In this video, we're going to look at the three states of matter, solid, liquid, and gas, and see how substances can change from one state to another. To do this, we're going to use a model called particle theory, or sometimes kinetic theory, which help us explain how the particles in each state behave by considering each of the particles as a small, solid, inelastic sphere. In solids, there are strong forces of attraction between the particles, which holds them all close together in a fixed position to form a regular lattice structure.
And because the particles are fixed, the overall substance keeps a definite shape and volume, so can't flow like a liquid. The particles can vibrate around though, so you can imagine them constantly jostling against one Now if we heat up a solid, its particles gain more energy and start to vibrate even more, which weakens the forces between them. And at a certain temperature, which we call the melting point, the particles will have enough energy to break free of their bonds, and so the solid melts into a liquid. In liquids there are only weak forces of attraction between the particles, so they're free to move around and are arranged pretty randomly.
However, the weak forces of attraction do mean that the particles tend to stick together, and are fairly compact. This means that they have a definite volume, even though their overall shape can change, allowing them to flow to fit a particular container. If we then heat up our liquid, the particles will again gain more energy, and this will make the particles move around faster, which weakens the forces holding the particles together.
Then once we reach the boiling point, the particles will have enough energy to break the bonds altogether, and so the liquid boils or evaporates into a gas. In gases, the force of attraction between the different particles is very weak, so they're basically free to move around by themselves. This means that gases don't keep a definite shape or volume, and instead will always fill a container, as they spread out as much as possible.
Now we normally say that gases are constantly moving with random motion, which is a bit confusing because gas particles actually move in straight lines, they don't randomly swerve. What we really mean by random motion is that the particles can travel in any direction and they'll end up being deflected by solid walls and other gas particles randomly. When we heat up a gas and the particles get more energy and so travel faster, the gas will either expand if the container it's in is expandable, like a balloon, or if the container is fixed, then the pressure will just increase. On the other hand, if we cool the gas down enough, then the particles won't have enough energy to overcome those forces of attraction between them, and so bonds will start to form between the particles, condensing the gas into a liquid.
As we cool down the liquid even further, the same thing happens. The particles won't have enough energy to overcome the attraction between the molecules, and this time even more bonds form, fixing the particles in place and freezing the liquid into a solid. Now the last thing we need to mention is that if we're working within a closed system, changes in state won't change the mass at all.
as they're still the same number of particles. However, the density of the substance will change, with solids having the highest density, liquids having slightly lower density, and gases having the lowest density of all three. That's everything for today's video, so if you enjoyed it then do give us a like and subscribe, and hopefully we'll see you soon!