Look at this diagram carefully. It shows how materials change between, solids, liquids and gases:
Now look at it again. What's happening to change a solid to a liquid to a gas? It's being heated.
What's happening to change a gas to a liquid to a solid? It's being cooled.
These are physical changes and they can be reversed, which means they go back to what they were before. When materials change in this way, it's called changing state. Do you notice each special word as the state changes (melting, condensing, etc.)?
Water is the most obvious example to demonstrate how it changes state. In the diagram above, replace:
Solid with Ice
Liquid with Water
Gas with Steam (or, better, water vapour).
Now, run through that diagram, and its words, again with all the watery words.
Does that make sense? Of course, many substances you come across will change like this - water's just an obvious example.
OK, to be able to talk about and describe these changes, we must understand the science vocabulary associated with each of the physical changes. So, let's check on that.