Objectives:

  1. To be able to use the particle model of matter to explain the density of materials.
  2. To fully understand that changes of state are physical changes; the change does not produce a new substance. If the change is reversed the substance recovers its original properties. Additionally, during a change of state the mass is conserved.

Firstly, it is important for you to understand the states of matter and how their structures change as more energy is supplied (via heat for example) or when their energy is removed (when they are cooled). The following shows and explains this quite nicely (click on the image to enlarge it if need be);
The changes of state can be laid out in the following way;

Note: A change of state is a physical change and not a chemical change. This means you do not end up with a new substance. It is the same substance you started with but in a different form.

Why is it different to say we have the same substance but in a different form?

  1. If you reverse a change of state (e.g. condense a substance that you previously boiled), the substance will return to its original form and regain the properties its original properties from when in the previous state.
  2. The number of particles does not change, they are just arranged differently. This means mass is conserved; none of it is lost when the substance changes state.

The density of a substance can also be used to explain behaviour of different states. For example, why when placed in a bucket of water does a diet coke can float and yet a can of regular coke sink? Additionally, why does lemon float and a lime sink? Watch the short video below to learn more.