Heating Curves of Pure Substances and Mixtures

Introduction

If you heat a substance steadily and plot its temperature every half minute, you get a heating curve . The shape of that curve is a fingerprint of purity. A pure substance produces flat steps where it changes state; a mixture produces sloping sections instead. Reading these graphs combines everything you know about melting points, boiling points and the particle model.

Core explanation

The heating curve of a pure substance. Suppose pure ice starts at −20 °C and is heated at a steady rate until it becomes steam. The graph has five sections:

1. Solid warming (−20 °C to 0 °C): the line slopes upwards as the particles vibrate faster. 2. Melting at 0 °C : the line is flat . This plateau lasts until all the ice has melted. 3. Liquid warming (0 °C to 100 °C): the line slopes upwards again as the particles move faster. 4. Boiling at 100 °C : a second, longer flat plateau until all the water has turned to steam. 5. Gas warming (above 100 °C): the line slopes upwards once more.

Why the plateaus are flat. During a change of state, the energy supplied is used to overcome the forces between particles rather than to make them move faster. The average kinetic energy of the particles, and so the temperature, stays constant. This hidden energy is called latent heat . The boiling plateau is longer than the melting plateau because far more energy is needed to separate particles completely into a gas than to loosen them into a liquid.

The heating curve of a mixture. For a mixture, such as a solid containing an impurity or salt water, the changes of state are not flat . Melting starts below the pure melting point and the temperature rises gradually as more melts, producing a gentle slope instead of a plateau. Similarly, a solution begins to boil above the pure boiling point and the temperature keeps rising slowly as it boils. The corners of the curve are rounded rather than sharp.

Cooling curves. The same ideas work in reverse. When a molten pure metal such as tin cools, its cooling curve shows a flat section at the freezing point, where latent heat is released as the particles form bonds and settle into a solid. An impure sample freezes over a sloping range. Metallurgists use cooling curves to study alloys.

Reading the evidence.

Feature of curve Pure substance Mixture --- --- --- Change of state Flat plateau Sloping section Temperature of change Matches data book Shifted (melting lower, boiling higher) Corners Sharp Rounded

Step-by-step reasoning

To interpret a heating curve:

1. Identify the sloping sections (one state warming up) and the flat or gently sloping sections (changes of state). 2. Read the temperature of each change of state from the axis. 3. Check whether each change is flat (pure) or sloping (mixture). 4. Compare the temperatures with data-book values to identify the substance.

Visual explanation

Sketch temperature on the vertical axis and time on the horizontal axis. For pure water, draw a staircase: slope, short flat step at 0 °C, long slope, long flat step at 100 °C, slope. For salt water, draw the same staircase but tilt the steps so they rise slightly, with the melting step starting below 0 °C and the boiling step starting above 100 °C.

Real-world analogy

Filling a staircase-shaped container is a little like heating. Water poured in raises the level steadily until it reaches a wide shelf, where the level stops rising while the shelf fills. Only when the shelf is full does the level climb again. Energy during a change of state goes into "filling the shelf" rather than raising the temperature.

Real-world example

Candle makers and chocolatiers watch temperatures closely because their materials are mixtures. Chocolate does not melt at a single temperature; it softens over a range because it contains different fats. Pure substances such as gallium, which melts at about 30 °C, show a sharp change instead and can melt in your hand.

Why?

Why does a mixture give a sloping change of state? As an impure solid melts, the liquid formed holds more of the impurity, while the remaining solid becomes purer and needs a higher temperature to melt. In boiling, the solvent leaves and the remaining solution becomes more concentrated. Either way, the composition keeps changing, so the temperature does too.

Common misconception

"On the flat part of the curve, no energy is being supplied." The heater is still supplying energy at the same rate. The energy is used to break the forces between particles during the change of state, so the temperature does not rise.

Worked example

Question: A solid is heated steadily. The temperature rises to 80 °C, then stays exactly at 80 °C for four minutes before rising again. Later it stays constant at 218 °C. What can you conclude?

Reasoning: Two flat plateaus show sharp changes of state at fixed temperatures, which is typical of a pure substance. The first is melting at 80 °C and the second boiling at 218 °C.

Answer: The substance is pure, with a melting point of 80 °C and a boiling point of 218 °C. (These match the values for naphthalene.)

Quick check

1. What does a flat section on a heating curve show? Answer: A change of state of a pure substance, taking place at a constant temperature.

Exam focus

Label heating curves with the state or states present in each section, for example "solid and liquid" on the melting plateau. Explain the plateau using energy: energy goes into overcoming forces between particles, not into increasing kinetic energy. Be ready to sketch the sloping version for a mixture.

Advanced insight

The length of each plateau is proportional to the latent heat. For water, the specific latent heat of fusion is about 334 J/g, while that of vaporisation is about 2260 J/g, almost seven times larger. This is why the boiling plateau of a water heating curve is much longer than the melting one when heating at a steady rate.

Summary

A heating curve plots temperature against time. For a pure substance, changes of state appear as flat plateaus at fixed temperatures, because energy goes into overcoming forces between particles. For a mixture, changes of state appear as sloping sections starting at shifted temperatures, because the composition changes as the process goes on. Cooling curves show the same features in reverse.

Practice questions

1. Why is the boiling plateau for water longer than the melting plateau when heated at a steady rate? Answer: Much more energy is needed to separate particles completely into a gas than to change a solid into a liquid. 2. Describe how the heating curve of salt water differs from that of pure water at the boiling stage. Answer: It starts boiling above 100 °C and the temperature slowly rises during boiling, giving a slope instead of a flat plateau. 3. Which states are present during the flat section at 0 °C on the heating curve of pure water? Answer: Solid and liquid, ice and water together. 4. A cooling curve for molten wax shows no flat section, only a gradual slope. What does this suggest? Answer: The wax is a mixture of substances, so it solidifies over a range of temperatures rather than at one fixed point.