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5.2.3Solids, Liquids and Gases

Core Practical- Investigating Changes of State

5 min read 5 parts Lesson 12 of 23 Unit quiz

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Section 1 of 5Setting Up

5.2.3 Core Practical- Investigating Changes of State

The equipment is chosen so that the substance heats gently and its temperature can be read clearly.

Substance
  • Use a substance with a clear melting point, such as wax, placed in a test tube.
Thermometer
  • Read the temperature at regular intervals, recording to a precision of 0.1 °C where the thermometer allows.
Water bath
  • Heating in a beaker of hot water keeps the heating gentle and avoids overheating the substance.
Stopwatch
  • Records the time for each temperature reading, usually taken every 30 seconds.

Section 2 of 5Method

Record data continuously through the change so that the whole graph is captured.

1
Place the solid in a test tube inside a beaker of water, and insert a thermometer into the solid.
2
Heat the water bath gently and record the temperature and time at regular intervals until the solid has fully melted.
3
Continue recording after melting until the liquid has warmed by a few degrees above its melting point.
4
Plot temperature on the y-axis against time on the x-axis, then draw a smooth curve through the average readings.

Section 3 of 5Interpreting the Graph

The shape of the curve shows which state the substance is in at each time.

Interpreting the Graph
Flat section
  • A horizontal section shows the melting point, where energy is used to break forces between particles.
Sloping sections
  • Rising sections show temperature increasing within a single state, as particles gain kinetic energy.
Impurities
  • An impure substance melts over a range of temperatures rather than at one sharp value.
Improving reliability
  • Repeat the experiment and use a lid to reduce heat loss, which improves the reliability of the readings.

Section 4 of 5Worked Example

Question Describe how to find the melting point of a solid from a temperature-time graph.

  1. Plot temperature on the y-axis against time on the x-axis.
  2. Identify the horizontal section where the temperature stays constant for a period of time.
  3. Read the temperature of that flat section, which is the melting point.

Answer The melting point is the constant temperature shown by the flat part of the graph.

Section 5 of 5Exam Checklist & Tips

5.2.3 – Key Points for the Exam

  • Melting point The temperature at which a solid melts, shown by a flat section on a heating graph.
  • Freezing point The temperature at which a liquid freezes, which for a pure substance equals its melting point.
  • Water bath A container of hot water used to heat a substance gently.
  • Impurity A substance mixed into a pure sample, which changes its melting behaviour.
  • Heating curve A graph of temperature against time as a substance is heated.

Common Mistakes

  • Stopping the recording as soon as melting starts, which misses the flat section of the graph.
  • Heating the substance directly with a flame, which causes uneven heating and overshoot.
  • Drawing a curve that does not follow the average of the repeated readings.
  • Stating that the temperature falls during melting; it stays constant until melting is complete.
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