Making Salts

    OCR
    GCSE

    The preparation of salts necessitates the selection of specific synthetic pathways governed by the solubility of reactants and products. Candidates must distinguish between the excess base method for insoluble reactants, titration for soluble alkalis, and precipitation for insoluble salts. The process integrates reaction logic with separation techniques—filtration, evaporation, and crystallization—to isolate pure, dry samples. Success requires linking experimental observations to underlying chemical equations and ionic interactions.

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    Objectives
    4
    Exam Tips
    4
    Pitfalls
    5
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for explicitly stating 'add excess' insoluble base to warm acid to ensure full neutralization
    • Credit 'filter the mixture' to remove the unreacted excess solid residue
    • Award 1 mark for heating the filtrate only until the 'point of crystallization' or 'crystals start to form' (do not accept 'heat to dryness')
    • Credit responses that describe drying crystals using 'filter paper' or a 'desiccator' rather than an oven (to preserve water of crystallization)
    • Award 1 mark for correctly identifying the specific acid and base required to form the named salt

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly listed the steps, but you must specify 'add excess' to ensure the acid is fully used up"
    • "Good identification of the reactants. Now, explain why we stop heating when crystals start to appear"
    • "You used the titration method here, but since Copper Oxide is insoluble, the excess base method is required"
    • "Excellent use of technical terms like 'filtrate' and 'residue'. To improve, include the specific equation for this reaction"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for explicitly stating 'add excess' insoluble base to warm acid to ensure full neutralization
    • Credit 'filter the mixture' to remove the unreacted excess solid residue
    • Award 1 mark for heating the filtrate only until the 'point of crystallization' or 'crystals start to form' (do not accept 'heat to dryness')
    • Credit responses that describe drying crystals using 'filter paper' or a 'desiccator' rather than an oven (to preserve water of crystallization)
    • Award 1 mark for correctly identifying the specific acid and base required to form the named salt

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to describe the method, always specify 'warm the acid' but never 'boil the acid' (safety implication)
    • 💡Memorize the solubility rules: if the metal is Group 1, use titration; if it is a transition metal oxide/carbonate, use the excess base method
    • 💡For 6-mark Level of Response questions, structure your answer chronologically: Reaction → Filtration → Crystallization → Drying
    • 💡Use the phrase 'heat until crystals form on a glass rod' to demonstrate understanding of the crystallization point

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Suggesting 'evaporate all the water' which leads to anhydrous powder rather than hydrated crystals
    • Attempting to use the excess base method for soluble alkalis (like Sodium Hydroxide) instead of titration
    • Failing to mention 'warming the acid' to ensure the reaction proceeds at a sufficient rate
    • Omitting the filtration step, resulting in crystals contaminated with unreacted base

    Key Terminology

    Essential terms to know

    Selection of synthetic method based on solubility rules
    Excess base method (neutralization of acids with insoluble bases)
    Titration method (neutralization of acids with alkalis)
    Precipitation reactions for insoluble salts
    Separation and purification techniques (filtration, crystallization)

    Likely Command Words

    How questions on this topic are typically asked

    Describe
    Explain
    Suggest
    Calculate
    Write

    Practical Links

    Related required practicals

    • •{"code":"PAG C1","title":"Preparation of a pure, dry sample of a soluble salt","relevance":"Direct assessment of the excess base method"}
    • •{"code":"PAG C2","title":"Titration","relevance":"Alternative method for making salts from soluble alkalis"}

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