The state of a matter is a variable property of matter. The state of a substance can be influenced by heat and pressure, thus changing the substance's state of matter. As this is thought to be an introduction, we focus on the classical states of matter, although there are many more states of matter to be discovered!
There are three classical states of matter:
The matter is rigid and does not change form or volume.
The matter is not rigid. It easily changes form but not volume.
The matter is not rigid. It easily changes form and volume.
For rememberance, here a the scientific terms to describe changes in states of matter:
1. A solid can melt to a liquid or sublimate to a gas.
2. A liquid can freeze to a solid or evaporate to a gas.
3. A gas can deposit to a solid or condense to a liquid.
You see that each state of matter can change into one of the two others. This can best be visualised through a triangle with 6 corresponding arrows for each term of change. Try to draw one yourself to understand this concept better.
Every chemical substance binds their molecules chemically in one or the other way. These bonds have an energy associated to them. If external energy is greater than bond energy, then these bonds may break. This may also occur the other way round, if energy is extracted from the substance, for example, by cooling.
This is also always the point where the change in the state of matter will take place. The level of energy is also connected to temperature, as temperature increase also increases particle movement, and thus kinetic energy. Pressure comes into equation as well. Pressure is able to reduce volume, and therefore higher pressure leads substances towards the solid state, lower pressure towards the gaseous state. For example, you can bring water to boil easily in near-vacuum conditions.
But when is the very energetic point when the state of matter begins to change slowly? You first need to note that not all matter changes it state instantaineously, but rather gradually. Temperature and energy are nearly equivalent as you've seen above. Similarly, if you bring water to boil, not all of it will evaporate instantly.
For each term of change in state of matter, there is a certain temperature or energy associated with it, called "point" for temperature or just "energy". So to bring water to boil, you'll need to reach the boiling point or boiling energy for this specific substance to boil. Every substance has its specific temperatures and energies where change in state of matter begins to take place. So you'll have specific freezing points, condensation points, etc. for every substance imaginable.