Matter comes in three states: solid, liquid or gas. Each state has a different particle arrangement, which allows particles to move (or not move), and sometimes this particle arrangement can change, altering the state of matter. Adding 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 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. Once the particles have sufficient energy to move around each other, the state changes from a solid to a liquid. The amount of kinetic energy needed to change a solid into a liquid depends on the makeup of the solid and it's "melting point".
In a liquid, the particles are still very close together, but have a random arrangement. They still vibrate, but can move past each other, which allows 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. There's lots of space between the particles, meaning that gasses can be compressed. 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 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, a characteristic that is not typical of other substances. This peculiarity has allowed living creatures to survive in the water insulated by the ice and allowed life to evolve the way it has.
The activities in this lesson plan use the simple ball model of particles to explain more complicated molecules in order to provide students with a solid base of understanding. 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.