Properties of Matter

Intensive properties do not depend on the amount of matter that is present. Intensive properties do not change according to the conditions. They are used to identify samples because their characteristics do not depend on the size of the sample.

In contrast, extensive properties do depend on the amount of a sample that is present. A good example of the difference between the two types of properties is that mass and volume are extensive properties, but their ratio (density) is an intensive property. Notice that mass and volume deal with amounts, whereas density is a physical property.

Phase Changes

There are six phase changes: condensation, evaporation, freezing, melting, sublimation, and deposition.

Condensation is the change of a gas or vapor to a liquid. A change in the pressure and the temperature of a substance causes this change. The condensation point is the same as the boiling point of a substance. It is most noticeable when there is a large temperature difference between an object and the atmosphere. Condensation is also the opposite of evaporation.

Evaporation is the change of a liquid to a gas on the surface of a substance. This is not to be confused with boiling, which is a phase transition of an entire substance from a liquid to a gas. The evaporation point is the same as the freezing point of a substance. As the temperature increases, the rate of evaporation also increases. Evaporation depends not only on the temperature, but also on the amount of substance available.

Freezing is the change of a liquid to a solid. It occurs when the temperature drops below the freezing point. The amount of heat that has been removed from the substance allows the particles of the substance to draw closer together, and the material changes from a liquid to a solid. It is the opposite of melting.

Melting is the change of a solid into a liquid. For melting to occur, enough heat must be added to the substance. When this is done, the molecules move around more, and the particles are unable to hold together as tightly as they can in a solid. They break apart, and the solid becomes a liquid.

Sublimation is a solid changing into a gas. As a material sublimates, it does not pass through the liquid state. An example of sublimation is carbon dioxide, a gas, changing into dry ice, a solid. It is the reverse of deposition.

Deposition is a gas changing into a solid without going through the liquid phase. It is an uncommon phase change. An example is when it is extremely cold outside and the cold air comes in contact with a window. Ice will form on the window without going through the liquid state.

Molecular Traits

Adhesiveness and Cohesiveness

Because of polarity, water is attracted to water, a property called cohesion. The typical water molecule has a polar configuration, as seen here.

Notice that there is a negative end and a positive end. This means it is a polar molecule. In a polar molecule, one end of the molecule is slightly negative and one end is slightly positive.

Inside a plant, water has to travel up, against gravity, to reach all the leaves. Because the water molecules are attracted to each other, or demonstrate cohesion, they also adhere to the sides of the xylem vessels that transport water up to where it is needed in the plant. This is possible because of adhesion. Adhesion is water’s ability to be attracted to other substances.

Diffusion and Osmosis

When a bottle of perfume is opened, perfume molecules diffuse throughout a room. Diffusion is the tendency of molecules and ions to move toward areas of lower concentrations until the concentration is uniform throughout the room (that is, it reaches equilibrium). This random movement of individual particles has an important consequence. Because the movement is random, a particle is more likely to move from an area where there are a lot of molecules (area of high concentration) to an area where there are fewer molecules (an area of lower concentration). In the human lungs, oxygen diffuses into the bloodstream because there is a higher concentration of oxygen molecules in the lungs’ air sacs than there is in the blood.

Solute and solvent particles tend to diffuse from areas where their concentration is high to areas where their concentration is lower. Imagine that a membrane separates two regions of liquid. As long as solute particles and solvent (water) molecules can pass freely through the membrane, diffusion will equalize the amount of solute and solvent on the two sides. The sides will reach equilibrium.

But what if a polar solute that cannot pass through the membrane is added to one side? This situation is common in cells. An amino acid cannot cross a lipid bilayer, and neither can an ion or a sugar molecule. Unable to cross the membrane, the polar solute particles form hydrogen bonds with the water molecules surrounding them. These “bound” water molecules are no longer free to diffuse through the membrane. The polar solute has reduced the number of free water molecules on that side of the membrane. This means the opposite side of the membrane (without solute) has more free water molecules than the side with the polar solute. As a result, water molecules move by diffusion from the side without the polar solute to the side with the polar solute.

Eventually, the concentration of free water molecules will equalize on the sides of the membrane. At this point, however, there are more water molecules (bound and unbound) on the side of the membrane with the polar solute. Net water movement through a membrane in response to the concentration of a solute is called osmosis. Stated another way, osmosis is the diffusion of water molecules through a membrane in the direction of higher solute concentration.

Let’s Review

• The difference between physical and chemical properties is that chemical properties involve a change in a substance’s chemical composition and physical properties do not.
• Physical properties can be categorized as either extensive or intensive properties. Intensive properties do not depend on the amount of matter present, such as color or density. Extensive properties do depend on the amount of a sample present, such as mass and volume.
• Water is considered a polar molecule, a molecule where one end is slightly negative and the other end is slightly positive.
• Cohesiveness is the attraction of water to itself, and adhesiveness is the attraction of water to other substances. Adhesion is the property that allows for water to travel against the force of gravity within a plant.
• Diffusion is the tendency of molecules and ions to move toward areas of lower concentrations until it is uniform.
• Osmosis is the diffusion of water and the movement of molecules from an area of high concentration to an area of lower concentration.