1. Carbon dioxide gives Coke its magic
Have you ever left an open can of Coke overnight and tried it the morning after? It tastes dull and flat, as though life and flavours have been sucked out of it. Drama aside, what has escaped the drink is actually carbon dioxide, which makes it fizzy and gives it a more exciting taste. This is because carbon dioxide is an acidic oxide that dissolves in water to produce a sour-tasting acid.
2. Carbon dioxide dissolves in and reacts with water
CO2(g) + H2O(l) ⟶ H2CO3(aq)
Compared to other gases like nitrogen and oxygen, carbon dioxide dissolves well in water. But it does not stop here. When carbon dioxide dissolves, it reacts with water to form carbonic acid, H2CO3.
3. Acidic oxides neutralise alkalis
Carbon dioxide reacts with alkali to form salt and water via neutralisation.
CO2(g) + Ca(OH)2(aq) ⟶ CaCO3(s) + H2O(l)
An important example is the reaction underlying the test for carbon dioxide. Limewater is calcium hydroxide solution, an alkali. When we bubble carbon dioxide into limewater, it neutralises the calcium hydroxide to form white precipitate of calcium carbonate salt and water.
The property of carbon dioxide to behave like an acid and react with an alkali makes it an acidic oxide.
Acidic oxides are a type of oxides that react with alkalis to form salt and water.
4. Most non-metallic oxides are acidic oxides
| acidic oxide | chemical formula | acid(s) formed in water |
|---|---|---|
| carbon dioxide | CO2 | H2CO3 |
| nitrogen dioxide | NO2 | HNO2 and HNO3 |
| silicon dioxide | SiO2 | Insoluble |
| sulfur dioxide | SO2 | H2SO3 |
| sulfur trioxide | SO3 | H2SO4 |
Besides carbon dioxide, most non-metallic oxides are also acidic oxides. For example, sulfur dioxide is an acidic oxide that can also react with limewater via neutralisation.
Silicon dioxide presents a curious case. Because of its giant molecular structure, it cannot dissolve in water. However, we still classify it as a acidic oxide because it can react with hot, concentrated alkalis.
Acidic oxides are non-metallic oxides.
5. Water and carbon monoxide are pesky exceptions
Sadly, annoying Chemistry has no simple rule.
A few non-metallic oxides are acidic oxides. These important exceptions include water and carbon monoxide, which are neutral oxides that neither react with acid nor alkali.
