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Lesson 1.3: Properties of Matter

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Phase Changes: An example of a physical change

One thing that makes the classification scheme of all matter into elements, compounds, and mixtures, is that it does not directly account for what the natural phenomena of phase changes. The uniformity of phase changes are used to establish the purity of a substance and cannot then be regarded as a chemical change. It is a physical change.

In general, chemists regard physical changes as changes in which no chemical bonds are broken or formed. This means that when a substance changes phase, goes from a solid to a liquid by melting, or from a liquid to a gas by evaporating, 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 changing the spatial arrangement of the basic units that make up a substance, without changing their identity.

Under normal conditions we distinguish three phases – solid, liquid, and gas.

Solids.
- At the molecular level a solid has parts that are ordered, the parts are close together. The motion of the parts s restricted – they vibrate back and forth in a fixed place. At the macroscopic level solids have a fixed shape and volume (their parts stay in the restricted locations).
Liquids
- At the molecular level a liquid has parts but they are not ordered. They are free to move past each other and as such do not have a fixed place. The parts are close together, but usually not as close as in the solid (they are less dense than their solid phase). There are important exceptions to this. Water as a liquid is more dense than as solid. The open structure of the water as a solid collapses in the liquid phase. Since the motion of the parts of a liquid are not restricted, liquids do not have a fixed shape but take on the shape of their container. However, the mutual attraction of the parts results in the liquid having a fixed volume like solids.
Gases
-At the molecular level a gas has parts but they are not ordered. This is like a liquid. They do not have a fixed shape but take on the shape of their container. However, since there is little attraction of the parts for each other, gases do not have a fixed volume. Their volume is simply the size of their container. The parts do not attract each other but can be forced together by increasing the external pressure on the container.

Changes of state.

When we heat a solid the parts vibrate more vigorously. This increases the volume of the solid and makes it less dense. At some point, the energy of the vibrating parts is great enough to overcome the attractive forces that keep them in place. They loose their orderly ranking and turn into a liquid with parts sliding past each other. The solid melts. If we continue to heat the substance after it is all liquid, the parts gain more energy and usually increase their overall volume – they become less dense. At some point, the energy of all the parts is great enough to overcome the attractive forces that keep them in a cluster. The parts separate from each other – they have vaporized and turned into a gas.

How can substances be separated?

Filtration

For example, grains of sand are much larger than water molecules. To separate a mixture of water and sand we simply pour the mixture through a filter. The sand does not pass through but the water does.

Chromatography

Chromatography works in a similar way. A mixture of substances is passed through a stationary phase (similar to the filter paper) which interacts differently with the various substances. Those that interact more take longer to reach the bottom of the column than those that interact less, allowing for separation.

Distillation

Distillation works by heating the mixture. The more volatile substances, the one with the lower boiling point, will evaporate first. If the vapor is cooled by passing it through a condenser, the more volatile substance will be separated from the less volatile. For example, distillation can separate ethanol from water. The ethanol will boil off first leaving the water behind. Another example would be purifying salt water. In this case, the water will boil off first leaving the salt behind.