Metal and Non-Metal Oxides

When different elements react with oxygen, they produce an 'oxide'

For example;

Magnesium + oxygen --> magnesium oxide

2 Mg _(s) + O_{2 (g)} --> 2 MgO _(s)

Sulfur + oxygen --> sulfur(IV) oxide (sometimes called sulfur dioxide - think why)

S _(s) + O_{2 (g)} --> SO_{2 (g)}

Sodium + oxygen --> sodium oxide

4 Na _(s) + O_{2 (g)} --> 2 Na₂O _(s)

Notice that oxides can be made by reacting both metal elements (e.g. magnesium) with oxygen or non-metal elements (e.g. sulfur) with oxygen. It's important which is which, because they changes the properties of the resulting oxide.

• Metal oxides react with water and produce a solution which is alkaline (pH more than 7)

Even though you don't need to know the equations to explain why this happens, it is worth seeing why....

MgO _(s) + H₂O _(l) --> Mg(OH)_{2 (aq)} - a metal hydroxide is made and hydroxides are always alkali

• Non-metal oxides react with water and produce a solution which is acidic (pH less than 7)

Even though you don't need to know the equations to explain why this happens, it is worth seeing why....

SO_{2 (s)} + H₂O _(l) --> H₂SO_{3 (aq)} - this makes a chemical called sulfurous acid (not to be confused with sulfuric acid which you are used to!) But because it's an acid, it has a pH less than 7.