1. Water and Aqueous Solutions
Image by Fir0002/Flagstaffotos
Water and Aqueous Solutions
Photo by Chris73

2. Solutions
Solutions one of the most common forms of mixtures. They are created when substances are evenly mixed together.
Solution = Homogeneous Mixture
Aqueous solutions are formed with solutes are dissolved in water
Alloys are solid solutions formed when metals are evenly mixed together.
Gaseous solutions, like air, formed when gases mix together.

3. In order to understand solutions, you have to understand 2 words:
Solute: _______________________________________
Solvent: ______________________________________
The substance being dissolved
The substance doing the dissolving
solvent
water
saltwater
solution
salt
solute
In aqueous solutions, the solvent is always water. This is the definition of an aqueous solution.

4. Water
Hydrogen and oxygen share their electrons to create covalent bonds. They do this to have a complete set of valence electrons. This is what creates each covalently bonded water molecule.
However, hydrogen and oxygen do NOT share their valence electrons equally.

5. Oxygen is far more electronegative than hydrogen
Oxygen is far more electronegative than hydrogen. Oxygen is the second most electronegative element. (Fluorine is the most electronegative.) This means that oxygen pulls harder on the electrons in the covalent bond than hydrogen.

6. If an element pulls electrons towards itself, will it become more positive or negative?
Explain your answer.
It will become more negative because it is pulling negative electrons towards itself.
If an element has electrons pulled away from it, will it become more positive or negative? Explain your answer.
It will become more positive because it is losing negative charge.

7. Compounds like water where electrons are not shared equally are _______ molecules.
POLAR
Polar molecules have a positive pole and a negative pole. Which element, H or O, is more electronegative? _______
oxygen δ-
Which element, H or O, will be more negative? _______
oxygen
Which element, H or O, will
be more positive? _________ δ+ δ+
hydrogen

8. So what does the δ mean? δ- δ+ δ+
δ is the lowercase Greek letter delta. In chemistry, we use it to mean “partial”. Ions have full charges because the gain or lose electrons, but polar molecules are just sharing them unevenly, so their charges are partial charges. δ+ δ+

9. Important Fact:
The ratio of hydrogen to oxygen (H:O) is always 2:1. Water always has 2 hydrogen and 1 oxygen. This will not change without performing a chemical reaction.

10. Water is made of very polar molecules that have a positive pole and a negative pole. Since water is so polar it can dissolve most things that are polar covalent or ionic.
When solutes are dissolved in water, a solution is formed. Since water is the solvent, we refer to them as aqueous solutions.

11. When ionic compounds dissolve, they break apart into their ions.
Dissolution
When substances dissolve, they simply break apart into their smallest units. These are too tiny to see, so it often looks like they disappear.
When ionic compounds dissolve, they break apart into their ions.

12. PbI2 is an insoluble solid.
Solubility
Solubility is a physical property that describes how well a substance will dissolve in a solvent like water. While there are varying degrees of solubility, we will talk about things as though they are completely soluble or completely insoluble.
PbI2 is an insoluble solid.

13. Solubility
There are a list of solubility rules which allow you to determine if a substance will dissolve in water or not. These appear on your STAAR Reference Sheet

14. Solubility
The top section of the chart contains ions which are soluble in water.
Examples:
The right column contains exceptions. If a compound contains ion in the left column AND the ion in the right column, it is not soluble.
CaBr2 is soluble, because most compounds of Br- are soluble.
PbBr2 is insoluble, because Pb+2 is an exception to Br-’s solubility.
Pb(NO3)2 is soluble, because ALL NO3- compounds are soluble with no exceptions.

15. Solubility
Examples:
CaCO3 is insoluble, because most compounds of CO3-2 are insoluble.
The bottom section of the chart contains ions which are insoluble in water.
Na2CO3 is soluble, because alkali metals are an exception to CO3-2’s insolubility.
Mg(OH)2 is insoluble, because most compounds of OH- are insoluble.
Ca(OH)2 is soluble, because Ca+2 is an exception to OH-’s insolubility.
The right column contains exceptions. If a compound contains ion in the left column AND the ion in the right column, it is soluble.

16. Only look in the right hand column for exceptions AFTER you have found one of the ions in the left hand column. S I S S I S I S S S I S
If you don’t remember what elements are the alkali metals (group 1), write them out on your notes below the table.

17. Precipitation Reaction
A precipitation reaction occurs when aqueous solutions are mixed, and a reaction occurs which forms a solid product. (This solid product is known as the precipitate.)
Pb(NO3)2 (aq) KI (aq)
2KNO3 (aq) + PbI2 (s)

18. Precipitation Reaction
Notice the reactants [Pb(NO3)2 and KI] are both soluble. Inside each bottle of reactant, there are ions floating around in the solution. This is because ionic compounds break apart into ions when they are dissolved.

19. Precipitation Reaction PbI2 is an insoluble solid.
When the solutions are mixed, the ions from each solution are free to meet. When ions come together that form an insoluble compound, they become a precipitate and fall to the bottom of the test tube.
PbI2 is an insoluble solid.

20. Precipitation Reaction
There will also be a product that is soluble and still dissolved in the solution. These ions will remain dissolved in the water, and they are called spectator ions.
KNO3 is soluble and still dissolved in the water.

21. For each chemical equation on your notes, determine which product is the precipitate using your solubility rules.