Chapter 15 water and aqueous systems Objectives… Properties of water Solute, solvent, solution Electrolytes Suspension, colloid, emulsion

Many unique and important properties of water result from hydrogen bonding high surface tension low vapor pressure

Surface Tension In other liquids, hydrogen bonds form between molecules on all sides of the liquid (balanced) In water, hydrogen bonds only form on one side of the liquid (unbalanced) This creates a force or pull called “surface tension” Water’s surface tension is higher than most other liquids

Surface Tension and Surfactants On some surfaces, water can “bead up” rather than spread out. A “surfactant” is a substance that breaks up the hydrogen bonding between water molecules and surface tension is reduced. What happens when you add soap to water on a greasy surface?

Vapor Pressure Vapor pressure is caused by molecules of a liquid escaping from the surface as they change states from liquid to gas/vapor Vapor pressure is low in water because the molecules escape slowly as they break from the hydrogen bonds which are holding them together. This results in slow evaporation.

What would happen if lakes and oceans evaporated quickly???

Density of Water Solid water has a lower density than liquid water WHY? Hydrogen bonds hold the water molecules in place. The structure of ice is a regular open framework of water molecules arranged in a honeycomb. When ice melts, the framework collapses and the water molecules move closer together, making liquid water more dense than ice.

Solid, Liquid, Gas Water can take on any of these forms. An “aqueous solution” is water that contains dissolved substances. In a solution, the dissolving medium is the “solvent” and the dissolved particles are the “solutes.” Solvents and solutes can be solid, liquid or gas Typically the particles are so tiny they can pass through filter paper and cannot be separated.

What is an electrolyte? An electrolyte is a compound that conducts an electric current when it is in an aqueous solution or in the molten state All ionic compounds are electrolytes because they can dissociate or break into ions. Some examples include * sodium chloride * sodium hydroxide * copper sulfate

What is a non-electrolyte? A nonelectrolyte cannot conduct electric current in either aqueous solution or molten state. Most covalent compounds are nonelectrolytes Some examples are * Carbon compounds like sucrose or rubbing alcohol Some covalent compounds, however, can conduct electricity if dissolved in water (ammonia and hydrogen chloride are examples)

Strong vs Weak Strong electrolytes conduct electricity well. Sodium chloride is a great example. In water, the sodium and chlorine ions separate and the ions carry the current. Weak electrolytes cannot conduct electricity well. Ammonia in water will carry a small current, but it does not break apart into ions. Sucrose or glucose do not carry a current at all. They are covalently bonded and no current flows.

suspension It resembles a solution, but…the particles are large and cannot stay suspended indefinitely. If you let a suspension sit, gravity pulls the layers apart and the particles settle. You can visually see the layers in a suspension once it has settled

colloid Colloids have smaller particles than those in suspensions but larger particles than those in solutions. The particles here do not settle out in time. Colloids can however, appear thick or milky or cloudy

coagulation This is the action when particles clump together and a precipitate forms What happens is that ions of the opposite charge destroy the matrix of the colloid. As a result, the colloid changes form and clumps in reaction to its surroundings

emulsion This is a colloid formed from a liquid in a liquid An emulsion helps two different liquids stay together or break them apart Butter is an emulsification Soaps and detergents are emulsifying agents – one end is polar and the other end is nonpolar. They break apart oil or grease or other liquids that do not normally mix.