The rusting of iron requires the presence of three key conditions:

• Iron (Fe): Iron must be present in its metallic form.
• Oxygen (O₂): Oxygen is essential as it acts as the oxidizing agent.
• Water (H₂O): Water acts as an electrolyte, facilitating the flow of ions and completing the electrochemical process. The presence of water significantly speeds up the rusting process.

Therefore, rusting occurs when iron is exposed to both oxygen and water.

(a) Magnesium is used to protect the steel beam because magnesium is more reactive than iron. According to the reactivity series, magnesium is positioned above iron. This means magnesium has a greater tendency to lose electrons and is therefore more readily oxidized than iron. As a result, magnesium will corrode preferentially, protecting the iron from corrosion.

(b) When the magnesium strip is attached to the steel beam, it acts as the anode and the steel beam acts as the cathode. The magnesium atoms lose electrons to become magnesium ions (Mg²⁺), which dissolve into the solution. These electrons flow to the steel beam. This influx of electrons causes the iron atoms in the steel beam to gain electrons and remain in their metallic state, preventing them from oxidizing and corroding. The magnesium 'sacrifices' itself to protect the iron by providing a more readily oxidizable metal.

Control Variable: A crucial control variable is the amount of zinc coating applied to each piece of metal. The thickness of the zinc layer should be kept constant for both the iron and copper samples. This ensures that the only difference between the samples is the base metal being protected.

Independent Variable: The independent variable is the type of metal being protected. The student should have two groups: one group with iron coated in zinc and another group with copper coated in zinc. The presence or absence of iron in the base metal is what's being manipulated.

Analysis of Results: The student should calculate the percentage mass loss for each sample after a week. This is done by subtracting the mass of the sample after a week from the initial mass and dividing by the initial mass, then multiplying by 100. The formula is: Percentage Mass Loss = [(Initial Mass - Final Mass) / Initial Mass] x 100.

The student should then compare the percentage mass loss of the iron and copper samples. If the iron sample shows a significantly lower percentage mass loss than the copper sample, this would indicate that galvanising is effective in preventing corrosion of iron. The copper sample, being more resistant to corrosion than iron, would likely show a lower mass loss regardless of the zinc coating. The results should be presented in a table to clearly show the mass loss for each sample.