7.1.3Acids, Bases and Salts
Classifying Oxides — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 4 of 8 Unit quiz
Part 1 of 5What is an oxide
Definition
- Definition An oxide is a compound made of oxygen combined with just one other element.
- Examples include magnesium oxide, MgO, zinc oxide, ZnO, carbon dioxide, CO2, and water, H2O.
Why classify them
- Why classify them Oxides behave in different ways when they meet water, acids and bases, so we sort them by their acid-base character.
Part 2 of 5The four types of oxide
| Type | Made from | Reaction pattern | Examples |
|---|---|---|---|
| Acidic | A non-metal and oxygen | Reacts with bases to give a salt and water; dissolves to form an acidic solution | CO2, SO2, NO2, SiO2 |
| Basic | A metal and oxygen | Reacts with acids to give a salt and water; if soluble, forms an alkaline solution | CuO, CaO, MgO |
| Amphoteric (HT only) | Some metals | Reacts with both acids and bases to give a salt and water | ZnO, Al2O3 |
| Neutral | Certain non-metals | Does not react with acids or bases | CO, NO, N2O |
Part 3 of 5Predicting the type
1Identify the element
- Identify the element Look at where the other element sits in the Periodic Table.
2Metal or non-metal
- Metal or non-metal Metal oxides are generally basic, while non-metal oxides are generally acidic.
3Check exceptions
- Check exceptions Carbon monoxide, nitrogen monoxide and dinitrogen oxide are non-metal oxides that are neutral.
4Look for both behaviours (HT only)
- Look for both behaviours (HT only) If an oxide reacts with both acids and alkalis, it is amphoteric, as in zinc oxide and aluminium oxide.
Part 4 of 5Amphoteric oxides in detail (HT only)
Acting as a base
- Acting as a base With an acid, aluminium oxide behaves as a base: Al2O3 + 6HCl → 2AlCl3 + 3H2O.
Acting as an acid
- Acting as an acid With a strong alkali, it behaves as an acid: Al2O3 + 2NaOH → 2NaAlO2 + H2O.
- In both cases a salt and water are formed.
Their hydroxides
- Their hydroxides The hydroxides of zinc and aluminium are also amphoteric and can neutralise both acids and bases.
Part 5 of 5Testing an unknown oxide
1
Add a small amount of the oxide to dilute acid and warm gently. If it dissolves to form a salt solution, it is basic or amphoteric.
2
Add a separate sample to sodium hydroxide solution and warm. If it also dissolves, it is amphoteric (HT only).
3
If it dissolves in neither, shake it with water and test the solution with universal indicator.
4
A solution below pH 7 means an acidic oxide; no change from pH 7 and no reaction anywhere suggests a neutral oxide.
Key terms
| Term | Meaning |
|---|---|
| Oxide | A compound of oxygen with one other element. |
| Acidic oxide | A non-metal oxide that reacts with bases to form a salt and water and gives an acidic solution in water. |
| Basic oxide | A metal oxide that reacts with acids to form a salt and water. |
| Amphoteric oxide | An oxide that can react as either a base or an acid, depending on the other reactant (HT only). |
| Neutral oxide | An oxide that does not react with acids or bases. |
Avoid these mistakes
1
Saying all non-metal oxides are acidic. Carbon monoxide, NO and N2O are neutral.
2
Describing neutral oxides as oxides with pH 7 only. The key idea is that they do not react with acids or bases.
3
Forgetting that an amphoteric oxide reacts with an alkali as well as an acid (HT only).
4
Assuming all basic oxides dissolve to form alkalis. Only the few soluble ones, such as CaO, do; CuO is insoluble but still reacts with acids.
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