States of Matter
The history of atoms and particles dates back thousands of years. Empedocles was a Greek philosopher who is famous for his theory that all matter was made by four different elements: earth, air, water and fire.
Democritus is often cited as the father of atom theory and first used the term ‘atomos’ (which translates as indivisible) to describe his idea that matter was made of small particles. He came to this idea after thinking about continually breaking a rock into smaller and smaller pieces. He decided there must be a stage where you cannot break the rock into any more pieces. It wasn’t until the English Chemist John Dalton's work nearly 2000 years later that atomic theory became the accepted scientific viewpoint. Dalton proposed that all the elements were made of different types of atoms and that these atoms were unbreakable. Although now we know atoms can be broken through nuclear reactions, many of his ideas have stood the test of time. In 1827 botanist Robert Brown observed pollen grains moving around in water seemingly on their own. This Brownian motion was later put to bed by Albert Einstein who hypothesized that it was the movement of the water molecules that were making the pollen grains move.
Matter comes in three states: solid, liquid or gas. These lesson plans use the simple ball model of particles to explain more complicated molecules. A water ‘particle’ is actually made up of three atoms, but treating it as one particle makes it easier to understand when describing the arrangement of the molecules. It is important the students are able to define a pure substance as a substance that is made of one type of atom or molecule.
The addition of thermal energy to a system of particles increases the average kinetic energy. A decrease in kinetic energy can reduce the temperature of a system or change the state of a system from a gas to a liquid or a liquid to a solid. In a solid, the particles are arranged in a regular pattern and are very close together. They cannot move around each other, but can vibrate about a fixed point. Of the three states, particles in solids have the lowest kinetic energy. As the particles get more thermal energy (often by being heated), they vibrate more. There becomes a point when the particles have sufficient energy to move around each other. We describe this state as liquid. In a liquid, the particles are still very close together, but have a random arrangement. They still vibrate, but can move past each other, allowing liquids to flow. The particles' ability to move is also why liquids will fill the shape of whatever container they are in. If we heat these particles even more, the bonds between the particles break and they become a gas. The particle arrangement for gases is random and the particles are spread out. They fly around colliding with each other and the sides of their containers. Gases can be compressed as there is lots of space between the particles. The more they are compressed, the more they collide with their container and each other. The collision of particles and other material exerts a force known as pressure.
Pressure is affected by the by various factors, such as the temperature of the system, the number of particles, and the volume of the container. The pressure of the system can affect what state the matter is. With a high pressure, more thermal energy is needed for particles to change from a liquid phase to a gas phase. With a low pressure the opposite is true; less thermal energy is needed for particles to change from a liquid phase to a gas phase.
The most commonly used example to teach students about the states of matter is H2O, or water. This is one of the few substances that can be found naturally on Earth in all three states. Water has a melting point at 0° C (32° F, 273.2 K) and has a boiling point of 100° C (212° F and 373.2 K). Water is most commonly used because students have experience with all three states. Ice, water, and steam are all made from the same type of particle, but each of the substances look and feel very different. Water is quite strange, however; ice is less dense than water and the solid floats on top of the liquid, which is not typical of other substances. This peculiarity has allowed living creatures to survive in the water insulated by the ice and life to evolve the way it has.
How the particles of a substance are arranged determine the different properties of a material. Diamond and graphite are made of the same types of particles, but have completely different properties. Diamonds are hard transparent crystals, while graphite is dark, shiny, and can conduct electricity. This is all to do with how those particles are arranged.
Essential Questions for States of Matter
- What happens to the particles when we change from one state to another?
- Do particles change when they go into a new state? Are the particles in ice the same as the particles in water?
- How is energy related to state and state change?
Other States of Matter Lesson Plan Ideas
- Create a storyboard that imagines what the world would be like if there were no liquid state.
- Create a timeline to show how atoms were discovered.
- Show the state of different substances using temperature instead of time on a timeline.