 Today we're going to talk about chemical reactions and what I'd like to do first is to talk about the difference between a chemical change and a physical change. A chemical change is something that actually changes the nature of the materials that are involved in a reaction. The physical change is something like changing ice into liquid water, in other words melting, or having water evaporate into a water vapor. These are changes because when the changes are reversed you go back to water and you can actually go back to the ice. So today when we talk about chemical reactions we're talking about reactions where the chemical composition is actually changed. To illustrate this I'd like to give you a general reaction of A plus B gives C. And let's talk a little bit about the kind of things that we have in this reaction. The first thing is that on the right hand side of this reaction are reactants. And if you want to know where your reactants are essentially the arrow is pointing away from the reactants and toward the products. So the products are on this side. Now this is a very simple form of reaction and you may remember that reactions will also need to be balanced. So we want to make sure that when we balance a reaction that you have the same number of each type of element on both sides. When we do reactions we simply have to state what the conditions are. And for instance if you see a triangle which we can refer to as delta this can mean change. But in the case of a reaction typically it means that heat has been added to the process. And so we have heat essentially driving the reaction. You may see something like this and that is a symbol for a wavelength. You may also see this represented as something that looks like an H and a V but this is actually H nu which is also a form of light. And so in this instance instead of using heat to cause a reaction to occur you will use light to cause the reaction to occur. You also when you have a reaction you indicate what the states of your reactants are. And S stands for solid. L is for liquid. G is for gas. And AQ is for aqueous. Aqueous means that the chemicals are dissolved in water. They are in a solution with water as the solvent. And it's important to know the states of the reaction because that can affect things as well. There are several types of reactions that we consider. The first reaction that typically we consider is called a combination reaction. And the combination reaction is represented by the reaction that we listed earlier. You have two components forming a single product. The reverse of a combination reaction is decomposition. And the decomposition reaction would be C giving A plus B. Or in other words a product or a material is breaking down into individual chemicals. Another type of reaction that we have is called a single replacement reaction. In this reaction what you see is that A which would typically be a metal would replace a metal ion in the BC to form AC plus B. The last chemical reaction that we typically talk about is called a double replacement reaction. In this reaction what you see is that the cations exchange places and are bonded to a new anion. So these are your basic types of reactions. Examples of these reactions would be the following. For a combination reaction nitrogen plus hydrogen giving ammonia is a combination reaction which is a very important reaction for making fertilizers. Water can be broken down into its individual components of hydrogen and oxygens. And this reaction is typically done using an electrical reaction or as the electricity causes these to separate. And these reactions are quite common and there is probably going to be new technology that will use this to produce energy as we move into the future. A single replacement reaction would be something like zinc plus copper chloride giving zinc chloride plus copper. And in that instance zinc is replacing the copper ion in the solution. A double displacement reaction would be lead nitrate plus potassium sulfate giving lead sulfate. And this lead sulfate will be a solid and potassium nitrate. And the interesting thing about this reaction is that you get a very pretty yellow precipitant at the end of it. Summarize we've talked a little bit about reactions and we've talked about the individual type of reactions and given you examples of each one. Thank you.