Na & Mg with water
Sodium & Magnesium
- Both sodium, Na, and magnesium, Mg, are metals and are found in Group 1 and Group 2 of the periodic table respectively
- Both have high melting points, but magnesium has a higher melting point than sodium
- This is because of the 2+ charge of magnesium, meaning that it is has a higher charge density
- Both are silvery metals
- Sodium is quite a soft, silvery metal which tarnishes quickly in air
- Magnesium is harder than sodium and you will often see it as magnesium ribbon
Reactions with water
- Despite their similarities, sodium and magnesium will react with water quite differently:
- Sodium with cold water:
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
- This is a very vigorous, exothermic reaction
- The sodium floats on the surface of the water fizzing rapidly and melting as a result of the heat produced during the reaction
- The colourless sodium hydroxide formed will have a pH of around 13-14, so a very alkaline solution is formed
- The oxidation state of the sodium changes from 0 in its elemental state, to +1 in the sodium hydroxide
- Magnesium with cold water:
Mg (s) + 2H2O (l) → Mg(OH)2 (aq) + H2 (g)
- This is an extremely slow reaction - only a very small number of bubbles will form on the magnesium ribbon
- The magnesium hydroxide formed will have a pH of around 10 - it is less alkaline than sodium hydroxide because magnesium hydroxide is only partially soluble
- This is the key component in 'milk of magnesia'
- The oxidation state of the magnesium changes from 0 in the elemental state, to +2 in the magnesium hydroxide
- Heated magnesium with steam:
Mg (s) + H2O (g) → MgO (s) + H2 (g)
- This reaction is must faster than with cold water
- The magnesium burns with a bright, white flame
- The products of this reaction are different - magnesium oxide is produced instead of magnesium hydroxide
- The oxidation state of the magnesium changes from 0 in its elemental state, to +2 in the magnesium oxide
