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Chemistry · Practice book · Senior 2 · Unit 7 of 9

Preparation and classification of oxides

Senior 2 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 7 Preparation and classification of oxides

Key unit competence: Use an appropriate method for preparing oxides and classify oxides based on their properties.

Key ideas

  • An oxide is a compound of oxygen with one other element. Oxides are prepared by direct combination (2Cu + O2 → 2CuO), by heating carbonates, nitrates or hydroxides (CaCO3 → CaO + CO2, Cu(OH)2 → CuO + H2O).
  • Basic oxides (metal oxides): react with acids to form salt and water, e.g. CaO, MgO, CuO, Na2O. Soluble basic oxides form alkalis: Na2O + H2O → 2NaOH.
  • Acidic oxides (non-metal oxides): react with bases/alkalis to form salt and water and dissolve in water to give acids, e.g. CO2, SO2, SO3, P4O10, NO2: SO3 + H2O → H2SO4.
  • Amphoteric oxides react with both acids and alkalis: ZnO, Al2O3, PbO. Example: ZnO + 2HCl → ZnCl2 + H2O and ZnO + 2NaOH → Na2ZnO2 + H2O.
  • Neutral oxides react with neither acids nor alkalis: carbon monoxide (CO), water (H2O), nitrogen monoxide (NO), dinitrogen oxide (N2O).
  • Uses: quicklime (CaO) is slaked with water to make slaked lime, Ca(OH)2 (Ishwagara), used in building, whitewash and to reduce soil acidity; SiO2 in glass; ZnO in creams; TiO2 in paints; CO2 in fire extinguishers and soft drinks.

Common misconceptions

  • "All oxides are acidic." No: Metal oxides are basic, some oxides are amphoteric and some are neutral.
  • "Carbon monoxide is an acidic oxide like carbon dioxide." No: CO is a neutral oxide; it does not react with acids or alkalis.
  • "Slaked lime and quicklime are the same substance." No: Quicklime is CaO; slaked lime is Ca(OH)2, formed when water is added.

Worked examples

Example 1. Classify the following oxides as acidic, basic, amphoteric or neutral: SO2, K2O, ZnO, CO.

Solution

SO2: acidic (non-metal oxide, forms sulfurous acid).

K2O: basic (metal oxide, forms KOH).

ZnO: amphoteric (reacts with acids and alkalis).

CO: neutral (reacts with neither).

Example 2. Describe how slaked lime is made from limestone and give two of its uses.

Solution

Heat limestone strongly in a kiln: CaCO3 → CaO + CO2 (quicklime forms).

Add water carefully to quicklime: CaO + H2O → Ca(OH)2 (slaked lime); much heat is given out.

Uses: neutralising acidic soils, making mortar and plaster, whitewashing walls, water treatment.

Practice questions

  1. Define an amphoteric oxide.

    Show answer

    An oxide that reacts with both acids and alkalis to form a salt and water.

    Common mistake: Saying 'an oxide that is neither acidic nor basic' (that is neutral).

  2. Give two examples of neutral oxides.

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    CO, H2O, NO, N2O (any two).

    Common mistake: Giving CO2.

  3. Write the equation for the reaction of MgO with dilute HCl.

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    MgO + 2HCl → MgCl2 + H2O.

    Common mistake: Writing hydrogen as a product.

  4. Write the equation for CO2 reacting with NaOH.

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    CO2 + 2NaOH → Na2CO3 + H2O.

    Common mistake: Writing NaCO3.

  5. Name two ways of preparing a metal oxide.

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    Direct combination with oxygen; heating the metal's carbonate, hydroxide or nitrate.

    Common mistake: Saying 'dissolving the metal in water'.

  6. Why is sulfur dioxide described as an acidic oxide?

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    It dissolves in water to form an acid and reacts with alkalis.

    Common mistake: Saying 'because it smells'.

  7. State two uses of quicklime or slaked lime.

    Show answer

    Building (mortar/plaster), soil treatment, whitewash, water treatment (any two).

    Common mistake: Saying it is used as fuel.

  8. Write the equation for ZnO reacting with NaOH.

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    ZnO + 2NaOH → Na2ZnO2 + H2O.

    Common mistake: Writing no reaction.

  9. Explain why calcium oxide is used to dry ammonia but not hydrogen chloride.

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    CaO is basic: it does not react with basic ammonia but would react with acidic HCl.

    Common mistake: Saying CaO reacts with ammonia.

  10. Calculate the mass of CaO formed from 50 g of CaCO3.

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    0.5 mol × 56 = 28 g.

    Common mistake: Using Mr of Ca(OH)2.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Basic oxides react with acids to form:a. salt and water
    2. Farmers use slaked lime to:b. Carbon dioxide
    3. Which oxide is used in fire extinguishers?c. reduce soil acidity
    4. Which is the product when calcium oxide reacts with water?d. Calcium hydroxide
    Show answer

    1 → a (salt and water) · 2 → c (reduce soil acidity) · 3 → b (Carbon dioxide) · 4 → d (Calcium hydroxide)

    • This is neutralisation.
    • It neutralises acids in the soil.
    • CO2 does not support burning and is denser than air.
    • CaO + H2O → Ca(OH)2.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    sulfuric acid  ·  sulfurous acid  ·  calcium oxide  ·  sodium hydroxide
    1. Sulfur trioxide dissolves in water to form .
    2. Sodium oxide dissolves in water to form .
    3. Quicklime is .
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    1: sulfuric acid · 2: sodium hydroxide · 3: calcium oxide

    • SO3 + H2O → H2SO4.
    • Na2O + H2O → 2NaOH.
    • Quicklime is CaO.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which reaction prepares an oxide by heating a hydroxide?a. Cu(OH)2 → CuO + H2O
    2. Which oxide is amphoteric?b. Zinc oxide
    3. Which oxide is neutral?c. Sulfur trioxide
    4. Which is an acidic oxide?d. Carbon monoxide
    Show answer

    1 → a (Cu(OH)2 → CuO + H2O) · 2 → b (Zinc oxide) · 3 → d (Carbon monoxide) · 4 → c (Sulfur trioxide)

    • Heat removes water from the hydroxide.
    • ZnO reacts with both acids and alkalis.
    • CO reacts with neither acids nor alkalis.
    • Non-metal oxides are acidic.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    Ca(OH)2  ·  a metal  ·  amphoteric  ·  carbon dioxide
    1. Aluminium oxide reacts with sodium hydroxide because it is .
    2. Slaked lime (Ishwagara) has the formula .
    3. Heating copper(II) carbonate gives copper(II) oxide and .
    Show answer

    1: amphoteric · 2: Ca(OH)2 · 3: carbon dioxide

    • Amphoteric oxides react with alkalis too.
    • Slaked lime is calcium hydroxide.
    • CuCO3 → CuO + CO2.

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