AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    Phosphate rock is treated with nitric acid or sulfuric acid to produce…

    Full specification wording

    Production and uses of NPK fertilisers: Phosphate rock is treated with nitric acid or sulfuric acid to produce soluble salts that can be used as fertilisers.

    Phosphate rock contains insoluble calcium phosphate, meaning plants cannot absorb it.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 7 of 8
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Phosphate rock contains insoluble calcium phosphate, meaning plants cannot absorb it. To produce soluble NPK fertilisers, the rock is treated with acids. Reacting phosphate rock with nitric acid produces a mixture of phosphoric acid and calcium nitrate. The phosphoric acid produced can then be reacted with further phosphate rock to manufacture triple superphosphate (calcium phosphate). Alternatively, treating the rock with sulfuric acid produces single superphosphate, which is a mixture of calcium phosphate and calcium sulfate. These acid treatments convert the insoluble rock into soluble salts, allowing phosphate ions to dissolve in soil water and be absorbed by plant roots for healthy growth.

    What earns the marks

    • Phosphate rock is insoluble, so it must be treated with acid to produce soluble salts for plant uptake.
    • Reaction with nitric acid produces phosphoric acid and calcium nitrate.
    • Reaction with sulfuric acid produces single superphosphate, which is a mixture of calcium phosphate and calcium sulfate.
    • Phosphoric acid reacts with additional phosphate rock to produce triple superphosphate (calcium phosphate).

    Common mistakes and how to put them right

    • Stating that phosphate rock dissolves directly in water.

      Put it right: It is insoluble and must first be reacted with an acid to form soluble salts.

    • Confusing the products of the different acid treatments.

      Put it right: Sulfuric acid produces single superphosphate, whereas nitric acid produces phosphoric acid and calcium nitrate.

    • Forgetting how triple superphosphate is made.

      Put it right: Phosphoric acid must react with additional phosphate rock to produce triple superphosphate (calcium phosphate).

    Exam tips

    • Memorise the specific names of the fertiliser products: single superphosphate (from sulfuric acid) and triple superphosphate (from phosphoric acid).
    • Always specify that the purpose of the acid treatment is to make the phosphate soluble so plants can absorb it.

    Try the lesson

    The start of the Production and uses of NPK fertilisers: Phosphate rock is treated with nitric acid or sulfuric acid to produce soluble salts that can be used as fertilisers. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Phosphate rock is treated with acids to produce soluble salts for use as fertilisers. Which two acids are commonly used for this purpose alongside phosphoric acid?

    Listen

    Phosphate rock contains insoluble calcium phosphate, meaning plants cannot absorb it. To produce soluble NPK fertilisers, the rock is treated with acids. Reacting phosphate rock with nitric acid produces a mixture of phosphoric acid and calcium nitrate. The phosphoric acid produced can then be reacted with further phosphate rock to manufacture triple superphosphate (calcium phosphate). Alternatively, treating the rock with sulfuric acid produces single superphosphate, which is a mixture of calcium phosphate and calcium sulfate. These acid treatments convert the insoluble rock into soluble salts, allowing phosphate ions to dissolve in soil water and be absorbed by plant roots for healthy growth. A strong answer earns marks for these points. Phosphate rock is insoluble, so it must be treated with acid to produce soluble salts for plant uptake. Reaction with nitric acid produces phosphoric acid and calcium nitrate. Reaction with sulfuric acid produces single superphosphate, which is a mixture of calcium phosphate and calcium sulfate. Phosphoric acid reacts with additional phosphate rock to produce triple superphosphate (calcium phosphate). A common slip: Stating that phosphate rock dissolves directly in water. Instead, it is insoluble and must first be reacted with an acid to form soluble salts. Exam tip: Memorise the specific names of the fertiliser products: single superphosphate (from sulfuric acid) and triple superphosphate (from phosphoric acid).

    See it

    A top-down flowchart. At the top, a node labelled 'Phosphate rock (insoluble)' branches into two paths. The left path goes to 'Treated with nitric acid', which leads to 'Phosphoric acid and calcium nitrate'. From there one arrow goes to 'Phosphoric acid reacts with further phosphate rock', which leads to 'Triple superphosphate (calcium phosphate)'. The right path from phosphate rock goes to 'Treated with sulfuric acid', which leads to 'Single superphosphate: calcium phosphate and calcium sulfate'. The nitric acid products node, the single superphosphate node and the triple superphosphate node all lead to a node labelled 'Soluble salts'. That node leads finally to 'Phosphate ions dissolve in soil water and are absorbed by plant roots'.
    It shows how insoluble phosphate rock is treated with nitric acid or sulfuric acid to make soluble salts, including single and triple superphosphate, which release phosphate ions for plant roots.

    Key words

    Say what each word means before you turn the card.

    Phosphate rock

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    Soluble salts

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    Single superphosphate

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    Triple superphosphate

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    See every definition
    Phosphate rock
    A substance containing insoluble calcium phosphate, meaning plants cannot absorb it.
    Soluble salts
    Products formed when phosphate rock is treated with acid, allowing phosphate ions to dissolve in soil water and be absorbed by plant roots.
    Single superphosphate
    A mixture of calcium phosphate and calcium sulfate produced by treating phosphate rock with sulfuric acid.
    Triple superphosphate
    Calcium phosphate produced by reacting phosphoric acid with further phosphate rock.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top, a node labelled 'Phosphate rock (insoluble)' branches into two paths. The left path goes to 'Treated with nitric acid', which leads to 'Phosphoric acid and calcium nitrate'. From there one arrow goes to 'Phosphoric acid reacts with further phosphate rock', which leads to 'Triple superphosphate (calcium phosphate)'. The right path from phosphate rock goes to 'Treated with sulfuric acid', which leads to 'Single superphosphate: calcium phosphate and calcium sulfate'. The nitric acid products node, the single superphosphate node and the triple superphosphate node all lead to a node labelled 'Soluble salts'. That node leads finally to 'Phosphate ions dissolve in soil water and are absorbed by plant roots'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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