Chemical Change (1958)

[music] [music] [music] >> The wonderful world in which we work and play is constantly changing. Some of the changes going on around us are easy to understand. We can understand how moving water changes the surface of rocks. And it's easy to see how an axe changes the shape of wood. Although we know that all things are made up of atoms and molecules arranged in chemicals and compounds, it is not always easy to understand chemical change. For example, in lighting a fire, this man is starting a chemical change. As the wood burns, it changes first into charcoal, which is one form of carbon. As burning continues, the wood becomes smoke and ashes, which are entirely different. A chemical change produces one or more substances that are different from those that were present before the change. Watch the bubbles of gas which rise as pancake batter cooks. Are the bubbles like any of the materials from which the batter was made? No, they are not. So, the bubbles are result of a chemical change. Note how the bubbles increase in size as the change goes on. Changes are chemical changes. This is formed. When ice cream melts, it changes only from a solid to a liquid. And when a tomato is sliced, it looks different, but it's still a tomato. These are physical changes. This thermometer contains the element mercury, which can help us understand the difference between physical and chemical change. Mercury undergoes physical change when it expands and rises with rising temperature. Or when it contracts and falls with falling temperature. In spite of these physical changes, the mercury is still mercury. However, if mercury is heated gently in the presence of oxygen, a new substance slowly begins to form on top of the mercury, a substance which is different from mercury itself. What has happened is that mercury has combined with oxygen to form a substance called mercuric oxide. A chemical change has taken place. Mercuric oxide is a compound made up of two elements, mercury and oxygen. You can see that the mercuric oxide is different from the silvery metal mercury or the colorless gas oxygen contained in this bottle on the right. We have seen how mercury and oxygen combined to form mercuric oxide. Now, let's see if we can take mercuric oxide apart, separating the mercury and oxygen. As we heat mercuric oxide in this special apparatus, oxygen is driven off, collecting in this tube. And as the oxygen is driven off, we find tiny beads of mercury deposited on the cooler parts of this tube. Here, we started with a compound, which in a chemical change yielded two elements, mercury and oxygen. Let's try some other elements this time. We'll use a spoon made mostly of silver and the hot solution containing a compound of sulfur. Do you suppose we will get a chemical change? And if so, what sort of change will it be? In a short while we see that a chemical change has taken place. The silver in the spoon has combined with the sulfur to form a dark coating of silver sulfide, commonly called tarnish. Here again, we have seen the type of chemical change in which two chemicals, silver and sulfur, combine to form a new compound, silver sulfide. Now we'll try an experiment which may remove the sulfur from the compound. In a pan lined with aluminum foil, we have water in which baking soda and salt are dissolved. We bring the water to a boil. If the experiment is successful, the sulfur should be removed from the silver. The silver sulfide compound will be taken apart. The experiment is successful. Once again, we have seen a chemical change in which a compound is broken down into elements. Here's a compound you've all seen, rust. Iron has combined with oxygen from the air to form iron oxide. You already know that iron is attracted by a magnet. Now let's look at iron oxide. Pure iron oxide is soft and reddish, entirely different from either oxygen or iron. And this iron oxide is not attracted by a magnet. Remember that iron combines with oxygen to form the compound we have just seen, iron oxide. In order to take the compound iron oxide apart, we have to mix it first with another element, carbon. And then we'll heat the mixture to a very high temperature. It may take some time to get the mixture hot enough so that the chemical change takes place. The container must be covered so that we keep out as much oxygen from the air as possible. We've heated the mixture for some time now, so perhaps the chemical change has taken place. Perhaps some of the iron oxide and carbon have combined to form different compounds. At first glance, the mixture looks much as it did when we began to heat it. But the carbon should have taken oxygen away from the iron oxide to leave pure iron. We can see if this is so by testing the mixture for iron. We can do this by holding a magnet under the paper. And sure enough, there is iron attracted by the magnet. We can see the iron particles following the magnet as it is moved about under the paper. Suppose we didn't want iron to rust. Could we keep the chemical change from taking place? Of course. We could keep oxygen from reaching the iron by painting it as this man is doing. Since fire is the result of a chemical change in which some material combines with oxygen, how could we put out a fire? That's right, by shutting off the supply of oxygen. In this case, by covering the material with soil. These farmers know that the crops they plant need certain chemicals to grow well. To find out if the soil contains these chemicals, they'll send a sample of the soil to a chemist. The chemist's bottles of chemicals have labels which tell him what they contain, but there are no labels on the chemicals in the soil. The chemist must discover what chemicals are present by what he knows of chemical change. He knows that one group of chemicals called acids, which might be in the soil, always produce a certain chemical change. That change is this. Acids turn blue litmus paper pink. In order to test the soil, it is first mixed with distilled water. Testing the mixture, he finds that it does turn blue litmus paper pink. So, he knows that this soil contains some acid compound. Of course, an acid does not have to be in soil to cause this chemical change in blue litmus paper. Let's try some other substances. Tomato juice turns blue litmus pink, so it is an acid. Vinegar turns blue litmus pink. It, too, is an acid. But ammonia does not affect the color of the litmus paper. So, ammonia is not an acid. If a substance does not turn blue litmus pink, it may be a base. We test for bases by using red litmus paper. A base turns red litmus blue. Ammonia does cause this change. Ammonia is a base. From a number of tests such as these, the chemist can determine exactly what chemicals the soil contains and what ones it may lack. Following the chemist's recommendation, the farmer can apply fertilizer containing whatever chemicals are necessary for best plant growth. And so, out of the knowledge of chemical change may come better crops for the farmer. But this is only one of many rewards which come from understanding chemical change. The beauty of nature is more appreciated by one who understands chemical change. The green material on this copper downspout is a compound resulting from chemical change. Outdoors, indoors, everywhere you look, you can find examples of chemical change and of some of the marvelous uses to which man has put his knowledge of it. Uses such as this, where elements combine and compounds separate to form substances that were not present before. Another example of the chemical changes that constantly surround us in our changing world. >> [music] [music] [music] >> Hey.