Define the term oxidising agent. Explain why the solution turns blue by referring to the relative strength of oxidising agents.

The solution turns blue because copper metal ($\text{Cu}$) is oxidized to copper(II) ions ($\text{Cu}^{2+}$), which form the blue copper(II) nitrate solution ($\text{Cu(NO}_3)_2\text{(aq)}$).

Silver ions ($\text{Ag}^{+}$) are stronger oxidising agents than copper ions ($\text{Cu}^{2+}$). This means $\text{Ag}^{+}$ ions have a greater tendency to gain electrons (be reduced) than $\text{Cu}^{2+}$ ions. Consequently, $\text{Ag}^{+}$ ions readily accept electrons from copper metal, causing copper to oxidize and the solution to turn blue.