What color is Fehling’s solution when it is not reacting?

Fehling’s solution consists of two parts, typically labeled as Fehling's A and Fehling's B, which contain copper(II) sulfate, sodium tartrate, and sodium hydroxide. When these two solutions are mixed, the solution is a deep blue color due to the presence of copper(II) ions in a complex with tartrate ions.

When no reducing sugar or aldehyde is present to react with the solution, it retains this blue color, indicating that the copper ions remain in their oxidized state. Upon reaction with a reducing agent, such as glucose, the solution changes color as the copper(II) ions are reduced to copper(I) oxide, which precipitates out and gives a reddish-brown color. However, in the absence of any reaction, the color remains unchanged as blue.

Thus, when Fehling’s solution is not reacting, the correct observation is that it is blue, confirming the presence of unreacted copper(II) ions in the solution.