Particles of a substance can be present in different physical states, the states of matter. They are solid, liquid and gas, and increasing or decreasing the kinetic energy of the particles changes which state the substance will appear in.
The properties of the different states can be clearly seen in the table below:
When a solid is heated to its melting point, the particles gain enough kinetic energy to break the forces of attraction between the particles, so the particles are no longer in a regular, densely packed and fixed pattern. The space between the particles has increased enough that the physical state has changed.
This is the same for boiling, the state of matter changes from liquid to gas due to increased movement of particles. Freezing and condensing work similarly, but the opposite way.
Sublimation is the change from a solid to a gas, skipping the liquid phase. This can happen due to a sudden increase in temperature, or at high pressure that means the particles spread quickly, forming a gas instead of melting.
Changes in state are also affected by pressure, the melting and boiling point can be increased by increasing the pressure. this is because increased pressure forces particles closer together and means it is more difficult for them to spread out at the same temperature.
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