Change of state

Level 1

What is a change of state?

Matter exists in three states: solid, liquid and gas.
When it changes from one state to another, it is called a change of state.

  Examples:

  • An ice cube melting changes from a solid to a liquid state; this is Melting
  • Boiling water changes from a liquid to a gas state; this is Vaporisation
  • Condensation on a cold window is water changing from a gas to a liquid state; this is Liquéfaction

The 6 changes of state

Noms des Changement d'états

Change of state temperatures

Each transformation takes place at a very specific temperature, called the change of state temperature.

  Example with pure water:

  • At 0 °C ➝ solid ↔ liquid
  • At 100 °C ➝ liquid ↔ gas

  The melting temperature is the same as the solidification temperature: 0 °C

Measuring temperature: the plateau

You can measure these temperatures yourself with a thermometer. A substance is heated (or cooled) and its temperature is recorded over time.

  Surprising observation: throughout the change of state, the temperature stops changing! For example, as long as there is melting ice left, the water + ice mixture remains stuck at 0 °C. This "pause" in temperature is called a plateau.

The temperature only starts rising again once all the solid has melted. The plateau is therefore the sign that a change of state is taking place.

Characteristic temperatures

Every substance changes state at its own temperatures. They are like "thermal fingerprints"!

Substance

Melting / Freezing

Vaporisation / Condensation

State at T°C

Water

0 °C

100 °C

 

Vinegar

-17 °C

118 °C

 

Aluminium

660 °C

2470 °C

 

Oxygen

-219 °C

-183 °C

 

 

Reading and describing a temperature graph

When studying a change of state, its temperature is recorded minute by minute to obtain a graph.
The temperature can:

  • Increase / Rise : the points are higher and higher. 
  • Remain constant : the points are at the same level. 
  • Decrease / Fall : the points are lower and lower. 

 

Description of the temperature graph

T (°C) t (min) 0 10 20 30 40 50 60 70 1 2 3 4 5 6 7 8 9 10 11
Hover over the graph
Explore the different heating phases

Let us take an example with a heated unknown solid. Here is a description of the graph:

  1. Phase 1 : the temperature increases from 5°C to 40°C
  2. Phase 2 : the temperature remains constant at 40°C (this is the change of state)
  3. Phase 3 : new temperature increase up to 70°C

The flat area is the temperature plateau. It allows us to identify when and at what temperature the change of state occurs.

Here, the change of state therefore takes place at 40°C.

Analysing a graph

Analysing a graph means searching for information about the matter being studied. We can extract 3 pieces of information from a change of state curve:

Analysis 1: Change of state temperature

Identifiable by the temperature plateau. We can find the temperature at which the substance changes state.

Analysis 2: State of matter"

  • Before the plateau ➝ a single state, the initial state
  • During the plateau ➝ two states coexist
  • After the plateau ➝ new state, the final state

Analysis 3: Purity of the substance

  • Pure substance ➝ sharp, horizontal plateau
  • Mixture ➝ no sharp plateau, gentle slope

Example of analyzing a solid melting curve.

Pas de palier = mélange Présence d'un palier = Corps pur Solide Solide + Liquide Liquide Palier de T : T fusion = 40°C
Hover over the graph
Explore the different possible analyses

What heat does to molecules

When we heat a substance, we supply it with thermal energy.
This energy is absorbed by the molecules, which:

  1. Move around more and more
  2. Break their bonds
  3. Become less organized

This is how a solid becomes a liquid, then a gas.

Conversely…

When we cool it:

  • The molecules lose energy
  • They slow down and can form bonds again.
  • They move closer together and become more organized

 

We therefore notice that a change of state only changes the organization and the motion of the molecules. Thus, the nature of the molecules does not change, so it is a physical change (see transformation)