Density

    OCR
    GCSE

    Density is defined as the mass per unit volume of a substance, mathematically expressed as ρ = m/V. It serves as a fundamental intrinsic property of matter, explained via the particle model by analyzing the spacing and arrangement of atoms or molecules in solids, liquids, and gases. Candidates must demonstrate competence in determining density experimentally for regular solids, irregular solids (using displacement), and liquids. Furthermore, understanding that mass is conserved during changes of state while density changes due to volume expansion or contraction is essential.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    4
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for recalling and applying the equation: Density = Mass / Volume (or ρ = m/V).
    • Credit responses that explicitly describe measuring the volume of an irregular solid using a displacement (Eureka) can and measuring cylinder.
    • Award 1 mark for correct unit conversion, specifically recognizing the factor of 1000 difference between g/cm³ and kg/m³.
    • Candidates must link low density in gases to the large spacing between particles relative to their size.
    • Award marks for stating that mass must be measured using a balance before the object is submerged in water.

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the value, but check the units required by the question — did you convert to kg/m³?"
    • "Good description of the displacement method. To improve, explain *why* we measure mass before volume."
    • "You identified the equation correctly. Now, ensure you show your substitution step clearly before writing the final answer."
    • "Excellent analysis of error. To reach the top band, suggest a specific improvement to the apparatus that would reduce this uncertainty."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for recalling and applying the equation: Density = Mass / Volume (or ρ = m/V).
    • Credit responses that explicitly describe measuring the volume of an irregular solid using a displacement (Eureka) can and measuring cylinder.
    • Award 1 mark for correct unit conversion, specifically recognizing the factor of 1000 difference between g/cm³ and kg/m³.
    • Candidates must link low density in gases to the large spacing between particles relative to their size.
    • Award marks for stating that mass must be measured using a balance before the object is submerged in water.

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When describing the density practical, always state that mass is measured *before* volume to avoid wetting the object and introducing error.
    • 💡Pay close attention to the units required in the answer line; OCR often asks for kg/m³ when data is given in g and cm³.
    • 💡In 6-mark Level of Response questions regarding the practical, ensure you cover both the method (equipment used) and the data processing (formula/calculation) to access Level 3.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Failing to convert volume units correctly, particularly assuming 1 m³ = 100 cm³ instead of 1,000,000 cm³.
    • Reading the volume from the top of the meniscus rather than the bottom when using a measuring cylinder.
    • Measuring the mass of an irregular solid after measuring its volume, resulting in a higher mass reading due to water retention.
    • Confusing the resolution of the measuring instrument with the accuracy of the result.

    Key Terminology

    Essential terms to know

    Mathematical relationship ρ = m/V
    Particle model explanation of density in different states
    Conservation of mass during physical changes
    Experimental determination using displacement and direct measurement

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Describe
    Explain
    Suggest
    Determine

    Practical Links

    Related required practicals

    • •{"code":"PAG P1","title":"Determination of Density","relevance":"Core practical requirement involving regular/irregular solids and liquids"}

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