Laboratory Activity: Teacher Notes

Major Chemical Concept

The products of a double replacement reaction can be determined experimentally. The amounts of reactants are equimolar.

Level

General or basic student.

Expected Student Background

Students should be familiar with filtration and be able to use a burner safely.

Time

45 min

Safety

Read the Safety Considerations in the Student Version.

Materials (For 24 students working in pairs)

Nonconsumables

• 24 Beakers, 100-mL
  
• 12 Funnels
  
• 12 Erlenmeyer flasks, 125-mL
  
• 48 Test-tubes, 13- x 100-mm
  
• 12 Burners
  
• 12 Stirring rods
  
• 12 Graduated cylinders, 25-mL
  
• 12 Blue cobalt glass squares, if available, for viewing the potassium flame test.

Consumables

• Calcium chloride, CaCl ₂ , 24 g
  
• Potassium carbonate, K ₂ CO ₃ , 30 g
  
• Filter paper
  
• 3 M Hydrochloric acid, HCl, 12 mL (25 mL conc. HCl diluted to 100 mL)
  
• 48 Splints

20 min to assemble the materials.

Pre-Laboratory Discussion

Review the evidence for chemical change and double replacement reactions. Emphasize that the only way we can be sure about the identity of products is to test them. Discuss flame tests if students are unfamiliar with them. The only use of the flame test in this activity is to identify potassium and calcium ions.

Teacher-Student Interaction

During the activity focus students' attention on the difference in properties of the reactants and products. Caution them to observe the flame test before the splint catches on fire. They should compare the colors of the flame tests on the known starting materials and the products.

Anticipated Student Results

SAMPLE DATA TABLE

Test-Tube Number

Test	1	2	3	4
Flame	Violet	Red	Violet	Orange
Acid	Bubbles	No Rxn	No Rxn	Bubbles

Answers to Data Analysis and Concept Development

1. A precipitate is formed.
2. Observations of reactants:
3. Potassium carbonate - white solid Calcium chloride - white solid Both dissolved in water Observations of products One substance precipitates and the other dissolves in water, passing through the filter paper.
4. The precipitate is calcium carbonate; the substance in the filtrate is potassium chloride.

Answers to Implications and Applications

2. K ₂ CrO ₄ (aq) or Na ₂ CrO ₄ (aq)
3. Make a solution of a small portion of each solid. Test each solution with a solution of calcium chloride. The one that precipitates is K ₂ CO ₃ . Alternatively, add dilute hydrochloric acid to both. The one that forms bubbles is K ₂ CO ₃ .

The discussion should focus on student answers to Data Analysis and Concept Development questions. The Pictures in the Mind section is particularly important in helping students develop the concept of the particulate nature of matter. These pictures can also be used to explain anomalous results arising from incomplete separations.

Extensions

1. Students may plan an investigation to determine the identity of products produced in one of the demonstrations that follow this section.
2. The use of flame tests as an identification tool may be studied further.

Assessing Laboratory Learning

1. Extension 1 could serve as an evaluation. Be sure to use reactions involving chemistry that students can reasonably be expected to know.
2. . Pictures in the Mind of different systems (Example: Demonstration 3) can be used to evaluate whether students have a reasonable concept of what occurs at the molecular level.

	TABLE OF CONTENTS	TOPIC OVERVIEW	CONCEPT/SKILLS DEVELOPMENT	LINKS/CONNECTIONS	APPENDEX