4PH1

Changes of State

Solids, Liquids & Gases · 2 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable
Low
Stable6%

Specific heat capacity, specific latent heat and heating/cooling curves tested consistently.

Aim

  • Measure how the temperature of a sample changes with time as it is heated steadily, and identify the flat region on the temperature-vs-time graph where the sample is changing state
  • For ice this is the period when the ice and water sit together at 0 °C while the rest of the ice melts; the thermometer doesn't climb even though the burner keeps supplying heat

Variables

  • Independent variable: time t (s)
  • Dependent variable: temperature T (°C)
  • Control variables: the same burner setting, the same starting mass of ice, the same beaker, the same thermometer position in the beaker

Apparatus

EquipmentPurposeResolution
400 ml beakerHolds the ice and water—
Crushed ice cubes (enough to fill about half the beaker)The sample being heated—
−10 °C to 110 °C thermometerReads the sample temperature0.1 °C
Tripod, gauze and Bunsen burnerHeat source under the beaker—
Heatproof matSits between the bench and the burner—
StopwatchTimes the readings0.1 s
Glass stirring rodStirs to keep the temperature even through the sample—
Apparatus for heating crushed ice: a beaker of ice cubes on a gauze and tripod above a Bunsen burner, with a thermometer standing in the ice and a stopwatch beside the setup
Source: Core Practical: Investigating Changes of State by Save My Exams

Method

  1. Fill the beaker about half-full with crushed ice. Add a small splash of cold water if needed so the bulb of the thermometer is properly surrounded by the sample
  2. Hold the thermometer vertically with its bulb in the middle of the ice (clamp it if possible, and don't let it touch the bottom of the beaker)
  3. Light the Bunsen burner and adjust to a small, steady flame. Start the stopwatch
  4. Stir the sample gently with the glass rod and record the temperature every 30 s
  5. Continue past the point where all the ice has melted and the water starts to heat
  6. Stop once the water reaches a temperature near 90 °C (don't take to boiling for safety)

Analysis

  • Plot a graph with temperature up the y-axis and time along the x-axis
  • The resulting curve splits into three sections:
    • A gentle rise from below 0 °C up to 0 °C, where the ice itself is warming
    • A flat plateau sitting at 0 °C while the ice and water coexist; the heat going in is overcoming the forces of attraction between particles (filling the potential store rather than the kinetic one), so the thermometer reading stops climbing
    • A steeper rise from 0 °C upwards once all the ice has melted and the water alone is being heated
  • The length of the flat region tells you how much energy was needed to melt all the ice (you can calculate this from energy = power × time)
A temperature-against-time graph for ice being heated steadily: a rising line labelled solid, a flat plateau labelled melting at the melting point, then a steeper rising line labelled liquid, with a note that the temperature stays constant at 0 °C while the ice melts
Source: Core Practical: Investigating Changes of State by Save My Exams

Sources of error and safety

  • Systematic, thermometer touching the bottom of the beaker. The glass is hotter than the ice during heating, so the reading is too high. Suspend the bulb in the sample
  • Systematic, burner flame turned up too high. A fast heat input drives the temperature past 0 °C before the flat region is clearly visible. Use a low flame
  • Random, temperature gradients within the beaker. Stir gently between readings so all the sample is at the same temperature
  • Safety. Wear goggles; stand throughout the experiment so any spill is dealt with quickly; ensure the heatproof mat is under the burner; let the apparatus cool fully before clearing away