Smart Wire

A piece of nitinol wire with spiral shape is placed in Petri dish. In contact with hot water the wire changes its shape.

Materials
One 100 cm³ beaker, Petri dish.

Chemicals
Nitinol wire ~ 10 cm long, hot water.

Experimental

Take hot water in the beaker. Show the nitinol wire to the audience (the best way is to use camera and video projector). Change its shape into spiral as it show on Fig. 1 and place it in the Petri dish. Add hot water in the dish and observe what happens. You can repeat the experiment many times.

Fig. 1. Before heating

Fig. 2. After heating

Observation and Discussion

The smart wire is made of metal alloy of nickel and titanium, also known as nitinol. The two metals are present in roughly equal atomic percentages.
This alloys exhibit unique property known as shape memory which means that it has ability to undergo deformation at one temperature and then recover its original, undeformed shape upon heating above its transformation temperature.
In this experiment one can observe shape change of the wire from spiral to slightly bended shape.
The memory shape phenomenon is based on a reversible, solid-state phase transformation known as a martensitic transformation. The simple explanation is related to rearrangement of the atoms of Ti and Ni.
Namely, at high temperatures, the memory alloy has crystal structure referred to  austenite and at low temperatures it spontaneously transforms to a more complicated monoclinic crystal structure known as martensite. At room temperature, the alloy is fully martensite. When it is subjected to heating, austenite starts to form.
When martensite is subjected to heating it is reverted to austenite and the original austenitic structure is restored. During the fabrication of the nitinol wire the austenite phase is remembered.

Fig. 3. Phase transformation of Martensite and Austenite