>> Here we have two bowling balls. One is blue and one is orange yellow in color. Please notice that they are the same size. In other words, they occupy the same volume. In case you are not living in a country where bowling is a popular sport, there's a link and a YouTube video that you can check out to get the idea of how these balls are used before you make any predictions. So feel welcome to pause the video and go to that link if you would like. Do you think these bowling balls will sink or float when we put them in a water bath? >> Okay, here we have two bowling balls. One's blue, one's orange. Notice that they're the same volume. Now, does a bowling ball sink or float when we put it in water, what do you think? >> Now that you've made your prediction, let's go ahead and do the experiment and make some observations. First, let's put the blue bowling ball in the water to see if it floats or sinks. That bowling ball definitely sank quite rapidly. And I would imagine that most people you meet on the street would say that bowling balls will sink rather than float in water, so that wasn't a surprise. Next, let's put the light orange bowling ball in the water to see if it floats, or if it also sinks. ha, this one floats. Even when Doctor Lyle tries to push it down, this bowling ball pops back up to the water's surface. Think about this. Why does one of the bowling balls sink, while the other bowling ball floats? Before answering this question, let's try another set of objects. One that probably a greater number of you will recognize, thanks to international marketing efforts. Both of these cans of soda are full and both occupy the same volume. Following the theme from the last set of objects, one of these will sink and one of them will float. Which one will float? Let's start this set of experiments by setting the regular Coke in the water bath. Regular cans of Coke do not float in the water. But what about Diet Coke? Again, they are the same volume but one sinks and the other floats. Why? Finally, let's test some objects that are different sizes. Here are two metal spheres. You can easily see that one is quite a bit larger than the other one, in terms of volume. The larger one also has a slightly larger mass than the smaller one. So, the one with the larger volume is also slightly heavier. Just to reiterate how these spheres compare to each other, the big one is relatively heavy and the small one is relatively light. Can you predict just by looking at them whether these spheres will sink or float in water? It's time to do the experiment and find out. First let's put the smaller sphere in the water. Doctor Lyle is being especially careful, because he doesn't want to break the fish tank glass on the bottom of the tank. No doubt about it, the smaller ball sinks. Now Mary Jane is putting in the larger sphere. Hmm, the larger sphere has more mass than the smaller sphere, but yet the larger one floats. Why is this true? It must be because the smaller sphere is more dense. In fact, the observations we made about the bowling balls and the cans of soda and the metal spheres tells us something about their relative densities. The density of an object is its mass divided by its volume. Rho is is the symbol most frequently used for density. Rho looks like that. So rho equals mass divided by volume. In chemistry calculations, densities are most frequently expressed in units of grams per cubic centimeter. Other units can be used of course, but the most common units used in chemistry are grams per milliliter or which is the same thing as grams per cubic centimeter. Elements and compounds have known densities in their pure state. At room temperature, water has a density of about one gram per mil. Some substances are much lighter than water. For example, when there's an oil spill in the ocean, the octane, or the petrol, floats on the water because that substance has a density of less than one gram per milliliter. Other substances that are less dense than water included gases, such as air. And the components of air, such as nitrogen, carbon dioxide, and helium, and oxygen, gas. Other substances have greater densities than water in their pure state. For example, many metals have densities approaching ten grams per milliliter. And then there are objects which are extraordinarily dense, such as the nucleus of an atom, which has a huge density. It's so large, it's unfathomable to me. Because of the way density relates a substance's mass to its volume, a substance's density can be used as a conversion factor. For example, if you know an object's mass, then you can figure out its volume, if the substance is pure, and you're able to look up its density in a reference material. Thank you for watching these demonstrations introducing the concept of density.
