Two graph skills carry heavy marks in A-Level Chemistry practical papers: drawing a tangent to find a rate, and extrapolating a cooling line to find the true temperature change. Practise both with a draggable ruler, plus gas-syringe readings, comparing curves and enthalpy calculations.
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A straight line that touches the curve at the stated time without cutting it, with a large triangle used for the gradient.
Rate = gradient of the tangent = ΔV / Δt, with units such as cm³ s⁻¹.
A best-fit line through the cooling points, extended back to the time of mixing, to correct for heat loss.
q = mcΔT using the total mass of solution, then ΔH = −q / n in kJ mol⁻¹, with the sign and three significant figures.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.