My driving question: Why did the solution in the boiling flask change from blue to pink/yellow?

Scroll down for more information and watch the video below for the answer:)

Scientific principle

HOW DOES COPPER REACT WITH NITRIC ACID?

Copper is an unreactive metal and doesn’t react in normal circumstances with dilute acids. However it does react with nitric acid. Why is this?

Nitric acid is an oxidizing agent and the reaction is not the usual acid + metal reaction. The products are oxides of nitrogen instead of hydrogen. The actual nitrogen oxide formed depends on the concentration and temperature of the acid.

There are actually two equations for the reaction of copper with nitric acid. It depends on whether the nitric acid is concentrated or not. If it is concentrated and in excess then the ratio is 1:4 copper to nitric acid. If it is dilute then the ratio is 3:8.

Cu + 4HNO3 –> Cu(NO3)2 + 2NO2 + 2H2O

3Cu + 8HNO3 –> 3Cu(NO3)2 + 2NO + 4H2O

Nitric acid when concentrated is a strong oxidising agent so it makes sense that a higher oxidation state of nitrogen (IV) oxide is formed when the nitric acid is concentrated.

The Chemistry...

Oxidation of copper metal with a strong oxidizing agent, conc. nitric acid.

In a classic experiment, copper metal is turned into copper(II) ion while the nitrogen(V) in the nitrate ion becomes nitrogen(IV) in the nitrogen dioxide gas.

Charles' Law

As the temperature from the reaction warms the gas, it expands. Later, as it cools, the gas contracts.

Nonmetal oxides are acid anhydrides (also link to acid rain)

Although the nitrogen dioxide gas is noxious and toxic, it dissolves readily in water and make the solution acidic. This can be shown by adding a little indicator to the water and making the water slightly basic before the copper is added to the acid.

Air pressure

As the pressure in the flask is decreased as it cools, the outside pressure pushes the water up the tubing toward the flask. The nitrogen dioxide gas is not pulling the water in.

Descriptive chemistry--copper solutions are green and blue

The colored solutions come from complexes of copper(II) ion in solution. Aqueous copper ion is blue, Cu(H2O)42+ The green must be copper surrounded by nitrates.

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