Final answer:
A relevant observation when heating copper nitrate crystals in a test tube is that they experience a color change and decompose, similar to the behavior of copper sulphate when it loses water of crystallization, resulting in the solid becoming white and formation of water droplets inside the tube.
Step-by-step explanation:
When crystals of copper nitrate are heated in a test tube, a relevant observation is that they undergo a chemical change. Initially, the blue copper nitrate crystals may decompose, releasing gases and potentially changing color as they lose their water of crystallization, much like when copper sulphate is heated.
The blue color fades as the water is driven off, and the solid may turn into a brownish or black substance, indicating the formation of copper oxide. Additionally, if it is similar to copper sulphate which loses its water of crystallization, the crystals can become white and you might observe the formation of water droplets on the side of the test tube.
We can relate this observation to the processes described in the reference materials. For instance, when copper sulphate is heated, it loses its waters of crystallisation, and blue crystals turn white. This is a classic inorganic chemistry experiment demonstrating the loss of water in a hydrated salt.
Similarly, the reference to the copper wire in a silver nitrate solution helps illustrate the color change associated with copper ions in solution. Copper compounds generally impart a blue color to their solutions due to the arrangement of electrons in the d orbital.