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A short H2 Chemistry revision video on H2 Chemistry 5 - The Periodic Table: Behaviour of Period 3 Oxides with Water, built for quick recap before tutorial practice or exam revision.
Read through the explanation after watching, or jump straight to the step you want to replay.
From pH 13-14 to 1-2
Across Period three, four oxides behave very differently in water.
From pH 13-14 to 1-2
Sodium oxide gives a strongly alkaline solution, often around pH thirteen to fourteen.
From pH 13-14 to 1-2
Phosphorus pentoxide gives a strongly acidic solution, often around pH one to two.
From pH 13-14 to 1-2
Why does the spread swing so far?
Na₂O
Sodium oxide is an ionic oxide.
Na₂O
In water, the oxide ion accepts a proton, forming two hydroxides.
Na₂O
The solution is strongly basic, typically around pH thirteen to fourteen.
MgO
Magnesium oxide is also ionic.
MgO
But the lattice energy is high, from small ions and two-plus, two-minus charges.
MgO
Only sparingly soluble. Solubility, not basicity, limits the pH.
SiO₂
Silicon dioxide has a giant covalent lattice.
SiO₂
Water cannot break silicon-oxygen bonds.
SiO₂
But silicon dioxide is still classified as acidic. It dissolves in hot, concentrated sodium hydroxide.
P₄O₁₀
Phosphorus pentoxide is a simple covalent oxide.
P₄O₁₀
Water attacks the discrete molecules directly.
P₄O₁₀
It reacts vigorously to form phosphoric acid.
P₄O₁₀
pH around one to two.
The rule
Across Period three, bonding shifts from ionic to covalent.
The rule
Ionic oxides are basic.
The rule
Covalent oxides are acidic.
The rule
Bonding decides.
The trap
One oxide catches everyone out on this.
The trap
Silicon dioxide is acidic.
The trap
But it does not react with water.
The trap
Acidic oxide means it reacts with a base, not that it reacts with water.