Chapter 17 Notes

The wonderful world of…

Water

• The seemingly simple molecule— made of 1 atom of oxygen and 2 hydrogens.

• But oh so interesting!

Water

• Water has polar bonds, and because of its bent shape, is a polar molecule.

• It also undergoes Hydrogen bonding due to the bond polarity between oxygen and hydrogen.

• These two factors give water some of its interesting properties!

Surface Properties

• Because the Hydrogen atoms cannot Hydrogen-bond to the air at its surface, molecules tend to be drawn towards the body of liquid.

Surface Properties

• This creates

surface tension

—the inward force or pull that tends to minimize the surface area of a liquid.

Though surface tension is what allows water bugs to walk on the surface of water, it does not have enough force to hold up this guy. Sorry!

On the hood of a car…

Surfactants

• A

surfactant

is a substance that decreases surface tension of a liquid.

• In water, it does this by interfering with the hydrogen bonding.

Detergent is a good example of a surfactant.

Phase Changes

• Water boils and freezes at much higher temperatures than most other substances that have similar masses. Why?

Molecular Polarity!

• Because the positive and negative side of water molecules act like tiny magnets, these forces have to be overcome in order for boiling to occur.

Phase Changes

• When water freezes, unlike most other substances, it becomes less dense! It floats! Why?

It is very important for life on earth that water floats when it freezes —it makes an insulating layer that keeps the ocean from freezing and killing everything!

Structure of Solid Water (ice) • What is this?

• Empty space! Just like a boat, where the average density makes it float, the empty space causes water to have a lower density as a solid than as a liquid.

Water—The Universal Solvent

• Water is called the universal solvent because so many different things will dissolve in it. How?

Water—The Universal Solvent

• Water dissolves polar and ionic compounds because it is polar.

• The negative side of water molecules collide with the positive side of a polar molecule (or the positive ion) and attract it, carrying it away from the crystal. The same thing happens with the negative side (or ion).

Like Dissolves Like

• Nonpolar substances are able to dissolve other nonpolar substances, due to the lack of repulsive forces.

• This leads to the saying “like dissolves like.”

Electrolytes

• • Any ionic compound will conduct electricity when dissolved in water. Compounds that do so are called

electrolytes

.

Non-electrolytes

—like sugar —will dissolve but do not conduct electricity.

Heterogeneous Aqueous Systems

• There are two types of heterogeneous aqueous systems — colloids and suspensions.

Suspensions

• Mixtures where particles settle out upon standing are called

suspensions

.

• They differ from solutions (which are homogeneous) because their particles are much larger.

Colloids

• A

colloid

is a mixture with particles intermediate in size between a solution and a suspension.

• They have properties that are different than both. For example, the particles in a colloid exhibit

Brownian motion

.

Brownian Motion • The random drifting of particles suspended in a fluid.

• This occurs when the particles are small enough that the effects of gravity are smaller than the interactions with other particles, resulting in a scattering of particles and no settling.

How can you tell what it is?

• A colloid can be discerned due to the

Tyndall effect

—light scattering by particles in a colloid or particles in a fine suspension.