1. Precipitate Formation: Formation of Lead (II) Iodide When mixed, two clear solutions produce a dense yellow precipitate

2. Purpose To observe the formation of a precipitate from ionic compounds in an aqueous state.

3. Demonstration The formation of PbI 2(s) from Pb(NO 3 ) 2 and 2KI solutions. Pb 2- NO 3 - K+ I-K+ I- + PbI 2(s) K+K+ NO 3 -

4. Concepts Double Replacement Reactions Solubility Precipitates

5. Double Replacement Reactions Occur when the elements in a solution of reacting compounds exchange places, or replace one another. AB + CD  AD + CB For this demonstration: Pb(NO 3 ) 2(aq) + 2KI (aq)  PbI 2(s) + 2KNO 3(aq)

6. Solubility Salts are substances composed of ions; many salts are soluble in water Solubility is the amount of a substance needed to make a saturated solution at a specific temperature. Solubility of solids and liquids varies widely, for example: 35.7g of NaCl dissolves in 100mL of H 2 O, but only 0.044g of PbI 2 dissolves in 100mL of H 2 O

7. Solubility Solubility is temperature dependent. For gases, an increase in temperature will decrease the amount of gas that can dissolve For salts, the amount and rate of dissolving will increase as temperature increases: Temperature Amount & Rate of Dissolving of Salt

8. Precipitates A precipitate is an insoluble solid formed when two ionic solutions react. Pb(NO 3 ) 2 and KI are ionic compounds that dissociate into ions as they dissolve in water, but when mixed together, undergo a double replacement reaction to form an insoluble yellow precipitate, PbI 2. Sometimes a downward arrow behind a compound is used to indicate that the compound is insoluble; sometimes the letter “s” in parentheses for “solid” is used: PbI 2 (s) or PbI 2 * Both mean “precipitate”

9. Conclusions Soluble ionic compounds dissociate into water and when allowed to react, may produce an insoluble compound known as a precipitate. If a precipitate is produced, the reaction that occurs is known as a double replacement reaction.