is heated, it gradually turns into a yellow-gold color.

2. Expansion: Like most metals, nickel expands when heated. This is due to the increased kinetic energy of its atoms, which causes them to vibrate more vigorously and occupy a larger volume. The expansion of nickel is relatively uniform and can be predicted using its coefficient of thermal expansion.

3. Phase Transformation: At a specific temperature known as the Curie temperature (approximately 355 °C or 671 °F), nickel undergoes a phase transformation. It changes from a ferromagnetic state, where the magnetic moments of its atoms are aligned, to a paramagnetic state, where the magnetic moments become disordered. This transition affects the magnetic properties of nickel.

4. Increased Strength and Hardness: Heating nickel can increase its strength and hardness, making it more resistant to deformation and wear. This property is utilized in various industrial applications, such as the manufacturing of tools, machinery components, and armor.

5. Oxidation: When heated in the presence of oxygen, nickel undergoes oxidation. A layer of nickel oxide forms on the surface, which can appear as a black or brown discoloration. The rate of oxidation depends on the temperature and oxygen availability.

6. Melting and Boiling Points: At extremely high temperatures, nickel eventually reaches its melting point (approximately 1,455 °C or 2,651 °F) and turns into a liquid. If the temperature is further increased, it will reach its boiling point (approximately 2,915 °C or 5,279 °F) and transitions into a gaseous state.

It's important to note that the specific changes observed in nickel upon heating may vary depending on factors such as the exact temperature range, heating rate, and surrounding environment.