A couple of weeks ago, Chris and I participated to a science fair at a local secondary school. We had some plastic tubes containing a red solution on our table. Students were curious about these and asked what it was… Their best guess was usually “Blood” although two suggested “Ribena”. When we were explaining that it was “gold” they were shocked. And gold it was, in the shape of very small particles of a few nanometers dispersed in water. Because of their small size, they have very different optical properties than bulk gold and a suspension of these appear red.
Picture: Synthesis of gold nanoparticles (reduction of a gold salt by sodium borohydride)