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Change is happening all around us all of the time. Just as les-grizzlys-catalans.orgists have classified elements and compounds, they have also classified types of changes. Changes are classified as either physical or les-grizzlys-catalans.orgical changes. les-grizzlys-catalans.orgists learn a lot about the nature of matter by studying the changes that matter can undergo. les-grizzlys-catalans.orgists make a distinction between two different types of changes that they study—physical changes and les-grizzlys-catalans.orgical changes.
Physical changes are changes in which no bonds are broken or formed. This means that the same types of compounds or elements that were there at the beginning of the change are there at the end of the change. Because the ending materials are the same as the beginning materials, the properties (such as color, boiling point, etc.) will also be the same. Physical changes involve moving molecules around, but not changing them. Some types of physical changes include:Changes of state (changes from a solid to a liquid or a gas and vice versa). Separation of a mixture. Physical deformation (cutting, denting, stretching). Making solutions (special kinds of mixtures).
As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change. Melting is an example of a physical change. A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. When liquid water is heated, it changes to water vapor. However, even though the physical properties have changed, the molecules are exactly the same as before. We still have each water molecule containing two hydrogen atoms and one oxygen atom covalently bonded. When you have a jar containing a mixture of pennies and nickels and you sort the mixture so that you have one pile of pennies and another pile of nickels, you have not altered the identity of the pennies or the nickels—you"ve merely separated them into two groups. This would be an example of a physical change. Similarly, if you have a piece of paper, you don"t change it into something other than a piece of paper by ripping it up. What was paper before you started tearing is still paper when you aredone. Again, this is an example of a physical change.
Label each of the following changes as a physical or les-grizzlys-catalans.orgical change. Give evidence to support your answer.Boiling water. A nail rusting. A green solution and colorless solution are mixed. The resulting mixture is a solution with a pale green color. Two colorless solutions are mixed. The resulting mixture has a yellow precipitate.
SolutionPhysical: boiling and melting are physical changes. When water boils, no bonds are broken or formed. The change could be written: (ceH_2O left( l ight) ightarrow ceH_2O left( g ight)) les-grizzlys-catalans.orgical: The dark grey nail changes color to form an orange flaky substance (the rust); this must be a les-grizzlys-catalans.orgical change. Color changes indicate les-grizzlys-catalans.orgical change. The following reaction occurs: (ceFe + ceO_2 ightarrow ceFe_2O_3) Physical: because none of the properties changed, this is a physical change. The green mixture is still green and the colorless solution is still colorless. They have just been spread together. No color change occurred or other evidence of les-grizzlys-catalans.orgical change. les-grizzlys-catalans.orgical: the formation of a precipitate and the color change from colorless to yellow indicate a les-grizzlys-catalans.orgical change.
Separating Mixtures Through Physical Changes
Homogeneous mixtures (solutions) can be separated into their component substances by physical processes that rely on differences in some physical property, such as differences in their boiling points. Two of these separation methods are distillation and crystallization. Distillation makes use of differences in volatility, a measure of how easily a substance is converted to a gas at a given temperature. A simple distillation apparatus for separating a mixture of substances, at least one of which is a liquid. The most volatile component boils first and is condensed back to a liquid in the water-cooled condenser, from which it flows into the receiving flask. If a solution of salt and water is distilled, for example, the more volatile component, pure water, collects in the receiving flask, while the salt remains in the distillation flask.
Mixtures of two or more liquids with different boiling points can be separated with a more complex distillation apparatus. One example is the refining of crude petroleum into a range of useful products: aviation fuel, gasoline, kerosene, diesel fuel, and lubricating oil (in the approximate order of decreasing volatility). Another example is the distillation of alcoholic spirits such as brandy or whiskey. This relatively simple procedure caused more than a few headaches for federal authorities in the 1920s during the era of Prohibition, when illegal stills proliferated in remote regions of the United States.
Another example for using physical properties to separate mixtures is filtration (Figure (PageIndex4)). Filtration is any mechanical, physical or biological operationthat separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass. The fluid that passes through is called the filtrate. There are many different methods of filtration; all aim to attain the separation of substances. Separation is achieved by some form of interaction between the substance or objects to be removed and the filter. The substance that is to pass through the filter must be a fluid, i.e. a liquid or gas. Methods of filtration vary depending on the location of the targeted material, i.e. whether it is dissolved in the fluid phase or suspended as a solid.
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