Elements, Compounds, Mixtures (1959)

[Music] throughout your course in chemistry you will be concerned with the nature of matter and the changes which matter undergoes along with the reasons and explanations in so far as they are known for these properties and changes therefore we might begin with a brief discussion of the classification of matter on the board you see a diagram illustrating one method for classifying matter first matter is subdivided into two broad categories homogeneous and heterogeneous matter homogeneous matter is the same throughout properties of one portion of it will be the same as properties of another portion heterogeneous matter is not the same throughout one portion will have one set of properties another portion may have other properties now homogeneous matter may be still further divided into Solutions and substances Solutions are of great importance in chemistry and will be considered in great detail later on in the course so we are not going to spend any more time on them today substances May in turn be subdivided into elements and compounds elements are substances which cannot be decomposed any further by ordinary chemical means and compounds are substances which are always comp composed of two or more elements United in a fixed proportion by weight and when two or more elements combine to form a compound the properties of the original elements disappear and the new compound with characteristic properties of its own appears now examples of elements are such materials as iron and copper and carbon and oxygen and many compounds also are known to you such as sugar and water and salt mixtures differ from compounds in that while they contain at least two or more elements or compounds or both in the mixture each element or compound retains its original property and therefore the mixture has more than one set of property in fact it has one set for each component and the mixture can be made to exhibit uh these different sets of properties and this is often of use when the mixture uh must be separated into its components in this film we're going to study the properties of two elements iron and sulfur we're then going to study the properties of the mixture of these two elements and finally we're going to study the properties of the compound which is formed when these two elements are united in my hand I have a small piece of sulfur you should note that this sulfur has a yellow color and you will have to take my word for the fact that it has no odor sulfur is hard but rather brittle as can be seen when I strike it with a hammer in this test tube we have placed some distilled water here is a lump of sulfur which I've removed from this bottle and we will drop the sulfur into the water you note that it sinks indicating that sulfur is heavier than water or more dense and that it does not appear to dissolve in this test tube I'll Place Another lump of sulfur and to this sulfur add some carbon disulfide we will now stir the sulfur in each tube for about 5 minutes after about 5 minutes we see that the sulfur in the test tube with the water has not changed its appearance at all and can be considered to be insoluble in water however the sulfur in the test tube with the carbon disulfide uh looks much different than it did before the large lump of sulfur has disappeared and only a few fluffy particles uh remain suspended in the solution the sulfur may be recovered from its solution in the carbon disulfide in this fashion first we will filter the solution containing the dissolved sulfur to remove any traces of impurities we will permit the filtrate which you can see is perfectly clear to run out onto this watch glass then we will permit the solution on the watch glass to evaporate to dry it after all of the carbon disulfide has evaporated the sulfur has been redeposited on the watch glass in Crystal form sulfur is hard as it was originally I think perhaps you can see the gleams of light from the crystal faces this is a good example of a physical change since the sulfur has been recovered after going into solution in essentially its original form some elements are attracted by a magnet While others are not when we apply the magnet test to sulfur we find that the lump of sulfur is not attracted by the magnet this is one more phys iCal property of sulfur which we will make note of for future reference since uh sulfur has shown to be uh insoluble in water and is more dense than water a simple method for determining the density of sulfur suggests itself on this balance we're weighing a piece of sulfur the sulfur weighs 30 uh 8 38 G in this graduated cylinder we placed exactly 25 milliliters of water and I'll now carefully Slide the 38 G piece of sulfur down into the graduated T the combined volume of the water and the sulfur is now 45 milliliters and since we originally placed 25 MLS of water in the graduated cylinder it's easy to see that the volume of the sulfur must be about 20 M density which is a specific physical property of a substance is defined as the mass of the substance divided by its volume for sulfur the weight of the piece of sulfur we use was 38 G and this mass or weight of sulfur occupied a volume of 20 [Applause] millit in carrying out the obvious calculation we find that the density of sulfur is 1.9 G per milliliter as determined by us experimentally when we examine the chemical properties of an element uh one of the properties that is usually examined is the reaction or lack of it of the element with acid in this test tube I will place a piece of sulfur then we'll add some concentrated hydrochloric acid in the case of sulfur no reaction takes place sulfur then does not react with concentrated hydrochloric acid in this test tube I've placed several pieces of sulfur which we will heat in the flame of the bumps and burner note that uh the sulfur melts rather readily indicating that its melting point is not uh too high you should also note that near the melting point molten sulfur is a mobile liquid it's rather low viscosity and it's yellow in color now as we heat the melted sulfur to a higher and higher temperature you can see the color becoming progressively darker and also the liquid becomes much more viscous in fact at this point the sulfur moves very sluggishly in the test tube and if we go to a little higher temperature we find we can scarcely pour the sulfur from the test tube at all this is unusual behavior because most elements become less viscous as they are heated while suur goes through a maximum uh viscosity Peak at a temperature well above its melting point next we will test in order to see whether sulfur Burns in air in a brass spoon called a deflorating spoon we've placed two lumps of sulfur we'll place the spoon the flame of the bunson burner cause the sulfur to melt and to ignite and I think you can see the blue flame with which the sulfur Burns in the spoon this is an important chemical property of sulfur its ability to combine with oxygen by burning we will now proceed to tabulate some of the physical and iCal properties of sulfur which we have just observed the color of sulfur was yellow brittle yes soluble in water no soluble in carbon disulfide yes react with hydrochloric acid no product of the reaction with hydrochloric acid obviously doesn't apply attracted by magnet no density 1.9 G per milliliter Burns in air yes melting point low we now turn to the consideration of the properties of another element I IR as an example of iron we will use these small Nails we note the color is silvery and metallic nails of course are well known to be hard but they are not brittle as sulfur was because they can be rather easily bent this property of strength with the ability to be bent is called malleability and is a characteristic of the element iron also Nails in contrast to sulfur are attracted by the magnet iron is one of the few elements it is attracted by a magnet so rather quickly we've developed several physical properties of iron in contrast with those of sulfur in the three test tubes on the screen we've placed three samples of iron two of the samples of iron consist of iron Nails the third sample is iron powder now to the tube on the extreme left we're going to add some distilled water and we see that iron is more dense than water appears to react with it very slowly if at all and certainly does not dissolve to the tube in the center we're going to add some carbon dulfi and again the iron does not react with the carbon disulfide at any appreciable rate this you remember is different than in the case of sulfur because sulfur dissolved in carbon disulfide to the tube on the extreme right we will add a few Mill of dilute hydrochloric acid in this case a vigorous reaction occurs the gas is evolved we will invert a second test tube over the reaction tube and collect a portion of this gas we will then test this gas with a burn in splint to see if it might be hydrogen because hydrogen explodes when ignited obviously the gas was hydrogen and is a product of the reaction between iron filings and hydrochloric acid we may calculate the density of Iron by the same method that we used to calculate the density of sulfur we have weighed out 94 gam of iron nails and these 94 gram of nails have been placed in this graduated cylinder in this cylinder we've placed exactly 25 mL of water and we'll now add the water to the nails in the fill the combined volume of the water and the iron Nails is now 38 ml the density of the iron Nails may be calculated using the same formula that we used for the density of sulfur our nails weighed 94 [Applause] G the combined volume of the nails and the walk water was 38 ml we had 25 M of water to start with therefore 13 ml represents the volume of 94 G of nails and the density of the nails is found to be 7.2 G per milliliter we will test the behavior of iron at fairly high temperatures by inserting a nail into the flame of this Meer burner you should observe that the nail soon glows the bright red color but that it does not melt or burn at these temperatures and under these conditions the properties of iron may now be placed on our table color plus silver ridle no soluble in water no soluble in carbon disulfide no react with hydrochloric acid yes product of the reaction hydrogen attracted by a magnet yes den 7.2 G per Mill Burns in air no melding Point [Applause] high on these small uh pieces of aluminum foil we've weighed out 4 G of sulfur and 7 G of iron the reasons for these proportions will become apparent later in the semester and we're not going to prepare a mixture of iron and sulfur the grind these two powders together in this mortar and this will give us a material grayish in color which appears to be homogeneous it is not however since mixtures of iron and sulfur actually contain thousands or millions of small particles of iron and of sulfur uh separately we'll now place a portion of this mixture in the test tube and treat it with some carbon dulfi we're doing this in order to see if the properties of the original sulfur have been altered by mixing the sulfur with the iron because as you'll recall we found that sulfur alone dissolved in carbon disulfide in the test tube we have our carbon disulfide uh solution of what we hope is the sulfur and the iron in the mixture we'll filter the solution to remove the suspended iron and catch the filtrate on a watch glass exactly as before we will then allow the carbon disulfide to evaporate after the carbon disulfide has evaporated we see that we have a deposit of crystals of sulfur entirely similar to that which we obtained when we treated pure sulfur with carbon disulfide obviously the properties of the sulfur have not been altered by mixing them with iron we will now um examine the mixture a little further to see if the iron has retained its properties in the mixture as well as the sulfur I'll place the portion of the mixture in this test to and add a few milliliters of dilute hydrochloric acid you'll recall that when we did this with pure iron hydrogen gas was obtained unvert test tube over the reaction test tube in order to catch any evolved gas we'll permit the reaction to proceed for just a moment you'll recall from our PR previous experience that sulfur does not react with hydrochloric acid and I believe you can see the uh scum of sulfur that's floating to the surface here reaction with between the iron and the sulfur is taking place at the bottom of the tube we'll now test the gas in the tube by lighting it and once again the explosion indicates the presence of hydrogen the properties of the iron have not been changed by mixing it with the sulfur therefore in the preceding experiment we saw that when hydrochloric acid is added to the mixture of iron and sulfur hydrogen gas is evolved in this experiment we will examine the properties of this gas a little further in this test tube we placed a small sample of our iron sulfur mixture in this test tube we've placed some cadmium sulfate solution the choice of cadmium sulfate for the reagent uh will become obvious later now I'm going to add some dilute hydrochloric acid to the iron sulfur mixture and cause the Evolve gas to Bubble into the cab sulfate solution and I think you can see that very little is happening we have previously studied the properties of iron and sulfur separately and we then studied the properties of the mixture of iron and sulfur we're now going to heat this mixture of iron and sulfur to cause these two elements to react chemically to form a new compound called iron sulfide I've placed some of our iron and sulfur mixture in this test tube we will now heat the test tube in the flame of the bunson burner heating is necessary in order to raise the iron and sulfur mixture to a sufficiently high temperature to cause the reaction to begin once the reaction starts however it gives off considerable heat and proceeds without further heating now the reaction has started you can see the bright red glow caused by the intense heat given off by the reaction this then is an exothermic reaction or a reaction which gives off heat don't be confused by the fact that we had to heat the mixture to cause the reaction to begin this simply was necessary in order to achieve a high enough temperature to cause the reaction to start once the reaction did start plenty of heat was produced to keep it going in order to recover our new compound of iron sulfide we have kept the test tube very hot and we will now quickly thrust it into this Beaker of cold water the glass breaks and our iron sulfide is now on the bottom of the beaker we will remove it from the beaker and examine both its chemical and physical properties having examined the solubility of iron and sulfur in carbon disulfide we now wish to test the solubility of our iron sulfide compound in that Sol solent here's a small piece of the iron sulfide which we've just made you can see that it's hard black solid obviously insoluble in water we'll place it in this test tube and then add some of the carbon disulfide we'll now wait about 5 minutes in order to give the iron sulfide a chance to dissolve and will then filter as before after 5 minutes we will filter the solution and then evaporate the carbon disulfide filtrate after the carbon disulfide has all evaporated the watch glass has disappearance you will note the complete absence of the yellow crystals of sulfur which were so obvious when free sulfur uh was dissolved in the carbon dulfi the sulfur and the iron sulfur mixture has obviously been converted into a form in which it is now insoluble in carbon dulfi we will now calculate the density of our iron sulfide by the same method that we have previously used for iron and for sulfur a sample of iron sulfide has been weighed out and placed in the graduated cylinder this sample weighs 12 G in this graduated cylinder we have 25 M of water I have the water to the graduated cylinder containing the iron sulfide agitate to remove a few Bubbles and note that the volume of the water in the cylinder is 28 and a half this is the combined volume of the water and the iron sulfide the density of the iron sulfide may now be calculated as before the mass of our iron sulfide was 12 G we found that this occupied a volume of 3.5 millit since the total volume of water and iron sulfide was 28.5 M and we had used 25 mL of water carrying out the division we find that the density of the iron sulfide is 3.4 G per [Applause] milliliter when we T Test the iron sulfide with the magnet we find that it is completely unaffected and this is in Mark contrast to the properties of the original iron obviously the magnetic properties of the iron have been destroyed when the iron was transformed into iron sulfite when we reacted iron with hydrochloric acid we observed that the gas hydrogen was produced we should now perform the same test on our iron sulfide in a test tube we've placed several pieces of the iron sulfide which we've just prepared and to this iron sulfide We'll add some dilute hydrochloric acid obviously a gas is being produced and we will replace the delivery tube and pass this gas into our solution of cadmium sulf the gas that's produced from hydrochloric acid reacts with the iron sulfide reacts with our cadmium sulfate solution to produce an instantaneous yellow precipitate this is a much different reaction than we obtained when hydrogen was passed through cadmium sulfate the formation of this precipitate is a good test for the gas hydrogen sulfide which is produced in the reaction between iron sulfide and hydrochloric acid finally we may place the properties of iron sulfide on our chart this color was black rle yes viable in water no soluble and carbon disulfide no react with hydrochloric acid yes product of this reaction hydrogen sulfide h2f attracted by a magnet no density 3.4 G per milliliter Burns in air no melting point High we saw earlier that properties of a substance are the characteristics by means of which we recognize describe and differentiate it from other substances in this experiment we have examined the properties of iron in the form of nails and of sulfur we have seen that these two elements retain their individual properties when they are combined in a mixture but that when the mixture is heated a chemical reaction takes place producing a new substance iron sulfide or Ferris sulfide and in ferrah sulfide the properties of the original iron and sulfur have disappeared and a new substance with new properties has appeared [Music]