Observing Evidence of a Chemical Reaction

Introduction

We live in a world of change—rapid technological changes, severe weather changes, more predictable physical changes, etc. Within our bodies, we depend on complex chemical changes to breathe, to see, to move, indeed—to grow. What is a chemical change? How can we identify a chemical change? What kinds of evidence can we gather to determine that a chemical change has occurred, whether in nature or in the laboratory?

Concepts

• Chemical change
• Chemical reaction
• Chemical properties

Background

A chemical change is defined as a change in the composition and properties of a substance. The transformation of old materials (reactants) into new substances (products) as a result of a chemical change is called a chemical reaction. Both in the natural world and in the laboratory we recognize that a chemical reaction has occurred by observing the appearance of products with physical and chemical properties different from the reactants from which they were made.

There are many types of observable changes that can be used to identify that a chemical reaction has occurred. Signs of chemical change include:

• Formation of solid particles, called a precipitate, upon mixing of two liquids
• Release of gas bubbles that are not due to a physical change (such as boiling)
• A color change that does not result from dilution or color mixing
• A temperature change that is not caused by external heating or cooling

These signs of change illustrate the dynamic nature of chemical reactions. What they do not show, however, is where the real action is taking place—at the level of atoms and molecules. Chemical reactions arise due to the rearrangement of atoms and molecules. Compounds are formed when atoms of different elements combine to form molecules. When the forces linking atoms together within molecules break, compounds can also decompose to their original elements. Molecules of one compound can exchange atoms or groups of atoms with other elements or compounds, generating new substances.

What evidence do we have for these rearrangements at the atomic level, which we cannot see? The law of conservation of mass states that in any physical or chemical reaction, mass is neither created nor destroyed—it remains the same, or is conserved. This implies that atoms are not gained or lost in a chemical reaction, they are only rearranged.

Experiment Overview

The purpose of this demonstration is to have the students examine the chemical properties of hydrochloric acid and copper(II) chloride and to identify the types of reactions they each undergo.

Materials

Safety Precautions

Hydrochloric acid and sodium hydroxide are corrosive to skin and eyes. Ammonia water solution is corrosive to skin and eyes and irritating to eyes and lungs. Copper(II) chloride is highly toxic by ingestion and silver nitrate solution will stain skin and clothes. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Unreacted aluminum foil may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. The contents of the beakers 2, 3 and 4 may be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b. Allow the solid material in the remaining beakers to settle. Decant the copper(II) chloride solution in beaker 4 and 6 down the drain according to Flinn Suggested Disposal Method #26b. Dispose of the solid copper and leftover aluminum foil from beaker 4 and the copper(II) carbonate from beaker 6 in the solid waste according to Flinn Suggested Disposal Method #26a. The contents of beaker 1 should be filtered to separate the insoluble silver chloride product. Silver chloride may be disposed of according to Flinn Suggested Disposal Method #26a. The remaining filtrate may be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Make copies and distribute Data Table A and Data Table B, including the Results Table, to each student.
2. Use a thermometer to measure the temperature of each solution before and after each reaction. Briefly rinse the thermometer with distilled water and pat it dry with a clean paper towel between steps.
3. Have students record all immediate changes in each reaction step. Continue to record observations as the reactions proceed—evidence of reaction may continue beyond 2–3 minutes.

Part A. Reactions of Hydrochloric Acid
Record all observations and answer all questions in Data Table A.

Reaction 1—Mixing of hydrochloric acid and silver nitrate solution

1. Add 100 mL of 2 M HCl to the 100-mL marking of a clean 400-mL beaker. Have the students record the color and appearance of the hydrochloric acid solution.
2. Measure and have students record the initial temperature of the hydrochloric acid solution in the 400-mL beaker. Note: Assume that all solutions used in the demonstrations are at the same initial ambient temperature.
3. Using a clean graduated cylinder, add 35 mL of the 0.1 M silver nitrate solution to the beaker. Have students record all observations.
4. Set the beaker aside. Tell students to monitor the beaker for any further changes over time.
5. Rinse the graduated cylinders using the wash bottle of distilled or deionized water into the 250-mL beaker.

Reaction 2—Mixing of hydrochloric acid and sodium hydroxide solution

6. Using a graduated cylinder, add 100 mL of 2 M HCl to a clean 400-mL beaker.
7. Test the solution in the beaker by taking the stirring rod, dipping it into the solution and touching the end of the rod to a piece of blue litmus paper. Have students record the initial and final color of the litmus paper.
8. With the stirring rod in the beaker, add 100 mL of 2 M NaOH solution using a graduated cylinder. Immediately place the thermometer in the beaker and have the students record any temperature change. After 1–2 minutes, test the final solution again with a fresh piece of blue litmus paper. Note the color change, if any.
9. Rinse the graduated cylinders with distilled or deionized water into the 250-mL beaker. Set the beaker aside for later disposal.

Reaction 3—Mixing of hydrochloric acid and solid sodium bicarbonate

10. Using a graduated cylinder, add 100 mL of 2 M HCl to a clean 400-mL beaker.
11. To the beaker, add ½ teaspoon (about 3 g) of solid sodium bicarbonate. Once any initial evidence for reaction has subsided, continue adding sodium bicarbonate in small amounts until a total of three portions have been added. Does the reaction continue at the same pace with each subsequent addition? Have students record all changes, including the temperature.
12. Rinse the graduated cylinder with distilled or deionized water into the 250-mL beaker. Set the beaker aside for later disposal.

Part B. Reactions of Copper(II) Chloride
Record all observations and answer all questions in Data Table B.

Reaction 4—Mixing of copper(II) chloride solution and aluminum foil

13. Using a graduated cylinder, add 100 mL of 0.5 M CuCl₂ to a clean 400-mL beaker. Have students record the color and appearance of the solution in Data Table B.
14. Add a small piece, approximately 2" x 2", of crumpled aluminum foil to the beaker. Immediately place the thermometer in the beaker. Have students record all observations.
15. Rinse the graduated cylinder with distilled or deionized water into the 250-mL beaker. Set the beaker aside for later disposal.

Reaction 5—Mixing of copper(II) chloride solution and ammonia water solution

16. Using a graduated cylinder, add 40 mL of 0.5 M CuCl₂ to a clean 400-mL beaker.
17. Using a clean graduated cylinder, add 100 mL of 1 M ammonia water solution to the beaker. Immediately place the thermometer in the beaker. Have students record all observations.
18. Rinse the graduated cylinders with distilled or deionized water into the 250-mL beaker. Set the beaker aside for later disposal.

Reaction 6—Mixing of copper(II) chloride solution and sodium carbonate solution

19. Using a graduated cylinder, add 100 mL of 0.5 M CuCl₂ to a clean 400-mL beaker.
20. Using a clean graduated cylinder, add 100 mL of 0.5 M sodium carbonate solution to the beaker. Immediately place the thermometer in the beaker. Have students record all observations.
21. Set the beaker aside. Tell students to monitor the beaker for any further changes over time.

Student Worksheet PDF

13941_Student1.pdf

Teacher Tips

• Enough chemicals are included to perform the demonstration as written seven times.
• The six 400-mL beakers can be labeled 1–6, with 100 mL of the hydrochloric acid and the copper(II) chloride solutions added to the correct beakers before starting the demonstrations (40 mL of copper(II) chloride solution in beaker 5).
• The solutions will probably need to be stirred with a stirring rod to get accurate temperature readings. Remind students, however, that they should never use a thermometer as a stirring rod.
• Digital thermometers are more effective than bulb thermometers in producing rapid and visible readings.
• The exact concentrations and amounts of solutions and reagents are not critical to the success of the demonstration. In the case of the reaction of HCl with NaOH, the overall concentrations and amounts should be approximately equal in order to observe that the final reaction mixture is neutral.
• One of the more surprising observations for students is the temperature decrease in the reaction of HCl with NaHCO₃. Students will almost always guess that the temperature should increase, given the vigorous fizzing and effervescence that takes place.
• This experiment is designed as a general introduction to the nature and kinds of chemical reactions. It integrates observation and measurement skills with concepts of chemical properties and chemical reactivity.
• Learning chemistry is often difficult for students, because it occurs at three levels. Students must relate observations made at the macroscopic level to abstract events at the molecular level, which they cannot see or feel. They must then translate both observations and abstract concepts to a new symbolic level, when they learn to use symbols and to write equations. This introductory experiment has been written to allow students to connect the first two levels of learning in chemistry. To maintain the introductory level, the use of chemical equations has not been included. Balanced chemical equations for all of the reactions are provided in the Supplementary Material.
• Ammonia water solution is also called ammonium hydoxide solution and can be written as NH₃(aq) or NH₄OH(aq).

Further Extensions

Supplementary Material

Reactions of HCl

1. HCl(aq) + AgNO₃(aq) → AgCl(s) + HNO₃(aq)
2. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
3. HCl(aq) + NaHCO₃(s) → NaCl(aq) + H₂O(l) + CO₂(g)

Reactions of CuCl₂

1. 3CuCl₂(aq) + 2Al(s) → 2AlCl₃(aq) + 3Cu(s)
2. CuCl₂(aq) + 4NH₃(aq) → Cu(NH₃)₄²⁺(aq) + 2Cl^–(aq)
3. CuCl₂(aq) + Na₂CO₃(aq) → 2NaCl(aq) + CuCO₃(s)

Sample Data

Data Table A. Reactions of Hydrochloric Acid
Initial appearance of HCl Solution? ___Colorless, clear solution; initial temperature: 22°C___

Data Table B. Reactions of Copper(II) Chloride
Initial appearance of CuCl₂ solution? ___Bright blue, clear solution; initial temperature: 22°C___ Results Table
Summarize the observations of chemical change in the reactions of HCl and CuCl₂, respectively. All reactions should be listed; some reactions may appear more than once.

Discussion

Predemonstration Discussion
There are many types of observable changes that can be used to identify that a chemical reaction has occurred. Signs of chemical change include:

• Formation of a solid precipitate upon mixing of two liquids
• Release of gas bubbles that are not due to a physical change (boiling or sublimation)
• A color change that does not result from dilution or color mixing
• A temperature change that is not caused by external heating or cooling

In the demonstrations that follow, the students are to write down their observations of each reaction and identify all signs of chemical change.

References

This kit has been adapted from Flinn ChemTopic^™ Labs, Volume 1, Introduction to Chemistry; Cesa, I., Ed; Flinn Scientific: Batavia, IL, 2002.