Lecture: Matter 1Preview The lectures in this unit cover an introduction to chemistry and matter, working with numbers and units, and an introduction to atoms and the periodic table. This lecture covers the nature and classification of matter. Matter In the previous lecture we discussed how researchers further their scientific knowledge and how they communicate (and validate) their findings. Since Chemistry is the study of matter, we want to know: what have researchers discovered about matter? Firstly, as they studied matter they determined it was helpful to classify matter based on easily observed criteria. We find it convenient to classify matter by state, by properties, and finally by composition. Once classified, we can make predictions about what qualities matter has at the microscale (the scale in which we cannot see the smallest particles) which give rise to each of these differences in the macroscale (the scale in which we can see things). I. Classification by state This method of classification refers to the physical state of the matter. Physical state refers to whether or not the matter is a solid, a liquid, or a gas. The properties of these three states of matter are as follows: Solids have a distinct shape and specific volume, and cannot be compressed. Liquids have a specific volume, but no distinct shape. They take the shape of whatever container they are in. They can only be compressed slightly. Gases have no distinct shape or volume. They conform to the space of the container and they are highly compressible. Now that we have identified the properties of each state of matter on the macroscale, let’s use that information to predict the nature of matter that we can’t see on the microscale. After all, just because we can’t see the particles doesn’t mean that we can’t tell something about them based on how they behave in aggregate. Here’s what we can predict about the smallest particles in solids, liquids, and gases based on what we just described previously: Solids: The smallest particles in a solid must be packed together very tightly because they cannot be compressed. They must be in fixed positions relative to one another because they have a distinct shape. If you were one of these smallest particles you might feel a little like a sardine in a tin can, or a passenger on a subway that is so packed you couldn’t even turn around: much less get off at the stop you needed.Lecture: Matter 2 Liquids: The smallest particles in a liquid must be packed together moderately because they can be compressed very little. But they must also be able to move past each other because liquids flow and have no distinct shape. If you were one of the smallest particles in a liquid you might feel like a passenger in a packed elevator – the kind where you can kind of slide out past the rest if you turn sideways. Gases: The smallest particles in a gas must be very far apart with space between them because they are highly compressible. They must have great ability to move because they also flow and expand and travel through space. If you were one of these smallest particles you might feel like one of just a few soccer players on an enormous field. This is a pretty straightforward example of how we can look at what we see in the world around us and use that to help us predict how the particles themselves are interacting. As we talk about other classification methods you might want to continue to ask yourself: what is it on the microscale that causes substances to appear or behave in these ways at the macroscale? II. Classification by properties The next method for classification is based on properties, or attributes. We’ll talk about four different types of properties in two different classifications: A. Physical vs. Chemical Physical Properties can be measured without changing the identity and composition of the substance. For example, I can tell that the color of a balloon is yellow, that the odor of a flower is sweet, or that the density of a liquid is 0.719 g/mL. Other physical properties include melting point, boiling point, mass, volume, and hardness. Chemical Properties describe how a substance may change, or react, to form other substances. As a result they can only be measured when a chemical undergoes a chemical change. For example, in order to determine if a substance is flammable, I have to try and light it on fire. If I want to find out if a substance is radioactive, I need to measure its radioactive decay. If I want to find out if something is edible, I have to find somebody willing to eat it (and then hope they don’t get sick or die). B. Intensive vs. Extensive Intensive Properties don’t depend on the quantity of a substance. Milk has a white color regardless of whether I have a cup of it or 500 gallons. It doesn’t get whiter the more of it I have. The density, melting point, boiling point, and hardness of a diamond are the same whether or not that diamond is .5 carats or 50 carats.Lecture: Matter 3Extensive Properties do depend on the quantity of a substance. The mass and volume of 500 gallons of milk varies greatly from that of 1 cup. The amount of heat energy released by burning 1 gallon of gasoline is much less than can be released by burning 30 gallons. III. Classification by composition Finally, we can try and classify substances by composition: what is matter fundamentally made out of? This is a bit of a trickier question since it is hard to guess what the smallest particles look like and how they interact to form different substances. But we can start by classifying anything that is an easily separated combination of multiple substances as a Mixture, and anything that cannot be easily separated into multiple pieces as a Pure Substance. A. Mixtures Any mixture can be separated into its component substances by physical means, because each of the mixture’s component parts retains its original properties. For instance, salt water is a mixture of salt and water. Salt is naturally a solid which melts at >800 ˚C and water is naturally a liquid which boils at 100 ˚C. If we heat up the salt water above 100 ˚C, the water evaporates and the salt is left behind. This is because even though the salt is mixed with the water, it still doesn’t ‘melt’ until > 800 ˚C, and the water, even though mixed with the salt, still boils off at ~ 100 ˚C. We exploit the physical properties of