1. A solid formed in an aqueous solution of ionic compounds is called a precipitate. a. Ionic compounds, which consist of a metal bonded to a nonmetal, are collectively known as salts. b. Solutions of dissolved salts consist of separated cations and anions. Chemists know that separated ions are present when ionic solids dissolve in water because the solutions become excellent conductors of electricity. Pure water does not conduct electricity. (See Fig 7.2 in text.) c. When solutions of dissolved salts are mixed together, the process of solid formation is called precipitation. A solid forms because some salt products are insoluble in water. d. The reaction is known as a precipitation (or a double-displacement) reaction . e. Example: when a solution of potassium chromate is mixed with a solution of barium nitrate a yellow precipitate forms.
K
2
CrO
4 (aq)
+ Ba(NO
3
)
2 (aq)

yellow precipitate f. What is the composition of the yellow precipitate? How does it form?
2. To answer the above, remember the basic form of a precipitation reaction
AB + CD

AD + CB
K
2
CrO
4
(aq ) + K
2
CrO
4
(aq)
The cations switch anion partners to form the possible products. The potassium ion is paired with the nitrate anion and the barium cation is paired with the chromate anion to produce the possible products.
3. How to determine the formula of the actual precipitate formed when the two aqueous solutions are mixed? a. Use the Solubility Rules handout (or Table 7.1 in text) to determine the solid product.
1) A soluble solid is one that dissolves readily in water
2) An insoluble or slightly soluble solid does not dissolve, or dissolves very little, in water. b. According to solubility rules, ______________ is insoluble in water, whereas
_______________ is soluble. Therefore, the solid product is ___________ and ___________ remains as dissolved ions in solution.
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Summary: Steps for predicting the precipitate product in aqueous reactions.
Step 1: Write reactants as ions.
Step 2: Exchange the anions of the dissolved compounds (salts) to determine possible products.
Step 3: Use solubility rules to determine if a solid forms.
Questions:
1. Use the solubility rules to predict which of the following substances will be soluble or insoluble in water. a. PbS ___________ i. ammonium sulfide
________________ b. Mg(OH)
2 ________________ j. cobalt(III) nitrate ___________ c. Na
2
SO
4 ________________ k. cesium ___________ d. (NH
4
)
2
S ___________ l. magnesium sulfite ___________ e. BaCO
3 ________________ m. aluminum nitrate ___________ f. AlPO
4 ________________ n. tin(II) hydroxide ___________ g. PbCl
2 ________________ o. manganese(II) iodide ___________ h. Au(NO
3
)
3 ________________ p. iron(II) nitrate ___________
2. Apply what you’ve learned. Write the balanced molecular equation for each reaction below. Show the appropriate states of matter. Use your solubility table to determine if a precipitate is formed.
Note: if no precipitate is formed, there was no reaction. All you have is a mixture of dissolved ions. a. H
2
SO
4
(aq) + BaCl
2
(aq)
 b. AgNO
3
(aq) + KCl (aq)
 c. KNO
3
(aq) + CaCl
2
(aq)
 d. Aqueous ammonium carbonate reacts with aqueous magnesium sulfate
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e. Aqueous cobalt (III) chloride reacts with aqueous sodium hydroxide f. Aqueous copper (II) sulfate reacts with aqueous sodium carbonate
Answer problem 17 (a, c and e) on page 202 and problem 71 on page 205 in your text.
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For this section all you need to know is that the complete equation showing all formulas for reactants and products and their states is called the molecular equation for the reaction. It treats all compounds in the reaction as molecules.
Ex:
1. An acid is a compound that produces H + ions (protons) and an anion when dissolved in water.
2. A strong acid is one in which every molecule dissociates to give ions. Since they completely dissociate they are strong electrolytes
Ex:
3. A base is a compound that produces hydroxide ions (OH
-
) and a cation when dissolved in water.
4. A strong base is one that completely dissociates into ions when dissolved in water. Strong bases are, therefore, strong electrolytes.
Ex:
5. When strong acids and strong bases mix, the fundamental chemical change that occurs is when H
+ ions combine with OH
-
ions to form water. The formation of water is the driving force for this reaction.
The cation from the base and the anion from the acid generally remain as a dissolved salt (spectator ions) or may precipitate out as a solid salt, depending on their solubility. (See Summary chart on page 188.)
6. Every strong acid-strong base reaction produces ______________________________.
Example. Write the molecular equation of the following reactions
Nitric acid reacts with sodium hydroxide to produce water and a dissolved salt
Aqueous sulfuric acid is mixed with aqueous sodium hydroxide
7.
Solve problem 39 (a to d) on page 203. Water and a salt are the products of any acid-base reaction. These problems ask you to identify the formula of the salt produced. (For problem 39d please note the correct formula for calcium hydroxide is Ca(OH)
2
.)
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