Physical and Chemical Changes

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Physical and Chemical Changes

 

this is about physical and chemical changes I’m going to do some things here on my kitchen table and you need to be a science detective and identify the physical and chemical changes that you see here and then we will finish off with our top three exam oriented questions on this topic before we start let’s do a quick recap on physical and chemical changes in a physical change no new substance is formed and it’s reversible but in a chemical change a new substance is formed and it’s irreversible so the two important questions to ask yourself when deciding the type of change are is a new substance formed is it irreversible if the answer to both of these questions is yes then it’s a chemical change otherwise it’s a physical change so watch carefully the things I’m doing here and you try to classify them as a physical change or a chemical change so are you ready let’s start I’ll start by lighting this candy here now I’m going to make some lemonade I’ll show you my recipe hopefully it turns out good so first cut the lemon I need powdered sugar so I’m going to crush this raw sugar in the grinder here then mix the powdered sugar and water stir it so that the sugar dissolves nicely in the water now squeeze some lemon and add a pinch of salt let’s drop in some ice cubes I’ll wait for the ice cubes to melt so that the lemonade becomes cool now let me enjoy the lemonade it’s perfect and refreshing the candle is still burning can you see the wax that has melted here are you ready with your answers in all the changes you saw these are the chemical changes in these changes some new substances are formed and here are the physical changes in these changes no new substances are formed a burning candle is an interesting example where both physical and chemical changes are taking place the physical change is the melting of the wax and the chemical change is the burning of the candle now that we’ve looked at the examples of physical and chemical changes let’s look more closely at the differences between the two for physical change we’ll use the simple example of melting of ice and for chemical change we’ll use the example of burning of a candle the first difference is that in physical change no new substances are formed so for example when ice melts you get water it may look different but chemically it’s the same it’s h2o if we try to represent the melting with a reaction it would look like this as you can see it’s water h2o on both sides only the state is changing from solid to liquid in chemical changes new substances are formed so in the burning of the candle what are the new substances that are produced the candle is a hydrocarbon and on burning it gives out carbon dioxide and water vapor of course we can’t see that because they are colorless gases here is the simple unbalanced equation for burning of a candle as you can see in the equation the hydrocarbon is combining with oxygen and producing carbon dioxide and water vapor the second differences that physical changes are reversible they’re temporary for example the melting of ice is temporary we can easily reverse it by refreezing the water back chemical changes on the other hand are irreversible they are permanent for example the burning of candle is a permanent change because once the candle is burnt there’s no way we can get it back let’s add these two important differences between physical and chemical changes on our concept board the third difference is regarding mass in a physical change the mass of the substance remains the same but in a chemical change the mass of the substance generally changes let’s go ahead and verify this with the help of a simple experiment in this experiment let’s measure the mass of the ice cubes before and after melting first let’s measure the mass of the ice cubes as you can see it’s 33 grams now I’m going to let the ice melt after all the ice has melted to water as you can see the mass is still 33 grams so in a physical change the mass remains same in a chemical change the mass of the substance generally changes let’s verify this with a simple experiment for the experiment I’m going to use this new candle and we are going to measure its mass before burning it and after burning the candle I’m using this small candle because otherwise it’s going to take a long time to burn so first let’s measure the mass of the unburned candle the mass of the unburned candle is 10 grams now I’m going to light the candle you now let’s measure its mass after burning we are left with only three grams of the candle so the mass of the candy has reduced by seven grams but doesn’t this violate the law of conservation of mass the law of conservation of mass states that matter can neither be created nor destroyed so the mass should remain constant even for a chemical change but

 in this case we saw that the mass of the candle is decreasing so how can you explain that well the reason is we’ve been only looking at the mass of the candle we haven’t accounted for the gaseous products because when a candle burns carbon dioxide and water vapor are produced and there are gases they go off into the atmosphere and we not accounted for their mass so this experiment is done in a different way let’s say we burn the candle in a closed container like this then what do you think the reading is going to be that’s right the mass of the container and the reactants will be the mass of the products and the container so then the law of conservation of mass is going to be satisfied the last difference is regarding energy now this is the confusing one because you find it expressed in different ways in different textbooks in both physical and chemical changes energy may be given up or it may be absorbed but the amount of energy in physical changes is much smaller compared to chemical changes for example when you burn wood you start the reaction with a small mass stick so with a little amount of energy but then we end up with a lot of energy in the form of heat and light if you compare that to a physical change like melting of ice or boiling of water energy is absorbed there but it’s much smaller compared to chemical changes let’s spin the last two differences between physical and chemical changes on our concept board so what do you think is the most important difference here that’s right new substances are produced during a chemical change but in a physical change nothing new is formed the irreversible point is not the most important one because a physical change can be irreversible what are some examples cutting of a lemon or breaking of a glass these are irreversible changes but since nothing new is formed they are

 classified as a physical change for a chemical change something new needs to be produced now that we are done with the concept of physical and chemical changes are you ready for the top three questions on this topic coming up for you right now.