Coach Coker's Chemistry



Unit 5: States of Matter

Content Outline: Liquids (5.3)

I. Liquid

A. A “state” of matter where the substance can flow and takes on the shape of its container.

1. The flow is usually in the direction of the force of gravity or channeling.

2. The flow is possible because the atoms/molecules are not fixed in position by strong attractive forces, like a solid.

3. The flow is possible also because the individual atoms/molecules possess more Kinetic Energy, than do solids.

a. This greater Kinetic Energy per atom/molecules helps them overcome some of the restrictive attractive forces.

II. Kinetic Theory of Matter Characteristics:

A. Expansion (can atoms/molecules “expand” their given “space”)

1. Liquids, while they can change shape, by changing the container, cannot expand their volumes.

2. Liquids have defined volumes, like solids.

B. Fluidity (the ease of movement past one atom/molecule by another atom/molecule because of attractive forces between atoms/molecules.)

1. Liquids atoms/molecules have greater ease of motion due to the possession of greater amounts of Kinetic energy per atom/molecule.

a. The greater Kinetic Energy makes it easier to overcome some of the restrictive (stronger) intermolecular attractions between atoms/molecules within the liquid, but some attractions still exist.

i. These would be London Dispersion Forces (a.k.a. Van der Waals Interactions), Dipole-Dipole Interactions, and Hydrogen Bonds.

ii. These attractive forces are what give the liquid “order” or “organization”, just like solids.

Example: Water molecules attracted to water molecules by Hydrogen Bonding.

• Cohesion – molecules binding to like molecules

• Adhesion – molecules binding to some other substance

2. At the surface of liquids, there exists a boundary between the liquid’s atoms/molecules and the atmosphere’s (air) atoms/molecules that are in a gaseous “state”.

a. Surface Tension (“tension” refers to “pulling strength”)

i. This is a cohesive attraction that tends to pull adjacent atoms/molecules of a liquids surface together into a more stable position with less surface area.

α. Greater attraction between atoms/molecules = greater tension.

b. Weaker attraction between atoms/molecules = weaker tension.

Remember, these stronger/more numerous attractions increase the boiling points of liquids, just like with solids.

ii. This is the attractive force that creates the concept of a drop.

α. Drop – the spherical (round) shape of a liquid in the air.

iii. Evaporation

α. This term refers to the “escaping” of surface atoms/molecules of a liquid to the

atmosphere.

b. The ability to “escape” from the liquid’s surface is because atoms/molecules

absorbed more Kinetic Energy from the surrounding environment.

For example, such as heating up a liquid, or putting a liquid in the sunshine, such as

you do when you sweat on a hot day.

c. This “escaping” of greater Kinetic energy containing atoms/molecules also takes

heat away from the liquid and thus providing a cooling effect known as

Evaporative Cooling.

• Vaporization - turning a solid or liquid into a gas.

• Vapor – a gas.

3. For liquids, there also exists a boundary between the liquid’s atoms/molecules and the solid surface or the container.

For example, water in a glass beaker or plastic cup.

a. Capillary Action (“capillary” refers to a “narrow tube”)

i. This is an adhesive attraction that exists between a liquid’s atoms/molecules and a solid container’s surface.

ii. This attractive force helps to “pull” liquids up the surface of the container.

For example, seen in the meniscus of a graduated cylinder or water moving up through the xylem tissue (which are dead, hollow plant cells).

iii. The narrower the tube ( greater the adhesive climb.

C. Density (how compact/close are the atoms/molecules to each other)

1. Most liquids are only slightly less dense than their solid “state”.

a. Water is a rare exception to the law. Water is more dense than ice; therefore ice floats.

i. The floating of ice is directly due to the Hydrogen bonds between water molecules, which the bond angle is fixed and maximized, in terms of distance… hence why it is less dense and floats.

ii. Water is most dense at 4OC… about 1g/cm3

iii. Water gets less dense as it approaches the boiling point (100OC).

iv. Items with greater densities sink in water. For example: Glycerol

v. Items with lesser densities float on top of water. For example: Oils and Alcohols

2. The density of a liquid can affect the boiling points of liquids.

a. Typically, less dense liquids boil at lower temperatures.

b. Typically, more dense liquids boil at higher temperatures.

c. This all due to the greater attractive forces between individual atoms/molecules within the liquids.

D. Compressibility (can the atoms/molecules be moved closer to each other)

1. Most liquids are only slightly compressible.

2. It usually requires extremely massive amounts of pressure to achieve a minimal compression.

a. Hydraulics

i. This is the study of fluids under pressure to move or lift objects.

3. Pressure diffuses through liquids in all directions evenly.

E. Diffusion (the ability of atoms to “mix” together over time)

1. Most liquids can diffuse from into another liquid, but it depends upon 2 dissolvability factors:

a. Polar versus non-polar – Like dissolves like; they do not “like” the other and keep separate.

b. Temperature – colder liquids dissolver slower than warmer liquids.

c. Stirring liquids can increase the rate of diffusion.

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