Preparation of Silver Tetraiodomercurate(II) for a demonstration.
Silver Tetraiodomercurate(II) is a quite special compound, since it’s thermochromic. It changes color from yellow to orange/red when heated: http://labphoto.tumblr.com/post/111304710632/yesterday-i-posted-about-fluorescence
Silver Tetraiodomercurate (Ag2HgI4) is yellow at room temperature, but when it’s heated above 50 °C, it turns orange. Since the color change is reversible, when it is cooled back to room temperature, the color changes back to its original color.
The compound was prepared from mercury(II)-iodide (that red powder on the first picture) and some potassium iodide with a little silver nitrate. The silver nitrate was added to the solution of potassium tetraiodomercurate (K2HgI4, what was previously prepared from mercury-iodide and potassium iodide). The addition of the silver salt caused the immediate precipitation of the yellow colored silver tetraiodomercurate (as seen on the gif).
What happened? This:
HgI2 + 2 KI + 2 AgNO3 –> Ag2HgI4 + 2 KNO3