CHEMISTRY 101: Classifying matter by state and composition

We're going to study chemistry from the perspective of an Atoms-First approach. Using this perspective, we have two fundamental postulates that we'll come back to again and again. The first is that matter is particulate, or made of particles. Matter is anything that has mass and takes up space. The second postulate is that the structure of those particles determines the properties of the matter as a whole. Particles that are the building blocks of matter, we'll call atoms. Two or more atoms can bond together to form a molecule. An example is water, which is a particle made up of two hydrogen atoms and one oxygen atom. Our postulate would say that the threedimensional arrangement of these three atoms, which we will call the molecular geometry (that's something we'll learn about when we discuss VSEPR theory), helps determine the properties of water. The geometry of water is called bent, making it a polar molecule (again, we'll learn more about this later), but it gives it particular properties. If water had a linear geometry like carbon dioxide, which is dry ice, it might have a much lower boiling point, and it could possibly even be a gas at room temperature. So the structure of water affects the boiling point, which is a property of water. In chemistry, we try to understand the properties of matter by studying the particles that compose it and their structure. When we classify matter, we can talk about its states, which are solid, liquid, or gas, and we can also talk about its composition or the type of particles that compose it. The state of matter depends on the relative positions of the particles that compose it. So for solids, the particles are close together in fixed positions with respect to one another, and they simply vibrate while maintaining those fixed positions. For liquids, the particles can move with respect to one another, but they stay relatively close. But for gases, the particles are far away from each other, so there's a great deal of empty space between particles. So the state of matter depends on the relative positions of particles that compose it and also the forces between the particles relative to temperature. We can begin to explain some of the properties of each state given the relative positions of the particles. For example, solids have a definite volume and a definite shape because the particles are in fixed positions with respect to one another. Liquids, on the other hand, have an indefinite shape because the particles move with respect to one another, but they still have a definite volume because the particles remain relatively close to one another. And gases have an indefinite volume and indefinite shape because they not only move with respect to one another but they're very far away from one another. They can take the shape of the container they're in. This is also why solids are not compressible, but gases are compressible—those molecules have lots of space between them in gases. We can also classify matter with respect to composition. Matter can be classified as either a pure substance or a mixture. And then we can further classify pure substances as either an element or a compound, and mixtures as either heterogeneous or homogeneous. Pure substances are composed of only one type of particle. For example, if the Goodyear blimp is filled with helium gas, helium exists as a single atom. It's a noble gas. So helium gas is just composed of these helium atoms. Water is also a pure substance. Water is made up of water molecules, which are all the same type of particle. Pure substances can be further classified as either an element or a compound. One difference between helium and water is that helium is an element. Elements are the simplest building block chemically for matter. Whereas water is considered a compound, and compounds contain two or more different elements. Mixtures contain two or more different particles. Examples would be mixtures of sand and water or a cup of hot tea. Homogeneous mixtures are uniform throughout. Our cup of hot tea is a homogeneous mixture. If I took just a teaspoon out of my cup of hot tea, it would have the same composition as the rest of the cup. Not so with sand and water. Sand and water do not have a uniform composition, so it's a heterogeneous mixture. There are clear areas in my mixture of sand and water that have more sand or more water. Another example of a heterogeneous mixture would be beef stew. A spoonful of beef stew might have a chunk of beef, and the next spoonful might have vegetables. So these are some ways we can classify matter by state and composition.