When copper is heated by a Bunsen burner, the particles of copper gain thermal energy and start to vibrate more rapidly. As the temperature increases, the particles move faster and faster, and the copper begins to glow. This is because the electrons in the copper atoms are excited to higher energy levels, and they emit light as they return to their original energy levels.

At the same time, the increased temperature causes the copper atoms to expand, and the density of the copper decreases. This is why copper floats on water when it is heated to a high enough temperature.

Finally, the increased temperature also causes the copper to soften, and it becomes easier to bend and shape. This is why copper is often used in jewelry making and other metalworking applications.