1. Water and Aqueous Systems
Chapter 15
Water and Aqueous Systems

2. 15.1 Water and Its Properties
Molecule has net polarity
Polar bonds δ– δ+
Water in the Liquid State
Water is a polar molecule. That means part of the water molecule has a partial positive charge while the other part of the water molecule has a partial negative charge.
This explains why water has a “bent” appearance
This also allows water molecules to attract to other nearby water molecules
(+ side of one to – side of another)
The oxygen atom acquires a partial negative charge (δ–).

3. Hydrogen Bonding
These strong attractive forces between 2 different water molecules is called Hydrogen Bonding
Hydrogen Bonding causes water to have several unique properties:
High Surface Tension
Low Vapor Pressure
High Boiling Point
Liquid water
Hydrogen bond

4. Surface Tension
Have you ever noticed that water forms nearly spherical droplets on a leaf?
The inward force, or pull, that tends to minimize the surface area of a liquid is called surface tension.
All liquids have a surface tension, but water’s surface tension
is higher than most.
The surface tension of water tends to hold a drop of liquid in a
spherical shape.

5. Vapor Pressure
Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure.
An extensive network of hydrogen bonds holds the molecules in liquid water to one another.
These hydrogen bonds must be broken before water changes from the liquid to the vapor state, so the tendency of these molecules to escape is low and evaporation is slow.

6. Boiling Point
Molecular compounds of low molecular mass are usually gases or liquids with low boiling points at normal atmospheric pressure.
Ammonia (NH3) has a molar mass of 17.0 g/mol and boils at about – 33˚C.
Water has a molar mass of 18.0 g/mol, but it has a boiling point of 100˚C.
The difference between the boiling points of ammonia and water is due to hydrogen bonding, which is more extensive in water than in ammonia.
It takes much more heat to disrupt the attractions between water molecules than those between ammonia molecules.

7. Water in the Solid State
Ice cubes float in your glass of iced tea because solid water has a lower density than liquid water.
This situation is not usual for liquids.
When the temperature of the water falls below 4˚C, the density of water actually starts to decrease.
Density of Liquid Water and Ice
Temperature (˚C)
Density (g/cm3)
100 (liquid water)
0.9584 50
0.9881 25
0.9971 10
0.9997 4
1.0000
0 (liquid water)
0.9998
0 (ice)
0.9168

8. Density of Ice Why is ice less dense than liquid water?
The structure of ice is a regular open framework of water molecules in a hexagonal arrangement.

9. 15.3Heterogeneous Aqueous Systems
Suspensions – A mixture where particles will settle upon standing.
Heterogeneous Mixtures
Large sized particles
Ex: Muddy Water
Colloids – A heterogeneous mixture where particles are intermediate in size.
Colloids can look cloudy or milky or might even look clear.
Tyndall Effect – The scattering of light by medium –large sized particles. Can be used to differentiate between colloids and solution
Flashlight
Solution Colloid Suspension

10. Some Colloidal Systems
Colloids
The first substances to be identified as colloids were glues.
Some Colloidal Systems
System
Type
Example
Dispersed phase
Dispersion medium
Gas
Liquid
Foam
Whipped cream
Solid
Marshmallow
Emulsion
Milk, mayonnaise
Aerosol
Fog, aerosol
Smoke
Dust in air
Sols, gels
Egg white, jelly, paint, blood, starch in water, gelatin
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

11. 15.2 Homogeneous Aqueous Systems
Solutions
Solvent – In a solution, the dissolving material
Ex: The Water
Solute – The dissolved particles in a solution.
Ex: The Salt
Aqueous Solution – A solution where water is the solvent. (as opposed to alcohol or oil)

12. 3 Types of solutions Gaseous solutions– air
Liquid solutions – vinegar (acetic acid dissolved in water); soft drinks (solutions of a gas, CO2, dissolved in water.
Solid solutions– alloys such as sterling silver – 92% silver, 8% copper; white gold – gold containing nickel, tin, zinc or copper.

13. Solvation
Solvated ions
Surface of ionic solid
Solvation – The process by which ionic compounds dissolve in a solvent
DRAW THE PICTURE

14. Electrolytes and Nonelectrolytes
Electrolyte – A compound that conducts an electric current when dissolved in water.
All ionic compounds are electrolytes
Strong Electrolytes – all of the ionic compound breaks apart into ions Ex: NaCl
Weak Electrolytes – only some of the ionic compound breaks apart into ions Ex: Vinegar
Nonelectrolyte – A compound that does not conduct an electric current in solution.
Solutes dissolve to form:
Ions Molecules
(electrolytes) (non-electrolytes)

15. Electrolyte Nonelectrolyte
To (+) electrode
To (–) electrode
To (+) electrode
To (–) electrode

16. Classification of Matter Chart
Pure Substances
Mixtures
Elements
Compounds
Homogeneous
Heterogeneous
Solutions
Colloids
Suspensions

17. 16.1 Properties of Solutions
Solution Formation
If you wish to dissolve a substance, you can help by…
Stirring it
Stirring sugar in cold tea vs not stirring at all
Heating it
Think sugar in cold tea vs sugar in hot tea
Crushing it – this increases the surface area of the solute
Think sugar packet instead of sugar cube

18. Solubility
Solubility – The amount of solute that dissolves in a given amount of solvent at a specific temperature and pressure
Saturated Solution – Contains the maximum amount of solute in a given amount of solvent
If even more solute is added to the solution, the salt will just sink to the bottom. No amount of stirring will get the solute to dissolve.
Unsaturated Solution – Contains less than the maximum amount of solute in a given amount of solvent.
If more solute is added to the solution, it salt will dissolve.

19. Factors Affecting Solubility
1. Types of solvents and solutes –“Like dissolves like”
Polar/ionic versus nonpolar
water oil
salt gasoline
sugar Styrofoam

20. Factors Affecting Solubility
2. Temperature
For most solids, solubility increases as temperature increases
Graph for solids:
Supersaturated Solution – Contains more solute than it can theoretically hold at a given temperature.
How is this possible? These solutions are created by saturating a hot solution and allowing it to cool undisturbed. Sometimes, the extra salt will stay dissolved after the temperature has been cooled back down.

21. Supersaturated Solutions

22. Factors Affecting Solubility
2. Temperature
For most solids, solubility increases as temperature increases
Graph for solids:
For gases, solubility decreases as temperature increases
Graph for gases:

23. Factors Affecting Solubility
3. Pressure (gases only)
As pressure increases, solubility increases
Henry’s law: solubility is proportional to pressure.

25. Solubility Problems
Ex1: What is the solubility of potassium chlorate at 50.0 oC in ml of water?
20.0 g of potassium chlorate in grams of water
Ex2: What temperature will result in a saturated solution of 80.0 grams of sodium nitrate and grams of water?
10.0 oC

26. Solubility Problems
Ex3: If 40.0 grams of ammonium chloride are placed in grams of water at 50.0 oC, is the solution saturated or unsaturated? If saturated, how much salt remains undissolved? If unsaturated, how much more salt can be dissolved?
The solution is unsaturated and can hold 10.0 more grams of ammonium chloride
Ex4: If 80.0 grams of potassium nitrate are placed in grams of water at 44.0°C, is the solution saturated or unsaturated? If saturated, how much salt remains undissolved? If unsaturated, how much more salt can be dissolved?
The solution is saturated with 5.0 grams of potassium nitrate undissolved

27. Solubility Problems
Ex5: What is the solubility of lithium sulfate at oC in 50.0 ml of water?
At this temperature the ml of water can hold 30.0 grams of this salt. So, if half as much water is present, half as much salt will dissolve.
30.0 g = x
100.0 ml ml
x = 15.0 grams of lithium sulfate in 50.0 grams of water

28. Solubility Problems
Ex6: What is the solubility of lithium sulfate at oC in ml of water?
30.0 g = x ml ml
x = 60.0 grams of lithium sulfate in grams of water

29. Solubility Problems
Ex7: What is the solubility of lithium sulfate at 90.0 oC in ml of water?
30.0 g = x
100.0 ml ml
x = 20.5 grams of lithium sulfate in 68.2 grams of water