Physical Changes

Physical Changes A physical change is any change NOT involving a change in the substance's chemical identity. Here are some examples: (1) any phase change. Moving between solid, liquid and gas involves only the amount of energy in the sample (this amount is the subject of future lessons). There is no effect on the chemical identity of the substance. For example, water remains water, no matter if it solid, liquid or gas. (2) grinding something into a powder. Or the reverse process of making a bigger lump of stuff, say by melting lots of small pellets of copper into one big piece. (3) iron (and other metals) can be made to be magnetic. This change in no way affects the chemical identity of the element. Iron that is magnetized rusts just as easily as iron that is not magnetized. (Yes, rusting is a chemical change. Rust is chemically different from iron.) Now would be a good time as any to list the names of the various phase changes: Change

Name of change

Solid to liquid melting, fusion Liquid to gas

boiling, evaporation

Solid to gas

sublimation

Gas to solid

deposition

Gas to liquid

condensation, liquefaction

Liquid to solid freezing, solidification An example of sublimation is dry ice. It is solid carbon dioxide and goes directly from the solid state to gas in the open atmosphere. You can make liquid carbon dioxide, but it must be done under about 5 atmospheres of pressure. Deposition is a bit of a non-standard word, but it fits better than using sublimation or condensation again. Ice cubes in the freezer undergo sublimation to water vapor, even with the ice is cold. The water vapor deposits back onto the solid ice without even going through the liquid phase. By the way, this is how ice cubes become "welded" together if they sit undisturbed in the freezer. Here is a great example of deposition. First a fact: solid water exists in nine different solid forms (at various combinations of temperature and pressure), called ice I to ice IX. (The one we use in our sodas is ice I.) There is a tenth solid form which is only obtained when water vapor is deposited onto a solid surface which is below -120 °C. At -80°C it spontaneously changes to ice I, however it cannot be obtained by cooling ice I.

Single Phase Changes Let’s focus on phase changes that involve materials changing from one state of matter to another. These phase changes can be brought about by heating or cooling. The terms used to identify the various changes are shown on this diagram. If you are already familiar with this terminology feel free to move on to the next topic. Phase changes can be brought about by heating or cooling. For example if water (a liquid) is heated enough, it evaporates and becomes water vapor or steam (a gas). This kind of phase change--liquid to gas--is called evaporation or vaporization. Water vapor can in turn be cooled to form liquid water. This kind of phase change--from gas to liquid--is referred to as condensation. If you were to continue to cool down the water and change it from a liquid to a solid, you would have another kind of phase change that is called freezing or crystallization. If you were to then take a solid and warm it up to change it into a liquid, that change is called melting. (It is also sometimes called fusion.) Another change that can occur for some solids is to change directly into a gas instead of changing into a liquid, and that is called sublimation. The reverse of this process can also occur. Some gases can be cooled down and changed directly into a solid. That process is also called crystallization, a second meaning for that word. Multiple Phase Changes In addition to these single phase changes, you can have a string of changes, one after another. There are three of these multiple phase changes that are very important and you should know them by name. One is distillation, changing a liquid to a gas and back to a liquid. The second is sublimation, which you already know means changing a solid to a gas. The term also applies to a double process in which a solid is changed to a gas and then back to a solid. The third is recrystallization, in which a

distillation LIQUID

GAS

LIQUID

sublimation (2nd) SOLID

GAS

SOLID

recrystallization (1st)

solid is changed to a liquid and then back to a solid. You will see another meaning for this term soon.

SOLID

LIQUID

SOLID

Dissolution and Crystallization Another type of phase change is dissolving or dissolution. This involves a material (usually a solid) seemingly disappearing when placed into a liquid. Sugar or salt dissolving in water are examples. The condition of the sugar and salt have definitely changed--they are no longer homogeneous, opaque, white solids. But they continue to exist as sugar and salt. (This can be verified by taste, in these cases.) They have changed to a different condition or phase, but they remain the same materials. Also, the water remains water even though the taste of sugar or salt has been mixed in with it. The reverse of this process (evaporating away the water to retrieve the sugar or salt) is called crystallization (another meaning for that term). Combining both processes (solid to solution back to solid) is a second type of recrystallization.

Cited from: http://www.chemteam.info/ http://www.virtualchembook.com/

dissolution SOLID

SOLUTION

crystallization (2nd) SOLUTION

SOLID

recrystallization (2nd) SOLID

SOLUTION

SOLID