 In this section we're going to go over some chemistry basics, many of which you may have encountered in junior years. The purpose of this section is to get everyone up to the same standard before we tackle a more complex stuff, and to fill in any little gaps that you may have in your chemistry knowledge so far. So let's begin. The universe is made of energy and matter. We chemists are interested in both of these things, but for us it all begins with matter. Matter is stuff that has mass that can be weighed. That's solids and liquids and gases and plasmas. And all matter is made of tiny, tiny particles. We can divide matter into two classes, pure substances and mixtures. In pure substances all the particles are identical. And in mixtures there are different kinds of particles rubbing shoulders with each other. For example, salt, table salt is a pure substance. All the particles in salt are salt particles. Water is also a pure substance. All the particles in it are water, as long as it's actually pure water. But if you mix salt and water then it becomes a mixture and the salt particles and the water particles are mixed together. They don't change their identity. The water particles remain water particles and the salt particles are still salt. So they can be separated back into pure substances. For instance, you could evaporate the water to leave the salt behind. They're simply jumbled up together like different LEGO blocks in a box. Depending on whether the different particles are mixed evenly or not we can distinguish two types of mixtures. In homogenous mixtures the different particles are thoroughly mixed, so that if you looked at one part of the mixture you'd see roughly the same types and numbers of particles as in any other part of the mixture. Salty water is a homogenous mixture. We would call it a solution, since the salt ions are evenly distributed through the water. Many metal alloys are like this as well, with atoms of one metal randomly distributed among atoms of another. In heterogenous mixtures, on the other hand, the particles are not evenly distributed. Particles of one kind can be clumped or organized. Muddy water is an example of this. The clumps of dirt in muddy water are large clusters of particles. And in whipped cream each bubble in the cream is a clump of air. If the clumps are small enough mixtures of these kind are called colloids, or well-mixed salad dressing, or smoke, and some gels are examples of colloids. So matter is made of particles.