[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe how the particles are arranged in solids liquids and gases and how this Arrangement changes during melting freezing boiling and condensing you should then be able to explain why some substances have higher melting and boiling points than others and finally if you're a higher tier student you should be able to describe the limit itations of the simple particle model now there are three common states of matter and I'm showing them here we've got solids liquids and gases as you can see the particles are arranged differently in these three states of matter and you need to be able to describe how they're arranged we're going to start by looking at the features of solids and then explain these features using particle theory firstly solids are extremely hard to compress in other words squeeze that's because the particles in a solid are packed together in a regular pattern with almost no spaces between the particles secondly solids have a fixed shape and they cannot flow from place to place that's because in a solid the particles can vibrate but they cannot move from place to place okay let's look at liquids now just like solids liquids are extremely hard to compress in and again that's because the particles are close together with not many spaces between them unlike solids liquids take the shape of their container and they can flow from place to place and that's because the particles in a liquid can move okay we're going to look now at gases firstly gases are extremely easy to compress that's because the particles and gases are widely spaced secondly gases spread out and fill the space of their container and that's because the particles in the gas move quickly and randomly now we can change the state of a substance by putting in or taking out energy if we heat a solid it can change state to a liquid this is called melting and this takes place at the melting point now as you can see the particles in the liquid are moving around so they've got more kinetic energy than the particles in a solid this means that we have to put energy in to convert a solid ID into a liquid this energy is needed to break the forces of attraction between the particles in a solid once those forces of attraction are broken the particles can now move around in other words we've changed the solid into a liquid now there's one really important fact that you need to understand the stronger the forces of attraction between the particles the more energy we have to put in to break these forces and the higher the melting point I'm showing you here a wax candle candle wax is a solid with a relatively low melting point in candle wax the forces of attraction between the particles are relatively weak so they're fairly easy to break this shows sodium chloride sodium chloride is also a solid however in sodium chloride the forces of attraction between the particles are very strong so sodium chloride has a very high melting point now if we cool a liquid then we can convert it back to a solid and this is called freezing freezing takes place when we cool a liquid back down to its melting point when a substance freezes the forces of attraction between the particles reform now if we heat a liquid we can convert it into a gas this is called boiling and boiling takes place at the boiling point just like before if there are strong forces of attraction between the particles then the boiling point will be high that's because takes a lot of energy to break those strong forces of attraction and boil the liquid now if we take a gas and cool it down we can convert it back to a liquid this is called condensing condensing takes place at the same temperature as boiling in other words the boiling point and when a substance condenses the forces of attraction between the particles reform okay now if you're a higher tier student then you need to be able to describe the limitations of the simple particle model of solids liquids and gases firstly the simple particle model assumes that all particles are solid spheres now this is not the case particles have lots of different shapes and they're not solid secondly in this simple particle model shown here it's assumed that there are no forces between the particles now as we've seen this is not correct in fact the forces of attraction between the particles have a major impact on the melting and boiling points of a substance you'll find plenty of questions on this topic in my vision workbook which you can get by clicking on the link [Music] above