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Notes On The Definition Of Physical Changes And Chemical Changes

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Published in: Chemistry
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The notes on the understanding of difference between Physical and Chemical changes.

Asha A / Al Ain

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Qualification: M.Sc-Physics

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  1. Definitions Physical Change Physical change is change in the appearance, smell, state or simple display of a substance without any change in its composition OR Any change that occurs without altering the chemical composition of a substance is aphysical change. Physical changes can include changing the color, shape, state of matter, or volume of a substance. Chemical change Chemical change is any change that results in the formation of new chemical substances Notes for preparation 1. Chemical change is any change that results in the formation of new chemical substances. At the molecular level, chemical change involves making or breaking of bonds between atoms. These changes are chemical: iron rusting (iron oxide forms) gasoline burning (water vapor and carbon dioxide form) • eggs cooking (fluid protein molecules uncoil and crosslink to form a network) bread rising (yeast converts carbohydrates into carbon dioxide gas) , milk souring (sour-tasting lactic acid is produced) • suntanning (vitamin D and melanin is produced) Physical change rearranges molecules but doesn't affect their internal structures. Some examples of physical change are: whipping egg whites (air is forced into the fluid, but no new substance is produced) magnetizing a compass needle (there is realignment of groups ("domains") of iron atoms, but no real change within the iron atoms themselves). boiling water (water molecules are forced away from each other when the liquid changes to vapor, but the molecules are still 1-120.) dissolving sugar in water (sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.) dicing potatoes (cutting usually separates molecules without changing them.) Classification of real processes can be tricky. Complex changes can be broken down into many simpler steps. Some of the steps are chemical and others are physical, so the overall process can't cleanly be placed in either category. For example, boiling coffee involves chemical change (the
  2. delicate molecules that give coffee its flavor react with air and become new, bitter-tasting substances) and physical change (the water in the coffee is going from liquid to gaseous form) 2. Question What is the difference between a physical change and a chemical change? Asked by: Tim Meyer Answer There are two possible definitions for Physical and Chemical changes which I use in school : 1. A physical change is reversible, a chemical change is not. For example, the freezing of water would be a physical change because it can be reversed, whereas the burning of wood is a chemical change - you can't 'unburn' it 2. A physical change is a change in which no new substance is formed; a chemical change results in the formation of one or more new substances. Again, consider the previous examples: Freezing water into ice just results in water molecules which are 'stuck' together - it's still H20. Whereas burning wood results in ash, carbon dioxide, etc, all new substances which weren't there when you started. Answered by: Michael Treacy, M.S., Physics Teacher, Ireland This is a good question! One would think that a change is a change is a change so why would there be a distinction between a physical change and a chemical change? Well, there is a big difference! If you change something physically you still have what you started with. For example, if you tear a piece of paper you still have paper. If you change something chemically you end up with something very different than what you started with. For example if you burn a piece of paper you end up with carbon and no more paper. But, these are obvious examples. Is it always that simple? What if you put sugar into water? Is that a physical or a chemical change? How about if you freeze water? Is that physical or chemical? For these examples we have to examine what it would take to get the original substances back after wo have made the change. With the sugar and the water we could evaporate the water and find that the sugar is left and there is no change, except in how it looks. It is still sweet and it can be remixed in water. The frozen water can be melted and we have the same water we started with. If we take that same sugar and water and mix it with flour and eggs and vanilla and chocolate chips and cook it we will have performed a chemical change making cookies. No matter what you do to the cookies you will never get the sugar and the water out of them as before. So, in summary, there are really only two criteria for whether a change is chemical or physical: 1) After the change does it look the same and 2) can you change it back? If the
  3. answer is yes, the change is physical every time. If the answer is no the change is chemical every time. 3. The difference between a physical reaction and a chemical reaction is composition. In a chemical reaction, there is a change in the composition of the substances in question; in a physical change there is a difference in the appearance, smell, or simple display of a sample of matter without a change in composition. Although we call them physical "reactions," no reaction is actually occurring. In order for a reaction to take place, there must be a change in the elemental composition of the substance in question. Thus, we shall simply refer to physical "reactions" as physical changes from now on. Notes for ppt Chemical changes explanations http://www.buzzle.com/articles/chemical-change-examples.html Fireworks detailed explanation http://www.scifun.org/chemweek/fi reworks/fireworks. htm Burning wood explanation Burning is a non-reversible chemical change. When you burn wood, the carbon in the wood reacts with oxygen in the air to create ash and smoke, and energy in the form of light and heat. This is a permanent change that cannot be undone — you cannot turn ashes back into wood. Activity Video for reference https://www.youtube.com/watch?v=zg9wmU7Z-6s Things needed 1. 2 beakers 2. 250 gms of sugar 3. Water 4. Sulphuric acid Things to do
  4. 1. 2. 3. 4. Take kids to lab Do the experiment Explain that a new substance is formed hence called as a chemical reaction The following happens What Happens Sugar is a carbohydrate, so when you remove the water from the molecule, you're basically left with elemental carbon. The dehydration reaction is a type of elimination reaction. C12H22011 (sugar) + H2S04 (sulfuric acid) 12 C (carbon) + 11 1-120 (water) + mixture water and acid Although the sugar is dehydrated, the water isn't 'lost' in the reaction. Some of it remains as a liquid in the acid. Since the reaction is exothermic, much of the water is boiled off as steam.

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