Heating Curves

Introduction

If you take ice straight from a freezer at −20 °C and heat it steadily until it becomes steam, what happens to its temperature? You might expect the thermometer to rise smoothly the whole time. It does not. The temperature rises, stops, rises again, and stops again. Plotting these readings gives a heating curve , a graph that reveals where changes of state happen and lets you read off melting and boiling points directly.

Core explanation

Plotting the curve. A heating curve shows temperature on the vertical axis and time on the horizontal axis , for a substance heated at a steady rate. For a pure substance, it has a characteristic shape made of sloping sections and flat sections.

The five sections for water heated from −20 °C to 120 °C:

Section What the graph does State(s) present What is happening --- --- --- --- A to B Slopes upwards Solid only Ice warms from −20 °C to 0 °C B to C Flat at 0 °C Solid and liquid Ice melts C to D Slopes upwards Liquid only Water warms from 0 °C to 100 °C D to E Flat at 100 °C Liquid and gas Water boils E to F Slopes upwards Gas only Steam warms above 100 °C

Sloping sections. Here the energy supplied makes particles move or vibrate faster, so the temperature rises. Only one state is present.

Flat sections (plateaus). Here the temperature stays constant even though heating continues. The energy is being used to overcome attractions between particles, changing the state. Two states are present together. The first plateau is at the melting point ; the second plateau is at the boiling point .

Plateau lengths. The boiling plateau is much longer than the melting plateau. Boiling needs far more energy than melting because the particles must be separated completely, so at a steady heating rate it takes longer.

Different slopes. The slopes differ between states because each state needs a different amount of energy to raise its temperature by one degree. Liquid water, for example, warms more slowly than ice or steam for the same heating rate.

Step-by-step reasoning

To interpret any heating curve:

1. Label the axes: temperature (°C) up, time across. 2. Find the flat sections. 3. Read the temperature of the first flat section: the melting point. 4. Read the temperature of the second flat section: the boiling point. 5. Name the state or states present in each section.

Visual explanation

Imagine a staircase drawn at an angle: a slope up, a short flat tread, a longer slope up, a long flat tread, then another slope. Beside each section, small particle diagrams show a regular solid lattice, a solid-and-liquid mixture, a liquid, liquid with gas bubbles, and finally widely spaced gas particles.

Real-world analogy

Imagine filling a bath with two overflow shelves. The water level rises until it reaches a shelf, then stays level while water spills across the shelf to fill it, then rises again. The level is like temperature; the shelves are like changes of state that absorb energy without any rise.

Real-world example

Food scientists record heating curves for fats such as butter and chocolate. The temperature at which the curve flattens tells them where the fat melts, which controls how chocolate snaps, how it melts in the mouth and how it must be stored and tempered in a factory.

Why?

Why does the curve have two plateaus and not one? A substance changes state twice as it goes from solid to gas: once when it melts and once when it boils. Each change needs energy to overcome attractions without speeding up the particles, so each produces its own flat section.

Common misconception

"On a flat section no energy is being supplied." Heating continues at the same rate throughout. The energy is still going in, but it is being used to change the state rather than to raise the temperature.

Worked example

Question: A heating curve for a pure substance has flat sections at −39 °C and 357 °C. Identify its melting and boiling points and its state at 25 °C.

Reasoning: The first plateau is the melting point, −39 °C; the second is the boiling point, 357 °C. Room temperature, 25 °C, lies between these values.

Answer: Melting point −39 °C, boiling point 357 °C; it is a liquid at 25 °C (this is mercury).

Quick check

1. What does the first flat section on a heating curve represent? Answer: The substance melting, at its melting point.

Exam focus

Be able to sketch a labelled heating curve with both axes named and units given, and to state which states are present in each section. Examiners often ask why the temperature stays constant on a plateau and why the boiling plateau is longer than the melting plateau.

Advanced insight

An impure substance does not give sharp plateaus. Its melting section slopes gradually over a range of temperatures, and its boiling section may also rise slowly as the composition changes. So the shape of a heating curve can reveal not only the melting and boiling points but also whether the sample is pure.

Summary

A heating curve plots temperature against time as a substance is heated steadily. Sloping sections show one state warming up; flat sections show a change of state at constant temperature, with two states present. The first plateau is at the melting point and the longer second plateau is at the boiling point, because boiling needs more energy than melting.

Practice questions

1. Name the two quantities plotted on a heating curve. Answer: Temperature (vertical axis) against time (horizontal axis). 2. Which states are present during the second flat section of a heating curve for water? Answer: Liquid water and gas (steam). 3. Explain why the boiling plateau is longer than the melting plateau. Answer: Boiling needs much more energy than melting because particles must be completely separated, so at a steady heating rate it takes longer. 4. A heating curve has plateaus at 0 °C and 100 °C. What substance could it be, and is it pure? Answer: Water; the sharp plateaus at exactly these temperatures suggest it is pure.