Experiment: Heating and cooling curve of a substance

The heating and cooling curve is the first quantitative experiment in the Matter and the Atomic Structure chapter, and it is where the kinetic theory of matter stops being an abstract idea and becomes something you can see on a graph. By tracking temperature against time while a substance melts and then freezes, you produce a curve whose shape explains what happens to particles during a change of state. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.

Aim

To study the heating and cooling of a substance, for example naphthalene, and to plot its heating curve and cooling curve, so that the melting point and freezing point can be determined and related to the arrangement and movement of particles.

Apparatus and materials

• Boiling tube
• Naphthalene (or another suitable solid such as octadecan-1-ol)
• Thermometer (for example a −10 °C to 110 °C thermometer)
• Water bath: a beaker of water on a tripod stand and wire gauze
• Bunsen burner
• Retort stand, boss and clamp
• Stopwatch
• Stirrer
• Heat-resistant mat

Procedure

1. Fill a boiling tube about one-third full with solid naphthalene and clamp it upright.
2. Place the boiling tube in a beaker of water to make a water bath, and suspend a thermometer in the naphthalene so that the bulb is fully covered but not touching the glass.
3. Heat the water bath gently with the Bunsen burner and start the stopwatch.
4. Record the temperature of the naphthalene at regular time intervals, for example every half minute, stirring gently and continuously.
5. Continue heating and recording, noting the time and temperature at which the solid begins to melt and the time at which it has completely melted.
6. Once the naphthalene is fully molten and its temperature has risen a little further, stop heating and remove the boiling tube from the water bath.
7. Allow the molten naphthalene to cool, and again record its temperature at the same regular intervals, stirring gently until it is safe to do so.
8. Continue recording until the naphthalene has solidified and its temperature is falling steadily again.
9. Plot a graph of temperature against time for the heating and for the cooling, and read the melting point and freezing point from the flat parts of the curves.

Expected observations

While the solid is being heated, the temperature rises steadily. When the naphthalene begins to melt, the temperature stops rising and stays constant even though heating continues; during this time solid and liquid are present together. Once all the solid has melted, the temperature rises again. On cooling, the temperature of the liquid falls steadily, then stays constant while the liquid freezes into a solid, and finally falls again once it is completely solid. The flat part of the heating curve and the flat part of the cooling curve occur at about the same temperature.

Inference and conclusion

The flat portion of the heating curve marks melting, and the flat portion of the cooling curve marks freezing. The temperature stays constant during melting because the heat absorbed is used to overcome the forces of attraction holding the particles in the fixed lattice, not to increase their kinetic energy. During freezing the temperature stays constant because heat is released as new forces of attraction form between particles. The temperature at the flat part is the melting point, which is equal to the freezing point of the same pure substance. A sharp, level plateau also indicates that the substance is pure; an impure substance melts over a range of temperatures.

Science process skills (Paper 3 style)

Making a hypothesis. As a solid is heated, its temperature rises until it reaches the melting point, where the temperature remains constant until melting is complete.

Identifying variables. The manipulated variable is time; the responding variable is the temperature of the substance; the controlled variables include the mass of naphthalene, the type of substance and the rate of heating.

Tabulating data. Draw a table with a column for time (recorded at fixed intervals) and a column for temperature, both to the precision the thermometer allows, so that every reading is entered as it is taken.

Plotting a graph. Plot temperature on the vertical axis against time on the horizontal axis, draw a smooth curve, and identify the horizontal plateau; read the melting point from the plateau.

Operational definition. The melting point is operationally defined as the constant temperature shown on the flat part of the heating curve, while solid and liquid exist together.

Safety precautions

• Heat the naphthalene in a water bath rather than directly, because naphthalene vapour is flammable and could catch fire over a naked flame.
• Work in a well-ventilated area or fume cupboard, because naphthalene vapour is harmful if inhaled in quantity.
• Do not let the thermometer bulb touch the wall of the tube, so the reading is the temperature of the substance and not of the glass.
• Handle the hot boiling tube with a clamp and let it cool on a heat-resistant mat, to avoid burns.

Common errors

• Reading the thermometer with parallax. Read it at eye level, or the temperature is wrong and the plateau is misplaced.
• Heating too quickly. A fast heating rate makes the plateau short and hard to see; heat gently through a water bath.
• Not stirring. Without stirring the temperature is uneven, so the recorded value does not represent the whole sample.
• Stopping recording too soon on cooling. Continue until the temperature is clearly falling again, so the freezing plateau is complete on the graph.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this experiment to make the kinetic theory concrete: a student who can explain the flat part of the curve in terms of energy and particle forces can answer the state-change questions that recur across SPM Chemistry. Because interpreting a heating or cooling curve is a classic Paper 3 skill (Paper 3 is a practical test assessing science process skills), practising the graph and its explanation here protects marks throughout Form 4.