APPLYING THE SCIENTIFIC METHOD TO SOLVE A CHEMICAL PROBLEM 5 Solution Unknown Lab APPLYING THE SCIENTIFIC METHOD TO SOLVE A CHEMICAL PROBLEM

5-Solution Unknown Lab Purpose: To use the scientific method to determine the relationship between a set of numbered solutions and a set of lettered solutions. Safety: Wear safety glasses. Avoid contact with chemicals. Wash your hands after the experiment.

Which letters match with which numbers? Initial Observations: All pipettes contain cl,c liquids A 1 B 2 C 3 D 4 E 5

Types of observations Clear, colorless (cl,c) clear, yellow liquid white precipitate (ppt) Yellow ppt

Data Table: Abbreviations: cl = clear, c = colorless, NR = no reaction, ppt = precipitate LETTERS: Initial observations: All chemicals are c,cl liquids. A B C D E X

Data Table for Lettered Solutions Initial Observations: All lettered solution are clear and colorless liquids. COMBINATION TESTED Final Observations A + B A + C A + D A + E B + C B + D B + E C + D C + E D + E

Data Table: Abbreviations: cl = clear, c = colorless, NR = no reaction, ppt = precipitate LETTERS: Initial observations: All chemicals are c,cl liquids. 1 2 3 4 5 X

Data Table for Numbered Solutions Initial Observations: All numbered solutions are clear and colorless liquids. COMBINATION TESTED Final Observations 1 + 2 1 + 3 1 + 4 1 + 5 2 + 3 2 + 4 2 + 5 3 + 4 3 + 5 4 + 5

Example Practice: 4 Solution Unknown Initial Observations: All pipettes contain cl,c liquids A 1 B 2 C 3 D 4

Data Table – 4 solution unknown Test Cross Outcome A + B Purple ppt A + C Green Clear A + D No Rxn B + C White ppt B + D C + D Test Cross Outcome 1 + 2 Green Clear 1 + 3 White ppt 1 + 4 2 + 3 No Rxn 2 + 4 Purple ppt 3 + 4

Data Table – 4 solution unknown Test Cross Outcome A + B Purple ppt A + C Green Clear A + D No Rxn B + C White ppt B + D C + D Test Cross Outcome 1 + 2 Green Clear 1 + 3 White ppt 1 + 4 2 + 3 No Rxn 2 + 4 Purple ppt 3 + 4 Purple ppts: A + B = 2 + 4; A = 2, B =4, C =1, D =2 Green, clear: A + C = 2 + 1;

A = 2, B =4, C =1, D =2 is a hypothesis Key Aspect of Science – hypothesis must be tested by experiments. A hypothesis must have predictive power – it must predict the outcome of experiments that have not yet been carried out. Example: If A + B = purple ppt and A =2, then 2 + B = purple ppt. Test 2 + B to see if purple ppt is produced.

5 – Solution Unknown 1) Combinations which produce brown ppt: + = + 2) Combinations which produce yellowish white ppt: + = + 3) Two Combinations which produce magenta pink: + = + AND + = + HYPOTHESIS: A = B = C = D= E =

TESTING HYPOTHESIS WITH NUMBER-LETTER CROSSES HYPOTHESIS MUST BE TESTED EXPERIMENTALLY:

Questions 1. In generating your hypothesis, what color was the most useful to begin the matching process and why? 2. Why is it important to have test crosses; that is, what purpose do they serve?