Particle Arrangement

If energy is continually added to a substance, there reaches a point where the particles cannot go any faster without changing into another state of matter - this is called a phase change. Recall that particles in gases are fast moving and spread out from each other. In liquids, particles are closely packed but can slide past each other. In solids, the particles are closely packed and held in a rigid formation. The reason each state of matter had different shapes is because of the particle attractions in each state of matter.

Particle Attraction

When energy is added and the particles cannot move any faster in the current state of matter, the energy is used to overcome the attraction between particles and causes a change of state. The additional energy increases the potential energy of the particles. Potential energy is stored energy due to the interactions between particles or objects. Potential energy increases as the distance between particles increases. Conversely, the potential energy decreases as the distance between the particle decreases. The particles that are farther apart have greater potential energy. The potential energy of the particles, determined by the state of matter present, contributes to the total energy of a substance.

Heating Curves

The graph below is the heating curve of water. Just as in the graphs you created, it shows what happens to temperature as energy added to a substance. As energy is transferred to a material, temperature increases when the state of material is not changing. The kinetic energy of the particles increases. This increases of the speed of the particles.

When substance is changing state, temperature stays the same at the melting and boiling points. The increases the distance between the particles.

Particles and Melting Points

Each substance has a unique melting and boiling point temperature. This is because the particles that make up that substance have greater attraction to each other. The more attracted these particles are to each other, the more energy it takes to increase the distance between particles. This results in higher melting and boiling points. The type of particles that make up a substance affect how much energy is needed to cause a phase change. This is why different substances are in different states at the same temperature.