Core Practical: Investigating Changes of State

First pass

Melt some crushed ice with a gentle flame, note the temperature every half-minute, and plot temperature against time. The graph should climb, sit flat at zero degrees while the ice melts, then climb again.

Analogy — Filming a staircase where one step is a landing: the temperature climbs, rests on the melting landing for a while, then resumes climbing.

Exam pointer — Quote the aim in the question's own terms — a temperature–time graph showing the constant temperature during the change of state — rather than a vague 'find the melting point'.

Going deeper

The design manipulates the heating rate (independent variable in time) and measures temperature at fixed intervals, with crushed ice chosen to minimise thermal gradients around the bulb. The plateau is the dependent behaviour of interest: energy supplied during melting is going into the change of state, not into temperature.

Analogy — A taxi meter that stops ticking while the passenger changes seats: the engine (heater) keeps running, the fare (temperature) pauses during the transition.

Exam pointer — If asked why crushed ice rather than a single block, answer thermal contact: small chips surround the bulb so the reading reflects the whole sample, not one warm corner.

First pass

Bury the bulb in the ice, light a small flame underneath, and take a temperature reading every fixed interval. Keep the flame gentle so the melting stage lasts long enough to be caught by several readings.

Analogy — Slow-roasting rather than searing: a low heat lets the cook watch every stage of the change instead of skipping past it.

Exam pointer — The words 'gentle' and 'steady' earn method marks — an uncontrolled flame is the classic reason a plateau shows only one reading.

Going deeper

The apparatus couples a low, steady heating rate to fixed-interval sampling so that the isothermal melt occupies a measurable stretch of the time axis. Starting readings before heating establishes the initial state, and continuing past full melt shows the resumed temperature rise — both anchor the plateau between two climbing limbs.

Analogy — A slow-motion replay of a sprint finish: only by slowing the action (gentle heating) can the judge (thermometer) log each frame (reading) of the decisive moment.

Exam pointer — Describe readings taken before, during and after the melt — the plateau is only convincing if the graph rises, flattens, then rises again.

First pass

Write each reading in the table, plot it, and look for the horizontal step. That step sits at zero degrees for melting ice and proves the temperature held constant while the state changed.

Analogy — A heartbeat trace with a flat pause: the pulse (heating) never stopped, but the trace held its line through the pause.

Exam pointer — When asked to 'use the graph', point at the flat region by name and read its temperature off the axis — that pairing is the analysis mark.

Going deeper

During the plateau the supplied energy raises the internal energy of the sample by weakening intermolecular attractions rather than by speeding particles up, so the mean kinetic energy — and hence the temperature — is unchanged. The gradient before and after the plateau reflects the specific heat capacities of ice and water respectively, which differ, so the two limbs need not be parallel.

Analogy — Paying workers to unbolt seats instead of pushing the rows faster: the money (energy) is spent, the seating chart (state) changes, but the row speed (temperature) pauses.

Exam pointer — If asked why the two rising limbs have different slopes, answer that ice and water have different specific heat capacities — equal heating raises their temperatures at different rates.

First pass

Watch out for heat sneaking straight into the thermometer or beaker rather than the ice, and for small wobbly readings. Use goggles, stand the hot beaker on a mat, and lift it with tongs.

Analogy — Checking a cake recipe: note where heat escaped, accept that every timing jitters a little, and keep the oven gloves on.

Exam pointer — Classify before you fix: systematic errors shift every reading the same way, random errors scatter — improvements should match the error type.

Going deeper

The dominant systematic threat is energy entering the bulb or glass rather than the sample, which lifts the apparent melt temperature; the control is insulating or repositioning the bulb. Random scatter comes from parallax and reaction-time variation between interval readings; repeating the run and smoothing the plot mitigates it. Safety mirrors any small-scale heating practical: eye protection, heatproof mat, tongs, and flame discipline.

Analogy — Calibrating bathroom scales on a tilted floor: no amount of re-weighing (averaging) fixes a tilt (systematic), but the jitter between steps (random) does average out.

Exam pointer — For the evaluation mark, name the error, name its effect on the plateau, and name the fix — all three, in that order.