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

2. Section 15.1 Water and It’s Properties
Objectives
Explain the high surface tension and low vapor pressure of water in terms of the structure of the water molecule and hydrogen bonding

3. Water in Liquid State The Water Molecule The Bonds The Molecule
Each O-H bond is a highly polar covalent bond because of the high electronegativity of the oxygen
The Molecule
Due to the bent shape, the O-H bond polarities do not cancel. This means water as a whole is polar molecule

4. The Water Molecule δ- δ+ δ+ δ- means a partial negative charge
Thus, water has a partial negative end (Oxygen) and a partial positive end (Hydrogen) – and it is called “polar” because of these areas of difference
δ- means a partial negative charge δ- O H H δ+ δ+
δ+ means a partial positive charge

5. The Water Molecule
Water’s bent shape and ability to hydrogen bond gives it many special properties
Water molecules are attracted to one another by intermolecular forces called hydrogen bonds
This hydrogen bonding gives water:
High surface tension
Low vapor pressure

6. High Surface Tension Surface Tension
The inward force or pull that tends to minimize the surface area of a liquid
Liquid water acts like it has a “skin” or a tight film stretched across it
In a glass water bulges over the top
Water can support a small amount of weight
All because water has ability to form hydrogen bonds

7. Surface Tension d- d+ d+ d- d+ d+ O H O H
One water molecule can hydrogen bond to another because of this electrostatic attraction between the oxygen of one molecule and the hydrogen of another molecule.
(electrostatic is just another way of saying + attracts to -)
Hydrogen bonding can also occur with other charged molecules surrounding water on all sides. H O d+ d+ d- H O d+ d+

8. Surface Tension Surface tension explains why water drops are round
All molecules on the edge are pulled to the middle - not outward to the air!

9. Low Vapor Pressure
Hydrogen bonding also explains water’s unusually low vapor pressure
Remember vapor pressure
Hydrogen bonds hold water molecules together, so they do not escape easily and evaporation is slow
This is a good thing, because lakes and oceans would evaporate very quickly due to their large surface area
Remember Reference Table H

10. Section 15.2 – Homogeneous Aqueous Solutions
Objectives:
Distinguish between a solvent and a solute
Describe what happens in the solution process
Explain why all ionic compounds are electrolytes
Demonstrate how the formula for a hydrate is written

11. Solvents and Solutes Solution Solvent Solute Aqueous solution
Uniform throughout
Solution
A homogenous mixture
Solvent
The dissolving medium
Solute
The dissolved particles
Aqueous solution
A solution with water as the solvent
Particle size less than 1 nm – cannot be separated by filtration
Not chemically combined

12. Practice Problem Which formula represents a homogeneous mixture?
H2O (l)
HCl (aq)
H2S (g)
NaH (s)
Compound – H & O are chemically combined
(aq) indicates aqueous solution
Compound – H & S are chemically combined
Compound – Na & H are chemically bonded
Remember (aq) stands for aqueous solution – always a homogenous mixture

13. Parts of an Aqueous Solution
Solute
A solute is the dissolved substance in a solution
Salt in salt water
Sugar in soda drinks
Solvent
A solvent is the dissolving medium in a solution
Water in salt water
Water in soda
Notice: in an aqueous solution – the solvent is always water

14. Aqueous Solutions
Water dissolves ionic compounds and polar covalent molecules best
The rule is: likes dissolve likes
Polar dissolves polar
Polar dissolves ionic
Nonpolar dissolves nonpolar
Examples
Oil and water do not mix (water is polar, so
Salt dissolves in water
oil must be nonpolar)
(salt is an ionic cmpd)
See REFERENCE TABLE F for compounds that will dissolve in Aqueous Solutions

15. Table F

16. The Solution Process H H O O O H H O O H O O H H H O O H
These ions have been pulled away from the main crystal structure by water’s polarity. H O H O H O H O H O H O H O H O H O
These ions have been surrounded by water, and are now dissolved!
How ionic solids dissolve in water

17. Practice Problem Which compounds will dissolve in water? MgCO3 BaSO4
Na2S
CaCrO4 X

18. Practice Problem Which compounds will dissolve in water? MgCO3 BaSO4
Na2S
CaCrO4 X X

19. Practice Problem Which compounds will dissolve in water? MgCO3 BaSO4
Na2S
CaCrO4 X X 

20. Practice Problem Which compounds will dissolve in water? MgCO3 BaSO4
Na2S
CaCrO4 X X  

21. Use your reference table!!
Practice Problem
Which of the following ions when combined with Cl- forms an insoluble substance in water?
Fe2+
Mg2+
Pb2+
Zn2+
Use your reference table!!
Remember if not on Table F
– is it ionic or polar covalent?

22. Practice Problem
Based on Reference Table F, which of these solutions has the lowest saturated concentration of dissolved ions?
NaCl(aq)
MgCl2 (aq)
NiCl2(aq)
AgCl(aq)

23. Why do some ionic compounds dissolve and others do not?
In some ionic compounds, the attraction between ions is greater than the attraction exerted by water
Solids will dissolve ONLY if the attractive force of water molecules is stronger than the attractive force of the ionic compound
If not, the solids are insoluble
Water does not dissolve nonpolar molecules (like oil)
Water has no positive or negative site on a Nonpolar molecule to which it would be attracted.

24. Electrolytes and Nonelectrolytes
Compounds that conduct an electric current in aqueous solution OR in the molten state
Conduction of electricity requires ions that are mobile and thus able to carry an electrical current
All ionic compounds are electrolytes because they dissociate into ions (also called salts)
Barium sulfate – will conduct when molten, but is insoluble in water
Nonelectrolytes
Do not conduct electricity
Molecular compounds because they do not have ions
Polar covalent molecules such as alcohols - methanol (CH3OH) do not fall apart into ions when they dissolve

25. The ions are free to move when they are molten or in aqueous solution, and thus they are able to conduct the electric current.