[Music] hey there welcome back to another mate science easy chemistry lesson in this lesson we are going to be looking at what purity is and how we can determine if the substance is pure or if it is not pure well the first thing we really need to understand is what do we mean by the term pure and a pure substance very very simply is something that is not mixed with anything else there is only one type of substance in something with pure we often use the word pure incorrectly in everyday language and we need to make sure that when we talk about pure things were using the correct chemical terminology so first of all all elements are pure and an element of pure because they only contain one type of atom hydrogen will only contain the hydrogen atom so two pure nitrogen will only contain nitrogen atoms so it's pure individual compounds are pure pure water will only contain h2o molecules and nothing else if it's pure sodium chloride salt will only contain sodium chloride molecules if they're pure nothing else so a pure substance only contains one type of substance we can look at it sampler mineral water when we talk about things that appear in everyday language we would say mineral water is pure but from a chemical point of view is mineral water is bottled water actually pure well the answer is no mineral water is not pure and the reason for this is that mineral water contains the following things it contains h2o it contains water molecules but it also contains potassium and calcium and many other dissolve minerals if we have a look at this little diagram here we can see a simplified version of what water is not pure through a bottle of water some mineral water and it contains water it contains potassium and it contains calcium there are three different substances in this water it is not pure pure substances only contain one type of substance mineral water is what we call a mixture when there is more than one type of substance that is not chemically joined together and it is not in a fixed ratio so what do we mean by pure we can see here we've got some equipment and this equipment is used for a process called distillation distillation is a method of obtaining pure liquids and we're going to look at how it works in later lessons but what we need to understand at the moment is the pure substance as we've already said is something that's not mixed with anything else so all elements are pure because they're not mixed individual compounds are pure because they're not mix but water is only pure if it contains molecules of water and nothing else so if we pour some mineral water into this container and we heat it the water molecules will evaporate the potassium and the calcium and all of the other minerals in there will not evaporate so we can create pure water by evaporating away the water molecules only and removing all of the impurities from that water now purity can be incredibly important but not always the water that we drink is impure but it doesn't matter as long as it's clean but since such as medicinal drugs and food additives need to be pure if there are any impurities in them then these substances may cause us harm so any unwanted substances that stop something from being pure unknown as impurities now we can work out to something pure through some very simple methods one of the simplest ways of finding out to some is pure is to check the substance melting point and its boiling point pure water boils at a hundred degrees Celsius and melts at zero degrees Celsius so they have a definite boiling point and a melting point if they are pure but if a substance is impure they might melt and it might boil over a range of temperatures so if water is impure it will not necessarily boil acid exactly a hundred degrees Celsius and it will not melt at exactly zero degrees Celsius so impurities reduce the melting point making it lower and they increase the boiling point making it higher but this is why if you live in a cold climate you might put salt on the roads if it's below zero degrees Celsius you'd normally expect water to freeze and become icy this is very dangerous if you're driving but if you put salt on the roads you lower the melting point water will now freeze at below zero degrees Celsius this means it can get much colder and the water on the roads will still not freeze meaning it is safer to drive also if you add salt to water it will boil at a slightly higher temperature in pure water boils at above a hundred degrees Celsius one of the simplest ways you can find out of a substance is pure then is to test its melting point and its boiling point if it does not melt or boil at the expected temperature and it melts and boils over a range of temperatures then we know it must be impure another way we can determine purity is to use a technique called chromatography chromatography can separate different colored dyes from their ink so you can separate colored liquids into all of the different colors that they're made from it can also be used to identify proteins and amino acids in order to test for purity using chromatography we use the following steps first of all we add some solvent to a beater a solvent is just a liquid that can dissolve another substance so water and alcohol are very good examples of solvent we then take a sheet of filter paper and using a pencil we draw a line at the bottom of the coma repaper the reason why we use pencil is because the graphite in the pencil is in soluble it will not develop in our solvent then using our pencil we draw two crosses on the line this is where we're going to mask on the ink that we are going to separate we then on our crosses mark on the ink we then place our chromatography paper into the solvent but we need to be really really careful the ink should not directly touch the solvent this is really important if the ink directly touches your solvent it's just going to dissolve into that water or into whatever so when you're using and the experiment will not work we then wait for the solvent to move and the solvent will move up the paper and as it does our inks will also dissolve and move up the paper and the different dyes will separate out from each other we then leave what we call a chromatogram to dry now we can see here that when we had a purple ink and a blue ink they separated out into different dyes our purple ink only had one die within it so this is a pure substance our blue ink on the other hand had a blue and a yellow dye that were part of it so it is an impure substance because there is more than one substance that makes it up so this is how we can use chromatography to test the purity we can also work out what substances we have within our chromatograms by calculating something called an RF value all different substances have a 6 RF value or retention factor and we can identify which substances we have in our mixture by calculating their RF value and it's very very easy to do so the RF value is equal to the distance moved by the substance divided by the distance moved by the solvent front the solvent front is the top of the solvent how far our X moves up our chromatography paper so the first thing we do is we measure our solvent front in this case I'm just going to call it a we then measure the distance moved by the purple dye and we measure the distance moved by the yellow dye and by the blue dye and we obviously if used to rule us to measure how far each one has moved I've taken the center point of the dye is where we're going to measure it moving from and we obviously measure to our pencil line because this is our starting point we can measure these values and record them in a table so the first thing we measure is our solvent front and the solvent has moved 88 millimeters so for all of our calculations in this example the solvent front is going to be 88 we then measure the distance move by diet B so 48 we repeat relation for C 29 and for D 62 millimetres so we measure the distance moves by each day and the solvent front so the equation is very simple for our purple dye 48 millimeters the distance moved / 88 millimeters our solvent front equals an RF value of 0.55 for the yellow dye the distance moves 29 millimeters divided by the solvent front 88 millimeters gives us an RF value of 0.33 just note there is no unit for an RF value it is just a number finally for our blue dye 62 millimetres the distance moved by the dye / 88 millimeters the distance moved by the solvent front gives us an RF value of 0.7 now it's worth pointing out the RF values must always be lower at than one if you have an RF value greater than one then you've done something incorrect the reason why an RF value must be less than one is because the dye cannot move silent in the solvent now there are other types of chromatography as well chromatography can be used to identify amino acids and these require a solvent to water nice acid and butanol all mixed together now amino acids are colorless which means we can't actually see how far they've moved so we need to add another chemical to them called a locating agent and this makes the amino acids visible so we can see them we can measure how far they've moved and we can calculate that RF value once we have their RF value we can work out what amino acid we're dealing with a good example of a locating agents are something called ninhydrin so in summary a pure substance is something that has no other substances mixed with it most substances even drinking water are impure medical drugs and food additives need to be pure in order to be safe pure substances have fixed boiling points and melting points in pure substances have a lower melting point and a higher boiling point than pure substances and chromatography can be used to identify impurities and substances and we can use an RF value to identify different substances substances that are colorless such as amino acid can be identified by using a locating agent so I hope you now understand what we mean by purity and you have some idea about how we can work out if a substance is pure or if it is not pure okay thank you so much for watching this lesson I hope you found it really useful don't forget to check out our other lessons on YouTube and if you want to learn even more about science then you can check out our website the URL is in the description below please give a like and subscribe thanks very much until next lesson keep on learning