[Music] hi and welcome back to frees 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 and this is a required practical so it's essential that you learn all the details now we've already looked at paper chromatography in the playlist on atomic structure in the periodic table I'd strongly recommend that you watch that video again before continuing with this one in this video we're looking at the method for carrying out paper chromatography we're going to use it to work out the colors in food coloring so let's get started we've got a sample of food coloring which is a mixture of chemicals we're going to call this U for unknown we also have four known food colorings that it could contain and we're going to label these a to d first we use a ruler to draw a horizontal pencil line on the chromatography paper the line should be around 2 cm from the bottom of the paper we now Mark five pencil spots at equal spaces across the line we need to leave at least 1 cmet clear at each side next we use a capillary tube to put a small spot of each of the known food colors and the unknown color onto the pencil spots the capillary tube is simply a very thin glass tube so here's the paper with the food colorings it's important that we keep the spots relatively small this prevents the colors from spreading into each other later now we pour water into a beaker to a depth of 1 cm remember that in this case the water is the solvent we now attach the paper to a glass rod using tape and we lower the paper into the beaker the bottom of the paper should dip into the water now there are three key points here firstly the pencil line with the spots of ink must be above the surface of the water otherwise the water will wash the ink off the line secondly the sides of the paper must not touch the side walls of the beaker if that happens then it will interfere with the way that the water moves and lastly we usually put a lid on the beaker to reduce evaporation of the solvent now at this stage the water will move up the paper and the colors will be carried off during this time we need to be careful not to move the beaker we remove the paper when the water's traveled around 3/4 up at this stage we use a pencil to Mark the point where the water reached and finally we hang the paper up to dry now as you can see the unknown color has separated into three spots telling us that this is a mixture of three colors the spots in colors a c and d line up with the spots in the unknown color so this tells us that the unknown color is a mixture of colors a c and d we can also see that the unknown color does not contain color B now what if we wanted to identify the chemicals in these colors well to do that we calculate the RF values so let's look at that now the first thing we need to do is measure the distance from the pencil line to the center of each spot I'm going to do that for the spot produced by color a in this case the chemical mve the distance of 14 mm we then measure the distance moved by the water from the pencil line in this case that distance is 70 mm to calculate the RF value we divide the distance moved by the chemical by the distance moved by the solvent like this so this gives us an RF value of 0.2 for the chemical and color a 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 chemical now I should point out that several different chemicals 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 chemical has never been analyed before then there will not be an RF value on the database so we'd need to carry out further analysis to identify it remember you'll find plenty of questions on this required practical in my vision workbook and you can get that by clicking on the link above [Music]