Differences Between Chemical & Physical Changes
- When you tear up a piece of paper, the smaller pieces are still paper. The paper in this case has undergone a physical change. In a physical change, the molecules which make up the substance don't change at all. You could also reverse the chemical process. However, the paper example may not be a good example for a reversible process. Water, on the other hand, physically changes from ice to water or to steam, and could change between the three easily by melting, evaporating, condensing or freezing. In rare cases, it could sublimate.
- Chemical changes often create all-new molecules. One good example of this is baking a cake. You mix all of the ingredients into that cake, and it changes into a cake after you bake it. The baking is when the chemical change takes place. Additionally, you can't get the milk, eggs and sugar back from the chemical change. On a microscopic level, the molecules that make up the original substance are different, whereas they would be the same if it had only been physical.
- If you were to dissolve sugar in water, it doesn't seem completely clear as to whether this is a physical or chemical change. It is, in fact, just a physical change. The sugar molecules are simply slipped in between the water molecules -- you could, in theory reverse the process by evaporating the water. A metal alloy is another weird example, because the new material takes on new properties. However, on a microscopic level, the metals are just arranged differently, and no new molecules were formed.
- The Law of Conservation of Mass states that through a chemical or physical change, you cannot create or destroy matter. So, regardless of what kind of change your materials are undergoing, they will never create or destroy more matter. So, if you mix a cup each of water and sugar, you will create two cups of sugar water. Sometimes the materials change state. In a reaction that uses a lot of heat, you may create a gas, but there will still be the same amount of molecules as when you started.
