hey class welcome to our third video here in chapter one uh we're going to talk briefly here about chemical versus phys uh sorry chemical versus physical changes um and then we'll talk uh briefly about energy and how we think about that so to start off physical changes are changes that don't involve a shift in composition so for example an example of a physical change would be when a substance melts or boils in those cases the say you have water and you have ice well the ice melts it was water before it was just frozen and then after it melts of course it's liquid water so the chemical identity of the substance never changes in a physical change so the atoms or molecules that compose that substance do not change their identity during a physical change as i said before when water changes state either from solid to liquid or in this case we're looking at liquid to gas the composition of the water does not change right in the liquid water we still have water molecules that make that up and then in the vapor phase we have water molecules that make that up as well it's just in a different state but its chemical composition is exactly the same when something happens that alters the composition of the substance that is considered a chemical change and for chemical to for a chemical change to occur a chemical reaction must occur so what you get at the end of that is something completely different it has completely different physical properties than it did before because it's a different substance so during a chemical change or we could say a chemical reaction the atoms rearrange which transforms what we had originally into something new and again with that we get completely new properties example of that would be when a nail rust so a new nail right is just made of iron and that's all it has right no oxygen nothing like that it's just iron and of course it may be some alloy or something right but here's kind of a simplistic version where it's a pure iron but over time as that iron is exposed to air and water that nail will begin to rust right metals have a tendency to rust and what that is is a reaction with oxygen so we start with something that's just iron and it has all of those properties that we would attribute to iron right it's silver it's hard it conducts electricity right any number of attributes we could give to iron but after it rusts and becomes iron oxide and we get those oxygen atoms embedded within i guess on the outside of that nail right we form that that rust that iron oxide that's gonna that's gonna change the properties quite a bit right uh iron oxide is not going to conduct uh electricity all that well it's not as strong it changes color right it's brown or orange as opposed to silver and and really there's no simple way to go back right you would have to it's not like you could just cool it down and it goes back to iron or heat it up and it goes back to iron you would actually have to perform a chemical reaction upon it in order to get rid of that rust so that would be an example of a chemical change another example we could give would be like gasoline in your car right that gasoline is made of hydrocarbons and then when it's burned when it's combusted in the presence of oxygen in the engine we create a new substance right we create carbon dioxide we create water from what was a hydrocarbon and oxygen it's not so simple right to take carbon dioxide and make gasoline from it all right you would have to do a series of chemical reactions in order to to get back to where you started there it's not like a physical change and i should mention there are other physical changes i i didn't mention um you could just change its shape right that would be a physical change so you could take a piece of paper and you tear it in half it's still paper right it doesn't change the chemical composition of that substance whatever it is or you could change its shape right you take aluminum foil and you want it into a bowl and you didn't change its identity right it's still aluminum foil it has the same properties it did before it still conducts electricity and all that jazz uh other physical properties the smell right so when you uh smell gasoline that's just that gas the gasoline molecules binding to your scent receptors but it doesn't change the identity of that gasoline it's still a molecule of gasoline some hydrocarbon that's bound to your scent receptor but it doesn't change its chemical composition back in the day one of the ways that we could tell if a solution is acidic or basic is that chemists would taste it right so if it's sour usually that indicates that it's an acid if it were was bitter that tend to hint that it was a base but you know different chemicals have different tastes right if you tasted sugar that's going to taste a lot different than acetone okay so uh taste the way that those molecules interact with our taste and our smell receptors we can consider those physical properties of the substance color appearance melting point boiling point density those are all properties that are inherent to that substance and all of those together right that's actually how we can identify stuff that and along with some spectroscopic evidence but back in the day right this is this would be how you would identify what something is by its odor taste color so on and so forth but i guess the point is we can test all of these right without changing the identity of that substance a chemical property is something that is really only on display when it is actually reacting with something um we would talk about flammability right that would be an example of a chemical property and that's just its propensity to participate in combustion reactions right how how easy is it to catch it on fire or flammability right um yeah corrosiveness would be an example of that so if you place it onto metal does it corrode the metal is it acidic or is it basic is it toxic right so if you ingest it what sorts of problems does that cause those are chemical properties of that substance and in organic chemistry which you'll take next year we get very very much into these chemical properties we talk about different groupings of atoms called functional groups and really the whole class is just exploring the chemical properties of those different functional groups and looking at the different reactions they can do energy is defined as the capacity to do work and there are different types of energy that we can think of thermal chemical so on so forth but the work when it's defined is the application or action of a force through a distance so energy basically says how much of that can you actually do and as i said before there's going to be lots of different ways that we'll think about energy we'll think about it in many different forms throughout the class all energy though can be thought of as either kinetic energy or potential energy now they come in different forms many times right so kinetic energy that you all are probably most familiar with is just i mean all kinetic energy is the energy of motion but in our day-to-day lives right we think about the movement of macroscopic objects so imagine we have this 10 kilogram weight at the top of a building that weight has potential energy due to its position right it's high up off the ground so if we were to let it go right we take it off of that ledge it would then fall to a lower energy position which is the ground and we see this in nature all the time things always tend towards lower energy it's actually one of the uh laws of thermodynamics okay that things tend uh towards lower energy it's stated in a little bit different way but that's in essence what it's saying but as we throw that weight off the potential energy is converted into kinetic energy so as it falls the block is going faster and faster and faster but we're not adding energy to it right energy is just being transferred from potential the energy of its position to kinetic the energy of its motion and then when it hits that block has to be at lower energy than what it was before but where does that energy go in a situation like this it's lost as heat now we see this a lot or we see this all the time in chemistry where we will say that a molecule or compound is of high energy and we really are talking about its position on this energetic landscape we're saying that that molecule or that compound is high in energy but in chemistry that energy is contained within the chemical bonds so when we do a chemical reaction and those bonds get rearranged into something that's more stable something that has lower potential energy we'll see that energy being released as heat in the chemical reaction now on the microscopic level what is kinetic energy it's just the energy that's stored in the motion of the molecules in the atoms so molecules are in constant motion or at least their atoms are always in constant motion through their vibrations now you heat uh you heat something up heat a sample up those atoms and molecules start moving faster and faster the bonds start vibrating faster and faster in those motions that is kinetic energy that is temperature okay temperature is just a measure of the kinetic energy within a sample and that's what this is saying right that the thermal energy is uh basically how hot something is uh and on a molecular level it's down to the motions of the atoms and molecules that make that up so energy is always conserved all right again one of our [Music] laws of thermodynamics and all it can do is just change forms so if we look here in a car you take gasoline and then in the engine it's being burned so we're taking this gasoline hydrocarbon we're turning it into carbon dioxide and water well gasoline is a high energy molecule there's a lot of energy that's stored within the bonds of gasoline carbon dioxide and water are at low energy so what we see is that difference in energy between the gas molecules and water and carbon dioxide which is the product i should also say oxygen right that difference in energy can be used to do work and in a car the work that's done is pushing pistons up and down which turns a crankshaft which is hooked to your transmission and then that's where i lose it right in terms of knowing how the heck a car works but anyways you're taking that heat energy and it's being turned into mechanical energy by the engine and then that drives your car forward but that energy ultimately comes from the chemical reaction of the hydrocarbon gasoline with oxygen in the engine of your car so you're turning that potential energy that was in the gasoline into two things heat and kinetic energy of your vehicle all right that's it for this part see you in the next one bye