Magnesium in water
Magnesium is not as reactive as sodium. But it behaves quite differently according to the temperature of the water.
Activity 16.5
- Put a cleaned magnesium (Mg) strip into a beaker of water. Record your observations.
- Now heat that beaker of water with a burner.
- Record your observations.
The observation
With cold water, magnesium gives no reaction that can be observed.
On reacting with hot water, gas bubbles are seen to be given off.
Three conditions, two products
Cold water
No observable reaction.
Hot water
Forms magnesium hydroxide and hydrogen gas.
Mg + 2H2O → Mg(OH)2 + H2
Steam
Magnesium reacts rapidly with steam to form magnesium oxide and hydrogen gas.
Mg + H2O → MgO + H2
Why the product changes
Hot water gives the hydroxide, steam gives the oxide. So before writing the equation, check which state the water is in.
Sodium reacts rapidly with cold water itself, while magnesium needs hot water or steam — which already tells us sodium is the more reactive of the two.