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 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 stage 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 you can't flow like a liquid the particles can vibrate around though so you can imagine them constantly jostling against one another now if we heat up a solid as particles that 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 they have a definite volume even though the 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 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 is 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 force 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 is 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