[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to describe how paper chromatography can be used to identify substances in a mixture we've been looking at the four ways that scientists use to separate mixtures we've seen filtration crystallization and distillation including both simple and fractional distillation another separation technique used by scientists is called chromatography this is really useful and there are several different ways of carrying this out in this video we're going to look at paper chromatography so let's get started paper chromatography allows us to separate substances based on their different solubilities so let's look at an example imagine I have a collection of colored pens such as these I want to know which pens contain only one color and which pens contain a mixture of colors we take a piece of special paper called chromatography paper and we draw a pencil Line near the bottom like this next we put a dot of our first color onto the pencil line and next to that we put a dot of our second color we can do this for several colors as long as there's enough space on the paper we now place the bottom of the paper into a solvent remember that a solvent is a liquid that will dissolve substances and there are lots of different solvents now here's a really important part the solvent now makes its way up the paper and it dissolves the ink in the two color dots and these are now carried at the paper as well now here are a couple of facts that you need to learn we call the paper the stationary phase because that does not move and we call the solvent the mobile phase and that's because the solvent moves so after a while the paper could look like this as you can see the red color formed a single spot here because there's only one spot we know that red is a single Pure Color however you can see that color B has now separated into two different spots and that tells us that color B was actually a mixture of two different colors so here's another key fact a pure compound will produce a single spot in all solvents whereas the compounds in a mixture may separate into different spots depending on the solvent I've shown you that with these two diagrams these show the experiment again but this time repeated with a different solvent you can see that the red color still produces a single spot even if we change the solvent the position of the spot may change but it's still a single spot paper chromatography works because different substances have got different solubilities a more soluble substance travels further up the paper than a substance Which is less soluble one question which is often asked in exams is why do we draw a starting line in pencil well that's because if we drew the line in pen the pen would move at the paper with the solvent coming up we're going to see how we can use paper chromatography to identify an unknown substance okay one of the real benefits of chromatography is that we can use it to identify an unknown substance I've got here a chemical I don't know what this chemical is and I'm going to find out using paper chromatography so just like before we place a DOT of our chemical onto the pencil line and we place the bottom of the paper into a solvent so here's where the chemical moves to and again we can see that the chemical is pure as we only have one spot here's where the solvent moved to and it's really important that we know that so the first thing we need to do is measure the distance moved by the unknown chemical to do that we measure from the pencil line to the center of the spot so in this example the substance moved a distance of 44 mm we then measured the distance moved by the solvent in this case 75 mm we now use these numbers to calculate the RF value for the substance we divide the distance moved by the substance by the distance moved by the solvent like this so this gives us an RF value of .57 for this substance notice that RF values do not have a unit we can now look this RF value up in a database and that will tell us the identity of the substance now I should point out that several different substances could have this RF value so we might need to repeat this experiment using a different solvent to narrow it down further also if this substance has never been analyzed before then there will not be an RF value on the database so we need to carry out further analysis to identify it remember that you'll find lots of questions on this topic in my revision workbook which you can get by clicking on the link above okay so hopefully now you should be able to describe how paper chromatography can be used to identify substances in a [Music] mixture [Music]