Magnesium

Magnesium (Mg) is a reactive Group II metal that runs through more of the 0620 practical syllabus than almost any other element — combustion, acid reactions, rates, displacement and an empirical-formula experiment all use magnesium ribbon.

Where magnesium appears in 0620

• Reactivity series and reactions of metals: magnesium sits above aluminium, zinc and iron.
• Rate of reaction: Mg + dilute acid is the classic rate experiment (measuring gas volume or mass loss over time).
• Stoichiometry: finding the empirical formula of MgO by burning a known mass of ribbon.
• Electrolysis and extraction: magnesium is extracted from molten MgCl2 because it is above carbon.
• Qualitative analysis: the Mg2+ cation test with sodium hydroxide.

Position in the periodic table

• Symbol: Mg
• Atomic number: 12
• Group 2 (II), Period 3
• Electron configuration: 2, 8, 2
• Forms Mg2+ ions by losing 2 electrons

Physical properties

Property	Detail
Appearance	Silver-white, shiny when freshly cut
Melting point	650 C
Density	1.74 g/cm3 (low for a metal)
Conductivity	Good
Hardness	Relatively soft

Reactivity

Magnesium is high in the reactivity series, above aluminium, zinc and iron.

Combustion in air

2Mg(s) + O2(g) -> 2MgO(s)

Observations: a brilliant white flame and a white powder (MgO). The reaction is strongly exothermic.

Safety: never look directly at burning magnesium — the intense light can damage eyes.

With dilute acids

Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

Mg(s) + H2SO4(aq) -> MgSO4(aq) + H2(g)

Observations: vigorous effervescence, the ribbon dissolves rapidly, a colourless solution forms, and the gas gives a squeaky pop with a lighted splint. Magnesium reacts faster than zinc or iron, confirming its higher position in the reactivity series.

With water and steam

Mg(s) + 2H2O(l) -> Mg(OH)2(aq) + H2(g) (very slow with cold water)

Mg(s) + H2O(g) -> MgO(s) + H2(g) (with steam — vigorous, bright white glow)

Note the different products: cold water gives the hydroxide, but steam gives the oxide.

With copper(II) sulfate solution

Mg(s) + CuSO4(aq) -> MgSO4(aq) + Cu(s)

A displacement reaction: magnesium is more reactive than copper, so brown copper deposits and the blue colour fades.

Magnesium oxide

• Formula: MgO — a white powder with a high melting point (giant ionic lattice of Mg2+ and O2-)
• Basic oxide: MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O(l)
• Slightly soluble in water, forming a weakly alkaline solution

See magnesium oxide for more detail.

Test for Mg2+ ions

Add sodium hydroxide solution: a white precipitate of magnesium hydroxide forms that is insoluble in excess NaOH. (This is how Mg2+ is distinguished from Zn2+ and Al3+, whose white precipitates dissolve in excess.)

Mg2+(aq) + 2OH-(aq) -> Mg(OH)2(s)

Extraction

Magnesium is above carbon in the reactivity series, so carbon cannot reduce it. It is extracted by electrolysis of molten magnesium chloride:

• Cathode: Mg2+ + 2e- -> Mg
• Anode: 2Cl- -> Cl2 + 2e-

Key facts

• Symbol: Mg — proton number: 12
• Position: Group II, Period 3
• Electron configuration: 2, 8, 2 (forms Mg2+)
• Key property: burns with a brilliant white flame to white MgO
• Reactivity: above aluminium, zinc and iron; extracted by electrolysis
• Main uses: low-density alloys, flares and fireworks, sacrificial protection

Uses

Use	Reason
Alloys (with aluminium)	Low density, strong
Fireworks and flares	Brilliant white flame
Sacrificial protection	More reactive than iron

Common exam mistakes

• Not balancing the combustion equation. It is 2Mg + O2 -> 2MgO. There is no such compound as “MgO2” — magnesium forms only Mg2+ and the oxide is MgO.
• Giving the same product for water and steam. Cold water -> Mg(OH)2 + H2; steam -> MgO + H2. Mixing these up loses the mark.
• Calling the bright white light a “flame test”. That glow is combustion; magnesium is not one of the cations identified by flame colour.
• Losing magnesium in the MgO practical. If white smoke escapes or the lid is not lifted to admit air, some magnesium is lost or unreacted, so the calculated formula is wrong.

Worked exam questions

Magnesium ribbon is cleaned with sandpaper before being burned in a crucible. Explain why. (2 marks)

Mark scheme

• Magnesium reacts with oxygen/moisture in air to form a layer of magnesium oxide on the surface [1]
• Cleaning removes this oxide so only magnesium reacts, giving a more accurate result [1]

Examiner note: the mark is for what the layer is (oxide) and why removing it matters (accuracy), not simply “to clean it”.

In an experiment, 0.24 g of magnesium reacts completely to form 0.40 g of magnesium oxide. Determine the empirical formula of the oxide. (Ar: Mg = 24, O = 16) (3 marks)

Mark scheme

• Mass of oxygen = 0.40 - 0.24 = 0.16 g [1]
• Moles of Mg = 0.24/24 = 0.010; moles of O = 0.16/16 = 0.010 [1]
• Ratio Mg:O = 1:1, so the formula is MgO [1]

Examiner note: always find the mass of oxygen by subtraction first. Candidates who divide the oxide mass by 16 instead of the oxygen mass get the wrong ratio.